Paleoecological dynamics of the Jurassic-Cretaceous transition: insights from the Vaca Muerta Formation marine biota

1 Introduction

Comprehensive reconstructions of past marine ecosystems are essential for understanding how communities reorganize across critical boundaries and during intervals of environmental change. Historically, ecological studies integrating both benthic and pelagic components during the Mesozoic have predominantly relied on a number of Northern Hemisphere sites with fine to exceptional fossil preservation. Remarkable examples come from the Triassic localities of south-eastern China (Hu et al., 2011; Ma et al., 2022), the Jurassic lithographic limestones of Solnhofen (Barthel et al., 1994; Ebert and Kölbl-Ebert, 2025) and Nusplingen (Dietl and Schweigert, 2004), the Jurassic of the Bighorn Basin in U.S.A (McMullen et al., 2014), and sections along the Cretaceous Western Interior Seaway (Kauffman and Caldwell, 1993), among others. In contrast, high-resolution paleoecological studies from the Southern Hemisphere are still heavily atomized. In Western Gondwana, research papers dealing with the systematics, paleobiology, and taphonomy of marine vertebrates collected from sections that also bear invertebrates have received substantial attention. However, rather than conducting a comprehensive paleoecological reconstruction of the entire ecosystem, these studies often take a compartmentalized approach; while vertebrates remain the primary focus of paleobiological and taphonomic interpretations, the associated invertebrates frequently serve only to provide the biostratigraphic constraint or to assist in the sequence stratigraphic framework (Stinnesbeck et al., 2014; Gouiric-Cavalli et al., 2018; Lazo et al., 2018; Fernández et al., 2019). Conversely, invertebrate paleoecology is treated separately from the vertebrate record (e.g. Lazo et al., 2005; Archuby et al., 2011). This compartmentalization has left a significant gap in our understanding of the coordinated trophic and eco-spatial responses of these marine ecosystems to autogenic and allogenic changes during the Late Jurassic and Early Cretaceous.

The Vaca Muerta Formation in the Neuquén Basin (west-central Argentina) provides an exceptional, continuous high-resolution sedimentary archive to address this bias. Deposited as an organic matter-rich carbonate ramp during the early Tithonian to the early Valanginian, this unit yields a remarkably well-preserved fossil record of both invertebrate and vertebrate associations. However, previous paleoecological characterizations focused on the reconstruction of sedimentary, stratigraphic and taphonomic dynamics (Doyle et al., 2005; Kietzmann and Palma, 2009) and their relation to vertebrate preservation (Spalletti et al., 1999; Scasso et al., 2002), neglecting the contribution of invertebrates and the relationships among all faunal components.

To bridge these gaps, the main goal of this contribution is to characterize the Vaca Muerta Formation marine biota in terms of its paleoecology through the evaluation of the trophic dynamics, guild structures, and eco-space occupation of benthic invertebrates, ammonites, fishes, and marine reptiles. To achieve this, we synthesize and analyze two well dated and stratigraphically constrained taxonomic datasets from distinct palaeogeographic settings within the basin:

• A detailed bed-by-bed collection of ammonite and benthic invertebrate associations from the lower Tithonian to the lower Valanginian at Sierra de la Cara Cura (southern Mendoza).
• A stratigraphically constrained synthesis of the vertebrate ichthyofauna and herpetofauna from Cerro Lotena/Cerro Granito (southeastern Neuquén).

By employing quantitative methods to assess morphology and possible life habits of the ammonite groups, alongside multi-level tiering, motility and feeding-strategy analyses for the benthic fauna, dentition and paleoecological preferences analysis of the vertebrate associations, as well as considering potential trophic interactions between all the groups, we aim to get an overall paleoecological picture of the whole marine ecosystem dynamics. This is the first research study integrating both macrofossil invertebrate and vertebrate marine associations across the Upper Jurassic–Lower Cretaceous transition in Western Gondwana.

2 Geological setting

2.1 Upper Jurassic-Lower Cretaceous in the Neuquén Basin

The Neuquén Basin is a large Mesozoic retro-arc basin related to subduction along the southwestern margin of Gondwana (Figure 1A). It is located along the eastern foothills of the Andes in west-central Argentina, bounded to the east by the Andean fold and thrust belt. An outstanding feature of the basin is the Huincul High, an east–west trending structure that divides the basin into two depocenters. The more important one is the Neuquén Embayment north of the Huincul High, and the less extensive Picún Leufú depocenter is situated to the south (Domínguez et al., 2020; Ramos et al., 2020).

The Mendoza Group represents the Upper Jurassic–Lower Cretaceous megasequence of the Neuquén Basin (Legarreta and Gulisano, 1989) that accumulated during a thermal subsidence phase (Ramos, 1999; Howell et al., 2005). It spans approximately the Kimmeridgian–Hauterivian interval and records the final major marine phase of the basin, when it was mostly connected to the west to the Pacific Ocean through marine seaways across an active volcanic arc (Legarreta and Uliana, 1991, 1996).

Although the names of stratigraphic units often vary regionally within the basin, the Mendoza Group within the Neuquén Embayment typically includes two major depositional phases reflecting the evolution from continental to open marine conditions, which in turn end with a progressively shallowing trend.

The lower phase includes the Tordillo and Vaca Muerta formations. The Tordillo Formation records non-marine clastic continental deposits of Kimmeridgian to lower Tithonian age (Spalletti and Colombo Piñol, 2005).

The Vaca Muerta Formation (lower Tithonian–lower Valanginian; Weaver, 1931) usually overlies the continental beds of the Tordillo Formation, and consists of thick successions of dark shales, mudstones, marls, and ash-fall tuffs. Limestones and calcareous nodules are usually highly fossiliferous, and the organic-rich basinal mudstones, deposited under oxygen-deficient conditions, form one of the most significant hydrocarbon source rocks globally (Legarreta and Villar, 2015). Within the Vaca Muerta Formation, the Los Catutos Member, of restricted regional distribution in western Zapala, Neuquén, is composed of fine marls and limestones reflecting orbital cyclicity (Scasso et al., 2002; Kohan Martínez et al., 2018). This late Tithonian member yields abundant and exquisitely preserved ammonites, fishes, and marine reptiles in lithographic limestones, representing a Southern Hemisphere analogue to classic European plattenkalk deposits (Gasparini et al., 2015).

In the western sector of the Neuquén Basin, the organic-rich shales of the Vaca Muerta Formation are intercalated with turbidite sandstone intervals and slump structures of the Huncal Member (Leanza et al., 2002), a Tithonian to Valanginian diachronic member representing turbiditic deposits developed in response to an array of controlling factors that interplayed differently across the basin (Naipauer et al., 2020).

Finally, the upper phase of the Mendoza Group comprises the Mulichinco/Chachao and Agrio formations (Valanginian–Hauterivian). These units record several second- and third-order sea level fluctuations, encompassing a wide arrange of environments, from fully marine to fully continental, and document the transition to the more restricted marine and continental environments that characterize the overlying Bajada del Agrio Group (Legarreta and Kozlowski, 1981; Legarreta and Uliana, 1999; Schwarz et al., 2011; Spalletti et al., 2011).

2.2 Sierra de la Cara Cura section

At Sierra de la Cara Cura, the Vaca Muerta Formation is 225 m thick (Figures 1B, 2) up to the Intravalanginian Discontinuity, after which lies the Mulichinco Formation (Gulisano et al., 1984; Schwarz and Buatois, 2012; Toscano et al., 2025). The geochronological framework relies on the interpretation of ammonite zones from the taxonomic and biostratigraphic analysis of 53 ammonite beds, ranging from the lower Tithonian Virgatosphinctes andesensis Zone to the lower Valanginian “Neocomites wichmanni” Zone (Vennari, 2016; Vennari et al., 2016; Vennari and Aguirre-Urreta, 2017; Marin et al., 2023). Geochronological spanning of the ammonite zones follows the recent revision of Vennari et al. (2026) and the systematic assignation of the ammonite fauna bed-by-bed is included as supplementary information (Supplementary Material S1) as well as an abridged taxonomic appendix accounting for taxa in open nomenclature (Supplementary Material S3; Marin, 2026).

The Vaca Muerta Formation deposits are typically represented by the rhythmic alternation of shaly to marly intervals with more weathering-resistant limestones, in the context of a westward-prograding homoclinal carbonate ramp system (Legarreta and Uliana, 1991; Kietzmann et al., 2014). Four facies associations are represented along the section (Figures 2A–E), corresponding to outer to middle ramp settings: FA-1) outer ramp facies association, FA-2) middle to proximal outer ramp facies association, FA-3) HCS-dominated middle ramp facies association, and FA-4) oyster-biostromes dominated middle ramp facies association. Facies were interpreted from bed-by-bed field data on strata contacts, lithology, textures, sedimentary structures, fossil content, and faunal taphonomical features, as well as on previous sedimentological and stratigraphical analyses of Schwarz and Buatois (2012); Kietzmann and Palma (2014); Kietzmann et al. (2014) and Toscano et al. (2025).

FA-1. The outer ramp facies association is represented within the lower third of the section. It is dominated by laminated shale/marls and bioclastic marls, that are intercalated with thin massive to laminated mudstones, wackestones, and floatstones. Benthic fauna is scarce and ammonites are mainly preserved as imprints or laterally-crushed shells, and by isolated Laevaptychus-type aptychi. Kietzmann and Palma (2014) reported ichnofossils of the Cruziana ichnofacies for this interval. Deposition in this setting occurred below the storm-wave base in an oxygen-controlled and low-energy environment (Spalletti et al., 2000; Kietzmann and Palma, 2009, 2014).

FA-2. The middle to proximal outer ramp facies association is well represented all along the section. It consists of the alternation of decimeter to meter-scale massive, laminar or laterally concretional bioclastic mudstones/wackestones and floatstones/rudstones, with mostly laminated bioclastic marl intervals which can bear mid to large spheroidal carbonate nodules. Benthic fauna is poorly/moderately abundant at the base to more abundant to the top of the section, showing signs of some reworking. Nektonic fauna is dominantly represented by ammonite shells that are either recorded as imprints, laterally crushed shells or as well-preserved casts in concretional beds. Occasionally, loose fish-scales, disarticulated ichthyosaur vertebrae and wood-fragments have been recorded. Kietzmann and Palma (2014) informed feeding trace fossils in wackestone beds.

This facies association has been interpreted as being deposited close to the storm wave-base, under oxygen-fluctuating conditions (Schwarz and Buatois, 2012; Kietzmann and Palma, 2014; Kietzmann et al., 2014).

FA-3. HCS-dominated middle ramp facies association is recorded at the middle portion of the section. It shows similar features to the previously described setting but it is characterized by hummocky cross-stratified grainstones with erosive bases. Concretional and laminar bioclastic mudstones/wackestones and floatstones are also recorded. Bioclasts, including disarticulated or fragmented mollusk shells, bone fragments, and fish scales, are frequently chaotically arranged. Three-dimensional ammonite casts are abundant on laminated and concretional floatstones/rudstones. This energy-fluctuating setting is interpreted as deposited above the storm-wave base under well-oxygenated conditions with development of Cruziana and Skolithos ichnofacies (Kietzmann and Palma, 2014; Kietzmann et al., 2014).

FA-4. Oyster-biostrome dominated middle-ramp facies association is well developed to the top of the section and encompasses the alternation of nodular and oyster floatstones and bioclastic mudstones and wackestones with fine calcareous sandstones and laminated to massive marls. The most distinctive facies of this association are the meter-scale oyster autobiostromes to autoparabiostromes (Kietzmann and Palma, 2014; Toscano et al., 2025) and the large ammonite shells preserved within meter-long sized nodules. Passively infilled Thalassinoides galleries are recorded in mudstone/wackestone beds.

This setting has been interpreted as a middle to outer ramp (Kietzmann and Palma, 2014; Kietzmann et al., 2014) or a distal-mid to proximal-outer ramp (Schwarz and Buatois, 2012; Toscano et al., 2025). In either case, it was deposited below but near the storm-wave base, under shifting energy and oxygen availability.

2.3 Cerro Lotena section

The exposures of the Vaca Muerta Formation at Cerro Lotena/Cerro Granito are 130 to 160 m thick (Windhausen, 1914; Weaver, 1931; Suero, 1942; Parent and Garrido, 2021) (Figure 1C).

Deposition of the Vaca Muerta Formation in the Picún Leufú depocenter was heavily influenced by the tectonic activity and siliciclastic input of the Huincul High (Ploszkiewicz et al., 1984; Naipauer et al., 2012; Domínguez et al., 2020). Sedimentary facies interpretations on the nearest Picún Leufú area to the west describe the evolution from a siliciclastic platform at the base of the Vaca Muerta Formation to a mixed siliciclastic-carbonate ramp to the top (Spalletti et al., 2000; Krim et al., 2017). The basal portion is mostly represented by yellow-greenish laminated shales interspersed with coarser beds reflecting distal storm and turbidite deposits (Spalleti et al., 2014) under anoxic to dysoxic bottom conditions (Hiriart et al., 2013). The upper portion of the Vaca Muerta Formation includes sandy limestones deposited on an outer-ramp setting (Spalletti et al., 2000; Leanza et al., 2011; Krim et al., 2017). Marine connections were limited and water circulation was partially restricted, but the water column was well oxygenated as demonstrated by the abundant record of nektonic vertebrate and invertebrate fauna (Windhausen, 1914; Leanza, 1980; Gasparini et al., 1997; Gouiric-Cavalli, 2017; Gouiric-Cavalli and Arratia, 2022) and microfossil associations (Hiriart et al., 2013). The abundant ammonite content points to a lower to upper lower Tithonian age for the Vaca Muerta Formation at this area (Weaver, 1931; Leanza, 1980; Parent and Garrido, 2021).

3 Materials and methods

3.1 Nektonic invertebrate faunas

Nektonic macroinvertebrate faunas of the Vaca Muerta Formation have been studied for a long time, with specific focus on ammonite taxonomy and biostratigraphy (Aguirre-Urreta et al., 2011; Riccardi et al., 2011; Leanza et al., 2020, and references therein), and also, albeit in a much lesser extent, on nautilids (Cichowolski, 2003) and belemnites (Vennari et al., 2023). At the Sierra de la Cara Cura, a single nautilid phragmocone assigned to Cymatoceras perstriatum was recorded loose at the base of the Vaca Muerta section, and undescribed belemnite remains were reported from thin-sections by Kietzmann and Palma (2014). Besides cephalopods, saccocomid microcrinoids occupied the water column and are usually preserved as disarticulated ossicles in outer-ramp facies of the Vaca Muerta Formation, and were reported from the Windhauseniceras internispinosum Zone at the studied locality (Kietzmann and Palma, 2009).

With more than 672 records at the Sierra de la Cara Cura section, ammonites are the most abundantly represented nektonic invertebrate group. Of these, 638 were able to be classified up to the species/genus-level, with 45 species ranging from the lower Tithonian to the lower Valanginian (Figure 2A), and 481 were suitable for the analyses described below. In order to explore ammonite ecospace occupation of the water column during that time span, we took advantage of the link between the ammonite shell external morphology and their possible mode of life (Westermann, 1996; Olóriz and Rodríguez-Tovar, 1999; Lukeneder, 2015 and references therein), as depicted by their morphological distribution in the ternary Westermann Morphospace diagrams designed by Ritterbush and Bottjer (2012) (Figure 3).

The Westermann Morphospace allows displaying planispiral ammonite numerical data on shell morphology relying on three parameters: umbilical ratio, whorl thickness, and whorl expansion rate. Maximum values for each of them characterize an end-member morphology: serpenticone, spherocone, and oxycone (Ritterbush and Bottjer, 2012; Yacobucci, 2018), which in turn can be connected to hypothetical modes of life as active swimmers (nekton), drifters (plankton), vertical migrants, or demersals (spending some time near the sea-bottom) as defined by Westermann (1996). As there is an important overlap between the demersal and the nektonic/planktonic fields (Ritterbush and Bottjer, 2012), both possible life modes are indicated for the species falling in that sector of the morphospace.

As most of the Sierra de la Cara Cura section ammonites are either incomplete (not preserving body-chambers), laterally crushed, or preserved as imprints, the morphological dataset to build the Westermann Morphospace was based on published type material or well-preserved specimens representing the species recognized at the study locality (Supplementary Material S1). To avoid potential interpretation biases, specimens closely resembling shell parameters and sculpture strength of the Sierra de la Cara Cura material were selected from previously published specimens, especially for morphologically variable species or for those classified in open nomenclature. Both representatives of a dimorphic pair, if known, or either one of them (microconch or macroconch), were considered for measurements according to their distribution in the studied section. Whenever possible, adult to subadult specimens were measured to avoid comparing possible different habitats along the ontogeny (Lukeneder, 2015). Adulthood was considered after the species’ known maximum size, and/or uncoiling of the body chamber, or the presence of mature apertural modifications. Measurements were digitally taken over escalated lateral and apertural views of each of the specimens, either from literature illustrations or photographs taken by the authors of the original type-material at their repositories. For plotting the ternary diagrams, the protocol described by Ritterbush and Bottjer (2012) was followed using the WestermannMorphospace3.R package at RStudio (Posit team, 2025).

In the morphological dataset, general shell parameters’ terminology (shell final size, whorl cross-section and coiling degree) follows Klug et al. (2015), and body-chamber length (brevidome, mesodome and longidome) accords to Westermann (1996). Shell ornamentation was qualitatively assigned to four categories as smooth (none to faint ribbing), medium (fine to medium ribbing/sculptural elements, like bullae), medium-coarse (medium ribbing with occasional lateral to ventro-lateral sculptural elements, like spine-like tubercles), and coarse (coarse ribbing/sculptural elements, like blunt-tubercles and spines). Shell sculpture variations can indicate the presence of deeper water inhabitants or “Leiostraca” and shallower waters dwellers or “Trachyostraca” (Westermann, 1996; Lukeneder et al., 2010).

To plot and quantify the variability in species richness, Westermann Morphospace distribution (and, consequently, in ecospace occupation), and ornamentation along the Sierra de la Cara Cura section, we considered four consecutive intervals (Figure 3): Va–Ap: lower Tithonian to lowermost upper Tithonian interval (Virgatosphinctes andesensis–Aulacosphinctes proximus zones); Wi–Ca: upper Tithonian interval (Windhauseniceras internispinosum–“Corongoceras alternans zones”); Sk–An: uppermost Tithonian to upper Berriasian interval (Substeueroceras koeneni–Argentiniceras noduliferum zones); and Sd–Nw: upper Berriasian to lower Valanginian interval (Spiticeras damesi–“Neocomites wichmanni” zones). With the exception of the first interval that involves three ammonite zones, minding that the lower Tithonian Pseudolissoceras zitteli Zone is poorly represented, the other intervals group together two consecutive ammonite zones, thus including all the ammonite taxa recorded at the section. Modes of life and ornamentation percentages for each interval were calculated over their actual representation in each of the intervals over the 481 specimens of the Sierra de la Cara Cura section.

3.2 Benthic invertebrate faunas

Benthic fauna is particularly rare in the Vaca Muerta Formation in comparison to the other marine units of the Mendoza Group, and thus has remained relatively understudied, contrasting with the prolific literature on the Agrio Formation (e.g. Lazo et al., 2005, 2020; Archuby et al., 2011; Milla Carmona et al., 2015; Cataldo and Lazo, 2016; Luci et al., 2016, 2025; Cataldo, 2017; Toscano et al., 2018; Andrada et al., 2020, 2022; Garberoglio et al., 2020, 2022; Schwarz et al., 2022; Lehmann et al., 2026), and to a lesser degree, with the Chachao and Mulichinco formations (Damborenea et al., 1979; Palma and Angeleri, 1992; Palma et al., 2000; Cataldo, 2017; Luci et al., 2019; Toscano et al., 2024). The paucity of benthic fauna along most of the Vaca Muerta Formation has been largely attributed to low oxygenation rates and an overall distal environment (Spalletti et al., 1999; Doyle et al., 2005). However, several authors have noticed an increase in the diversity and abundance of benthic fauna towards the top of the unit, associated with a slightly shallower environment, possibly influenced by storms (Spalletti et al., 2000; Doyle et al., 2005; Kietzmann et al., 2008, 2014). In the Sierra de la Cara Cura section, this overall trend is maintained, with the lower portion of the sedimentary column showing, for a uniform sampling effort along the section, little to no benthic fauna. In contrast, the upper section (from the base of the upper Berriasian Argentiniceras noduliferum Zone upwards) bears an abundant, quite diverse benthic invertebrate fauna (Figure 2A). For this study, previously analyzed data and newly collected material were synthesized to provide new and more complete insights regarding the Vaca Muerta Formation’s benthic fauna’s paleoecological variations. Given the different preservation quality of the fossil material and its strongly variable abundance throughout the sedimentary column, only presence/absence of taxa were considered per sampling level here, whereas abundance was considered qualitatively. See Toscano et al. (2025) for a more comprehensive analysis encompassing the topmost Vaca Muerta section and the lowermost part of the overlying Mulichinco Formation, spanning the Argentinicieras noduliferum–lower Lissonia riveroi ammonoid zones.

Identification of each specimen aimed to the lowest taxonomic level possible (Table 1), but in some cases the poor preservation precluded generic assignment, especially for Tithonian–lower Berriasian bivalves. Many of these are preserved in a very fragmentary state, and/or as outer views only, which prevents examination of the typical diagnostic features such as muscle scars, hinge and ligament areas. Several taxa have a very nonspecific outer morphology and thus could only be determined to a familiar or even suprafamiliar level. However, every specimen assigned to a broad taxonomic category (such as a family) was ensured to represent a single taxonomic entity, or were separated when not (e.g. Nuculanoidea A and B).

Table 1. Benthic taxa recognized in the Sierra de la Cara Cura section.

Distribution across the seven intervals determined along the section, detailing the tiering (T), motility (M) and feeding (F) strategies for each taxon. Tiering: SuE, surficial epibenthic; SeE, semi-endobenthic; SE, shallow endobenthic; DE, deep endobenthic; EE, erect epibenthic; Motility: NMU, non-motile unattached; NMA, non-motile attached; FU, facultative unattached; FA, facultative attached; M, motile; Feeding: FF, filter-feeder; D, depositivore; D/B, depositivore/browser; C, carnivore.

For each taxon, the most likely life habit (tiering and motility) was determined on the basis of literature review, when available, or extant living relatives, in the cases where no data on the fossil taxa’s life habits was available, or when the taxonomic assignment was suprageneric (Table 1). In addition, feeding was also characterized in order to evaluate the various trophic strategies represented along the section. These categories were determined as specified in Toscano et al. (2025).

Based on the taxa’s distribution along the section, seven distinct intervals were recognized (Figure 4). For each of these, richness was determined, and percentages of each paleoecological trait were calculated. In this respect, using presence/absence data instead of relative abundances to characterize the paleoecological traits implies that some abundant taxa may be underrepresented. However, given the scarcity of taxa in the lower portion of the section, we preferred to apply a uniform criterion along the succession in order to be able to compare across intervals confidently. As mentioned above, a quantitative analysis of the upper portion of the section bearing abundant enough fauna to perform statistical analyses is given in Toscano et al. (2025).

The defined intervals were plotted against the ammonite zonation in order to display their distribution along the section. A plot of the richness across the intervals was made in order to evaluate changes along the section.

3.3 Vertebrate faunas

The Vaca Muerta Formation preserves one of the most important Jurassic marine vertebrate assemblages in Gondwana. Fieldwork conducted over approximately the last five decades across several localities in Neuquén and Mendoza provinces has yielded an abundant and diverse record of fishes and marine reptiles (e.g. Gasparini and Fernández, 2011; Gouiric-Cavalli and Cione, 2015; Fernández et al., 2019, and references therein). However, many of the earliest discoveries of fossil fishes resulted from fieldwork primarily focused on invertebrates and marine reptiles (e.g. Leanza and Zeiss, 1990), leading to an underrepresentation in the literature. Some of these vertebrate findings lack proper contextualization, particularly regarding their precise geological and, in some cases, geographical provenance.

To organize the available information on marine vertebrates’ occurrences in the Vaca Muerta Formation, we built a table with a revised taxonomic distribution of fishes and reptiles across the Tithonian–Berriasian interval, pinpointing their occurrences within the current ammonite biozones framework whenever possible (Supplementary Material S2). We excluded the localities where reporting was insufficiently detailed. Furthermore, for specimens lacking a documented association with an accurate ammonoid-based biostratigraphic control, a conservative biozonal range encompassing their most likely interval of origin was assigned. Part of the ichthyofauna and herpetofauna has been studied in previous contributions (see references in Supplementary Material S2), while others have been recorded directly by some of the authors.

Based on records with reliable stratigraphic control, a presence/absence chart was generated to assess the distribution of major vertebrate clades across two broad intervals: lower upper Tithonian (Virgatosphinctes andesensis to “Corongoceras alternans” zones) and upper Tithonian–Berriasian (Substeueroceras koeneni to Argentiniceras noduliferum zones) (Figure 5). These intervals were defined according to the stratigraphic ranges inferred from the ammonite-based biozonal framework described above. Considering that patterns observed in vertebrate diversity, abundance, and stratigraphic distribution should be interpreted cautiously, as they may partly reflect historical collecting practices, uneven research effort, and collection biases rather than the original ecological structure of the ecosystem, we explore the ecosystem structure beyond taxonomic composition. The ecological assignments were inferred from previously published studies using a combination of direct evidence from specimens of Northern Hemisphere (e.g. Martill, 1992; Hua et al., 2024; Herrera et al., 2026) and direct (Gouiric-Cavalli and Arratia, 2022), and indirect evidence from specimens from the Vaca Muerta Formation (e.g. Herrera et al., 2021; Gouiric-Cavalli et al., 2023; Campos et al., 2025). This integrative approach provides a framework for evaluating temporal changes in trophic structure, ecospace occupation, and potential ecological interactions among marine vertebrates throughout the Tithonian–Berriasian interval.

3.4 Institutional abbreviations and collection numbers

All the studied material is deposited in official repositories. CPBA, Colección de Paleontología de la Universidad de Buenos Aires, CABA, Argentina; GPIT, Paläontologische Sammlung der Eberhard Karls Universität, Tübingen, Germany; GZG-INV, Geowissenschaftliche Museum der Universität Göttingen, Göttingen, Germany; MACN, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, CABA, Argentina; MCNAM, Museo de Ciencias Naturales y Antropológicas “Juan Cornelio Moyano”, Mendoza, Argentina; MHNSR, Museo de Historia Natural de San Rafael “Humberto A. Lagiglia”, Mendoza, Argentina; MLP-PV, Museo de La Plata, Paleontología Vertebrados, Buenos Aires, Argentina; MOZ-PI, Museo Provincial de Ciencias Naturales “Prof. Dr. Juan A. Olsacher”, Colección de Paleoinvertebrados, Neuquén, Argentina; MOZ-PV, Museo Provincial de Ciencias Naturales “Prof. Dr. Juan A. Olsacher”, Colección de Paleovertebrados, Neuquén, Argentina; SEGEMAR, Servicio Geológico Minero Argentino, Buenos Aires, Argentina; STIPB, Steinmann-Institut für Geologie, Mineralogie und Paläontologie der Universität Bonn, Bonn, Germany.

Studied invertebrate specimens are deposited under MCNAM-PI 24642–24645, 24686, 24696–24733, 25258-25311, 25314–25387; CPBA 22153, 23500. Vertebrate specimens are deposited under MCNAM-PV 2058–2060, 5089–5090, 5092–5100; MLP-PV 48-I-1-1, 72-IV-7-1, 76-IX-19-1, 80-V-29-1, 83-XI-15-1, 83-XI-16-1, 84-V-1-1, 85-1-12-1, 85-1-15-1, 86-III-30-1, 86-III-30-2, 86-XI-10-1, 95-IV-4-1; MOZ-PV 43, 58, 89, 103, 281, 1106, 1160, 1167, 1533, 1536, 1747, 1763, 1767, 1769, 1788, 1822, 1854, 2306, 2487, 2564, 2606, 2609, 2612, 2806, 3071, 3081, 3642, 3645, 3719, 3724–3725, 3728, 4077, 5787–5788, 6094, 6097, 6111, 6113, 6139–6141, 6144–6146, 6254–6255, 6260, 7260; MACN-N-32, MACN 18043; MHNSR-PV 344, 1195, 2130; CPBA-V 14206, 14209.

Supplementary Material S4 synthesizes data sources and collection localities for the major fossil groups studied in this work.

4 Results

4.1 Ammonites from the Sierra de la Cara Cura section

4.1.1 Va–Ap interval (Virgatosphinctes andensensis–Aulacosphinctes proximus zones)

Ammonite specific richness of this interval is of 13 species which plot dominantly over the demersal/nekton area of the Westermann Morphospace (75%), where they are mostly represented by medium-ornamented platycones, followed by planorbicones and discocones (Figure 3). Ataxioceratids are well represented in this area by the moderate to large sized macroconchiate Virgatosphinctes spp. and Pseudinvoluticeras douvillei, and by the moderate-sized microconchiate Choicensisphinctes choicensis and Indansites malarguensis. The smooth platycone haploceratid Pseudolissoceras zitteli also plots here but near the nekton region. The nekton area (10%) is occupied by smooth ammonites like the small platycone Hildoglochiceras? sp., the oppelid oxycone Cieneguiticeras falculatum, and the possible microconch of P. douvillei. The large and heavily ornamented aspidoceratid Pseudhimalayites steinmanni lies within this zone but very near the vertical-migrant area. The demersal/plankton region (8%) includes the moderate-sized macroconchiate platycones of Pseudinvoluticeras platyconus and the large-sized fasciculated-ribbed macroconchs of Ch. choicensis. Finally, the plankton corner (6%) bears two serpenticone groups, the large macroconchs of I. malarguensis and the moderate-size coarsely sculptured simoceratid Pseudovolanoceras aesinense krantzense.

4.1.2 Wi–Ca interval (Windhauseniceras internispinosum–”Corongoceras alternans” zones)

The specific richness of this interval is the lowest for the section, with 10 ammonite species recorded, of which the majority (81%) are plotted in the plankton corner of the Westermann Morphospace (Figure 3). That region includes coarsely tuberculated himalayitid platycones like the Corongoceras mendozanum dimorphs, the very large-sized finely-ribbed Windhauseniceras stipanicici, and also large serpenticones like Catutosphinctes rafaeli and the macroconch of Windhauseniceras internispinosum. The corresponding heavily sculptured small microconch of W. internispinosum plots in the demersal/nekton area (17%), which also holds the records of the medium-sized discocone neocomitid Blanfordiceras bardense. The ecospace is completed with two aspidoceratid inflated forms: the cadicone Schaireria longaeva, with mostly smooth whorls and a single row of tubercles on outer-whorls occupying the 1% at the nekton area; and the very large and stout tuberculated Aspidoceras quinchaoi, representing the remaining 1% and almost included within the vertical migrant region.

4.1.3 Sk–An interval (Substeueroceras koeneni–Argentiniceras noduliferum zones)

Specific richness (n=12) comparatively increases from the preceding interval. Most of the ammonites plot again in the plankton extreme (79%) that holds large coarsely ribbed and tuberculated neocomitid serpenticones like Argentiniceras bituberculatum and Hemmispiticeras steinmanni, large platycones with acute venters and conical tubercles such as the olcostephanid Spiticeras acutum macroconchs, and moderate-sized lateroventral-spine bearing Cuyaniceras transgrediens (Figure 3). In addition, 14% of the ammonites plot in the demersal/nekton region, where they are represented by the medium-ribbed “Thurmanniceras” duraznense and Tirnovella sp. cf. T. densestriata. Whereas the remaining 5% of the records are held by Pomeliceras? egregium, a large coarsely sculptured planorbicone that plots within the demersal/plankton area but very near its boundary with the plankton region. At this interval, undetermined Spiticeras sp. were recorded, but they were too laterally crushed to unambiguously assess their shell parameters.

4.1.4 Sd–Nw interval (Spiticeras damesi–”Neocomites wichmanni” zones)

Specific richness is again at its top with 13 species that continue to be mostly plotted in the plankton region of the Westermann Morphospace (69%) (Figure 3). Platycones are abundant and sculpturally diverse, with smooth forms such as the olcostephanid Negreliceras fraternum, medium/coarse ribbed shells as in Pseudoblanfordia spp., and coarse and tuberculated such as Erdenella paquieri. At the demersal/nekton area, Tirnovella sp. cf. T. densestriata represents the 13% of the possible modes of life, whereas another 13% of the records are held by the demersal/planktonic region, including the large to very large spiticeratids Spiticeras damesi and S. groteanum. Finally, at the spherocone corner of the vertical migrant region plots the small and smooth olcostephanidae Valanginites argentinicus, representing the remaining 1% of the association.

4.2 Benthic fauna from the Sierra de la Cara Cura section

Along the whole Sierra de la Cara Cura section, seven distinct benthic intervals are recognized (Figure 4). The first four cover the Tithonian–lower Berriasian part of the Vaca Muerta Formation, and the last three correspond with the lower three assemblages described in Toscano et al. (2025) spanning the upper Berriasian–lower Valanginian (pars) remaining part of the unit. These intervals are characterized below.

4.2.1 Interval 1 (surficial epibenthics, non-motile attached dominated)

This interval lies at the base of the Vaca Muerta Formation, spanning the Virgatosphinctes andesensis, Pseudolissoceras zitteli, and Aulacosphinctes proximus ammonite zones of early to early late Tithonian age. This interval’s richness is the lowest recorded for the Vaca Muerta Formation, with only four bivalve taxa. All of them are surficial epibenthics but they share the particularities of being invariably closely associated with ammonites, and having an epibyssate (three taxa) or cemented (one taxon) life habit. The only cementing taxon is the oyster Argustostrea, which is found cemented upon the ammonite shells. The epibyssate taxa are also recorded directly upon or “pending” from the ammonites, suggesting that they were also attached to the shells. Feeding is unimodal in this interval (with all taxa being filter-feeders) and so is motility, with all taxa being non-motile, attached. The great majority of all encrusted ammonites along the whole section is concentrated in this interval (over 85%), with encrusters occurring mostly on the umbilicus (over 80%).

4.2.2 Interval 2 (surficial epibenthics-shallow endobenthic dominated)

This interval spans the lower to mid Windhauseniceras internispinosum Zone of early late Tithonian age. Richness increases significantly, with nine recorded taxa. This interval is not only composed of bivalves (six taxa), but also of gastropods (three taxa). While surficial epibenthic is still the most represented life habit (55.55%), shallow endobenthics are also important (22.22%), occupying the shallowest layers of the substrate and its surface. Gastropods include epifaunal and semiinfaunal forms, as well as the facultatively semiinfaunal and motile epifaunal Protohemichenopus. Likewise, new feeding modes appear both through carnivorous and browsing/deposit-feeding gastropods, and two deposit-feeding nuculanoid taxa.

4.2.3 Interval 3 (surficial epibenthics-motile dominated)

It spans the remaining W. internispinosum Zone and the lowermost “Corongoceras alternans” Zone, of late Tithonian age. In this interval, the benthic richness drastically decreases again to the minimum of four taxa, mostly gastropods (three taxa) and one ammonite-encrusting oyster (Argutostrea). The most common taxon, Sinuarbullina, is a carnivorous semiinfaunal gastropod. This interval is dominated by surficial epibenthics (75%), followed by semiinfaunals (25%). Carnivores dominate this interval (50%), with the campanilid gastropods representing browsers/deposit feeders.

4.2.4 Interval 4 (shallow endobenthics dominated)

Spanning the remainder of the “C. alternans” Zone and the whole Substeueroceras koeneni Zone, of late Tithonian to early–late Berriasian age. This interval represents a marked increase in richness, with ten bivalve taxa and five gastropods, resulting in a total richness of 15 taxa. Some of them become more abundant, such as the epibyssate bivalve Huncalotis, and some shallow burrowers such as astartids and venerids. Shallow endobenthics dominate this interval (46.67% of taxa). A considerable increase in paleoecological diversity is also recorded, with five different tiering categories represented, including a deep endobenthic taxon, albeit very small-sized, which increases substrate exploitation. While all bivalves are suspension feeders except for the deposit-feeder nuculanoids, gastropods include two carnivores and three browsers/deposit feeders, adding dimensionality to the feeding strategies.

4.2.5 Interval 5 (surficial epibenthics dominated)

This interval spans the entire Argentiniceras noduliferum Zone, of late Berriasian age, and corresponds to the “ARV assemblage” of Toscano et al. (2025). Here, richness strongly increases again, with 21 taxa, reaching its maximum for the section. In this interval, in addition to bivalves and gastropods, free-living coiled serpulids are also quite common. Thus, recorded taxa consist in five gastropods and 15 bivalves; however, most taxa are newly recorded in this interval, and few were represented in the previous one. Oysters dominate several levels, conforming laterally extended concentrations corresponding to autoparabiostromes and autobiostromes. While filter-feeders strongly prevail over other feeding strategies (74%), tiering includes five categories, with surficial epibenthics (58%) followed by shallow endobenthics and semi-endobenthics (16% each). Regarding motility, facultative unattached predominates (31%), followed by motile and non-motile unattached (both with 26%).

4.2.6 Interval 6 (surficial epibenthics dominated)

Coinciding with the Spiticeras damesi Zone, of latest Berriasian age, and corresponding to the “NA assemblage” of Toscano et al. (2025). Richness descends slightly here with 18 taxa represented. Bivalves are the commonest, followed by gastropods. However, serpulids are still represented by free-living and attached taxa, and two brachiopods (a terebratulid and a rynchonellid) are recorded, being the only representatives of this phylum in the whole section. Oysters dominate several levels, forming extensive concentrations that are interpreted as autoparabiostromes and autobiostromes. The paleoautoecological characterization of this interval shows a strong dominance of surficial epibenthics (88%), followed distantly by semi-endobenthics and shallow endobenthics (both 6%). Regarding motility, there is a more even representation of non-motile unattached taxa (29%), followed by motile and non-motile attached taxa (23%). There is a higher proportion of filter-feeders (81%), followed by deposit-feeders/browsers (12.5%).

4.2.7 Interval 7 (surficial epibenthics dominated)

This is the topmost interval of the Vaca Muerta Formation, spanning the lower Valanginian “Neocomites wichmanni” Zone and coinciding with the “CPN assemblage” of Toscano et al. (2025). Richness is moderate (15 taxa, equal to Interval 4). Bivalves (11 taxa) and gastropods (three taxa) are accompanied by a single serpulid, the same free-living coiled form registered in the previous two intervals. Ecological categories present a less marked dominance within ecospace usage, with a majority of surficial epibenthics (54%), followed by shallow endobenthics and semi-endobenthics (23% each). Regarding motility, non-motile unattached predominate (38%), closely followed by facultative unattached (31%), and motile taxa (23%). Filter-feeders (77%) are commonest, followed by carnivores (15%). Fauna was found dispersed along a relatively thick sedimentary interval, with a low total abundance as compared to the two previous intervals, but still being much more abundant than in any of the lowermost four.

4.3 Vertebrate faunas

Over the past 15 years, studies have shown that the taxonomic diversity and morphological disparity of vertebrates from the Vaca Muerta Formation are considerably higher than previously recognized. This increase in knowledge applies not only to actinopterygian fishes but also to marine reptiles (plesiosaurs, ichthyosaurs, and metriorhynchid crocodyliforms). From 2013 to date, research has led to the description of several new species of actinopterygians, including the recognition of suspension-feeding pachycormids, a trophic guild previously unknown in the unit. Regarding herpetofauna, seven of the 11 marine reptile species currently recognized from the Vaca Muerta Formation were erected during this interval (Supplementary Material S2).

The currently known ichthyofauna is dominated by actinopterygians, particularly Pachycormiformes and Aspidorhynchiformes. Amiiformes, Lepisosteiformes, and Teleostei are also present, while chondrichthyans are comparatively scarce, represented only by Hybodontiformes and Batomorphii (Supplementary Material S2). This fish record is unevenly distributed, with most taxonomic diversity concentrated in El Ministerio Quarry, Neuquén.

Plesiosaurs, ichthyosaurs, metriorhynchid crocodyliforms, and turtles are well represented in the Vaca Muerta Formation. However, plesiosaurs and the turtle Notoemys laticentralis are currently unknown from Mendoza localities. Conversely, ichthyosaurs and metriorhynchids are widely distributed across all studied localities, with the former consistently exhibiting higher relative abundance (Fernández et al., 2019; Figure 5; Supplementary Material S2). Within these occurrences, Jurassic records are particularly prominent, whereas Early Cretaceous ones are much less frequent (Figure 5; Supplementary Material S2). Two localities in Neuquén, Cerro Lotena and El Ministerio Quarry (lower Tithonian–lower upper Tithonian), are particularly significant because they yield all the fish taxa and both turtle species recognized for the Vaca Muerta Formation. Furthermore, together with Cajón de Almanza or Almaza/Arroyo Trincajuera (upper Tithonian), both sites record all four marine reptile clades (e.g. Fernández et al., 2019). From those, Cerro Lotena preserves the most taxonomically diverse reptile assemblage, including both metriorhynchid subfamilies (Metriorhynchinae and Geosaurinae), and multiple Thunnosauria taxa. Nevertheless, it does not represent the richest ichthyofaunal assemblage within the Vaca Muerta Formation, as only a few fish groups are currently recorded there, namely large hybodontiforms, suspension-feeding and predatory pachycormids, and aspidorhynchids. Based on this diversity and the currently best-known vertebrate fauna from the Vaca Muerta Formation, we selected this locality for integrating results and paleoecological interpretation of the vertebrate assemblage.

Unfortunately, marine vertebrates are sparsely recorded in the Sierra de la Cara Cura section, which yields only isolated and poorly preserved fish scales, whereas marine reptiles are represented by disarticulated cranial and axial elements of two undetermined ichthyosaurs found in beds of the A. proximus to W. internispinosum zones (Figure 2A; Supplementary Material S2).

Despite the advances detailed here, the current picture remains incomplete. Several vertebrate groups have been identified in available collections, but detailed systematic revisions and formal taxonomic descriptions are still pending; several specimens have not yet been collected or prepared. Consequently, both the taxonomic diversity and ecological disparity of the Vaca Muerta Formation fauna are likely underestimated, and future studies will probably reveal an even more complex vertebrate assemblage than is currently recognized.

5 Discussion

5.1 Ammonite intervals paleoecology

Despite the fact that heteromorphs have been described from other sections of the Vaca Muerta Formation (Marin et al., 2023), all the studied ammonites from the Sierra de la Cara Cura section bear planispiral shells with a wide variety of ornamentation styles. Also, no true Leiostraca were recorded. In fact, open oceanic-dwellers, such as Phylloceratids, are unknown from the Vaca Muerta Formation, whereas Lytoceratids are only scarcely and sporadically recorded (Steuer, 1897; Krantz, 1926; Riccardi, 1991; Parent et al., 2015; Vennari and Pujana, 2017).

Along the Sierra de la Cara Cura section, specific richness remains mostly unchanged and with an average of 12 species in all four analyzed intervals. In contrast, morphospace occupation and shell ornamentation features show some interesting variations (Figure 3).

The lower Tithonian Va–Ap Interval covers the lowermost portion of the Vaca Muerta Formation. It starts with a sudden marine transgression and continues upwards with the establishment of a middle to proximal outer ramp setting. The comparatively wide morphospace occupation, covering the plankton, demersal, and nekton areas, reflects the sudden availability of new niches, which is expected during a dominantly transgressive phase (Westermann, 1996). In this interval platycone demersal/nektonic and nektonic ammonites predominate, among which those with more compressed whorl sections, less exposure of the umbilical area and shorter body chambers (brevidomes to mesodomes) would have been more stable in the water column and moderate to good swimmers (Westermann, 1996; Hebdon et al., 2020). That is the case of the genus Pseudolissoceras, whose smooth ribbing also aided in reducing drag and is known from off-shore to shallow platforms (Cecca, 1992). The best streamlined morphology of this interval is represented by the oxycone Cieneguiticeras falculatum (Figure 3), although its mesodomic body-chamber probably militated against its stability and maneuverability. Nevertheless, Cieneguiticeras would have been a more effective swimmer than the openly-coiled, longidome to mesodome, serpenticones Indansites malarguensis (Figure 3) and Pseudovolanoceras aesinense krantzense (Saunders and Shapiro, 1986; Westermann, 1996). The latter also bears a flat venter and coarse ornamentation, which increased drag and thus this form was unable of direct swimming. Interestingly, Pseudhimalayites steinmanni (Figure 3), heavily armored, with an inflated cross-section and long body-chamber, can be interpreted as a vertical migrant (Westermann, 1996; Lukeneder et al., 2010), though with a low overall stability in the water column (Saunders and Shapiro, 1986).

In the upper Tithonian Wi–Ca Interval a shift towards the occupation of the plankton area of the Westermann Morphospace occurs, while the remaining species plot in the demersal/nekton to nekton area. This interval initiates with a sea-level transgression near the base of the W. internispinosum Zone, defining an outer ramp that shallows upwards to a HCS-dominated middle ramp setting. Longidome to mesodome variably compressed serpenticones, platycones and planorbicones are interpreted as planktonic drifters, well-suited for dispersal and colonization of new settings (Smith et al., 2014). Notably, two contrasting morphotypes, the coarsely ornamented W. internispinosum microconch and the slenderer Blanfordiceras bardense plot very near in the ternary diagram and within the demersal/nekton zone (with B. bardense almost located in the nekton region), thus indicating that they were relatively good swimmers. The more evolute and depressed whorl-section of W. internispinosum microconch might however have affected its efficiency, showing that form-function relationships are not always as directly interpreted (Tendler et al., 2015; Hebdon et al., 2020, 2022). At this interval, Aspidoceras quinchaoi replaces P. steinmanni in the nekton/vertical-migrant corner, with a similar general armored morphotype and a very large size.

Coarse ornamentation held mainly by himalayitids in the Wi–Ca Interval, achieves a maximum representation in the overlying uppermost Tithonian–lowermost upper Berriasian Sk–An Interval with some strongly sculptured neocomitids like Argentiniceras. The Sk–An Interval spans the current position of the Jurassic/Cretaceous boundary, and it is mostly represented by middle ramp settings either influenced by storms (HCS-dominated) or by the development of oyster-biostromes. Platycone and serpenticone, longidome to mesodome, planktonic drifters dominate the association, among which, Spiticeras acutum, with a typically acute venter and conical periumbilical tubercles might have compensated the drag against propulsion exerted by its open umbilicus (Jacobs, 1992; Jacobs and Chamberlain, 1996). The demersal-nekton is represented by two discocone, subinvolute, rapidly expanding and medium sculptured forms, “Thurmanniceras” duraznense and Tirnovella sp. cf. T. densestriata, whose streamlined general form allows them to move effectively through the water column. No vertical-migrants were recorded in this association.

T. sp. cf. T. densestriata is also recorded in the upper-lying Sd–Nw Interval, where the general morphospace occupation concentrates on the plankton and demersal/plankton areas with a variety of longidome to mesodome platycones, both endemic (e.g. Cuyaniceras, Pseudoblanfordia) and more globally distributed forms (e.g. Erdenella, Spiticeras and Negreliceras). This interval begins in an oyster biostrome dominated middle ramp, continues with a middle to proximal outer ramp setting and ends with an abrupt sea-level fall signaled by the Intravalanginian discontinuity surface. This changing environment may have favored the establishment of variably ornamented planktonic drifters over other ecological strategies. Notably, the vertical-migrants are solely represented by the spherocone Valanginites argentinicus (Aguirre-Urreta and Rawson, 1999). Recent research on stable δ¹⁸O on other species of Valanginites in Poland are compatible with a planktonic or nektonic mode of life (Ploch, 2022).

5.2 Benthic communities’ paleoecology

Along the Vaca Muerta Formation, a marked contrast is evident in the richness and paleoecological composition of benthic faunas between the lower and lowermost upper Tithonian, in which a depauperate fauna is sparsely registered, and the upper Tithonian to lower Valanginian interval, in which richness, while presenting oscillations, is maintained at significantly higher levels, and paleoecological strategies are also more diverse (Figure 4; Table 1).

The depauperate fauna is represented by the three lowermost intervals, in which richness oscillates between four and nine taxa. In Interval 1, no isolated bivalves are recorded: they are always associated with ammonites. This suggests that the sea bottom was hostile to benthic fauna, and thus, the taxa encrusting ammonites may have been either pseudoplanktic (Wignall and Simms, 1990), or benthic island dwellers that fixated themselves upon to sunken ammonite shells. Several criteria have been established to determine whether benthic islands or pseudoplankton scenarios took place (see Schmid-Röhl and Röhl, 2003) that reflect the interaction of substrate consistency and oxygenation of the sea bottom. Densely colonized shells are more typical of benthic islands, while pseudoplanktonic encrustations tend to be more localized and involve fewer encrusters. The second scenario is more aligned with the observed encrustation patterns in this interval, suggesting that ammonites were encrusted in the water column and sunk to a dysoxic bottom. Likely, the dysoxic conditions favored the preservation of byssal threads, allowing the epibyssate bivalves to be preserved in situ or nearly so, which would not be the case if benthic colonization was controlled mainly/only by substrate consistency. Some ammonites presented encrusters on both sides of the shell, which is another indication that a pseudoplanktic encrustation was more likely, since benthic islands are more typically encrusted on one side only. However, many ammonites were not preserved in three dimensions, so the presence of encrusters on the left/right sides could not be evaluated in all specimens.

In Interval 2, the increased richness and the presence of new life habits, especially shallow endobenthics, suggest a relative improvement of benthic conditions as compared to Interval 1, specifically at the substrate-water interface. The novelty of shallow endobenthic bivalves and semi-endobenthic gastropods implies a less hostile environment, allowing a minimum colonization of the topmost infaunal level. Shallow endobenthics in this interval are represented only by nuculanoids, which are commonly reported in settings with organic-rich bottoms/high organic input (Damborenea and Pagani, 2019). Nuculoids are reportedly tolerant to dysoxic conditions (e.g. Taylor et al., 1995). In addition, semi-endobenthic gastropods recorded here, such as Protohemichenopus, have been reported previously in dysaerobic facies from the Mendoza Group (Cataldo and Lazo, 2016). The appearance of carnivorous gastropods could indicate the onset of a more complex trophic network.

The improvement seen in Interval 2 was reversed in Interval 3. The sudden drop in richness suggests the reinstatement of an overall inhospitable benthic environment, with bivalves restricted to oysters attached to ammonites, whereas gastropods were the only benthic taxa capable of inhabiting the sea bottom in this interval. While taxa were mostly surficial epibenthic, the presence of a semi-endobenthic taxon (Sinuarbullina) suggests less severe conditions than in Interval 1. Gastropods likely survived through browsing the fallen carcasses of ammonites and vertebrates, or by consuming organic matter accumulated in the substrate.

After this lapse, the remaining part of the section saw a comparative increase in richness and greater diversification of paleoecological strategies. Throughout the remainder of the analyzed interval, richness oscillated between 15 and 21 taxa, more paleoecological categories were recorded, and abundance became significantly higher. Interval 4 marks the richest, most ecologically diverse and highest abundance lapse in the section so far. Tiering and feeding strategies are more diversified than in the previous intervals: some of the recorded taxa also had larger sizes and thicker valves. This suggests a more pronounced and sustained improvement in benthic conditions, with an oxic sea bottom not only at the interface, but also in the shallow infauna. Five tiering modes are recorded: the maximum, seen only in Interval 5 as well. This is the only interval in which the prevailing life habit consists of shallow endobenthics, which may be indicative of firmer substrates than elsewhere in the succession.

Intervals 5 and 6 present common oyster floatstones, in which most taxa are mainly recorded. In Interval 5, benthic fauna is frequently recorded in nodules as well, which may have helped to facilitate their good quality preservation. Oyster floatstones, dominated by large, thick-shelled Aetostreon subsinuatum (Interval 5) and smaller Nanogyra brevisulcata, along with Aetostreon latissimum (Interval 6) are frequent. They are interpreted as autobiostromes to autoparabiostromes, presenting a mainly biogenic signature with minor-moderate reworking. These are a novelty for the section, indicating an extensive colonization of the sea bottom by large, thick-shelled organisms, together with occasionally increased energy conditions. Within Interval 7, although oysters were dominant, no biostromes were formed, indicating a higher sedimentation rate, reduced oyster proliferation, or a combination of both. See Toscano et al. (2025) for a more comprehensive analysis of intervals 5-7.

5.3 Vertebrate paleoecology: Cerro Lotena and surrounding areas as an example

Unfortunately, the current state of knowledge does not allow a quantitative interval-by-interval ecological reconstruction comparable to that achieved for invertebrate assemblages. Nevertheless, the Cerro Lotena section (lower Tithonian–lower upper Tithonian; Figures 1A, C) provides a suitable benchmark for exploring the vertebrate ecosystem structure within the Vaca Muerta Formation, as it preserves a well-documented diversity of marine vertebrates and includes representatives of several trophic and functional guilds.

The vertebrate assemblage from Cerro Lotena suggests a complex marine food web composed of apex predators, mesopredators, benthic-demersal consumers and specialized pelagic and benthic-associated feeders. Potential trophic interactions can be inferred from a combination of anatomical evidence, body size, tooth morphology (see e.g. Dunne et al., 2014), phylogenetic bracketing and quantitative ecomorphological analyses (Gouiric-Cavalli et al., 2023). Possible interactions involving invertebrate communities are discussed separately in sections 5.4.2 and 5.4.3.

At the top of the trophic web, the large pelagic plesiosaur Pliosaurus patagonicus likely occupied the apex predator niche. Its short mandibular symphysis and trihedral teeth indicate specialization toward a macropredatory (i.e., predation on large prey) lifestyle (Gasparini and O’Gorman, 2014). The possible occurrence of a geosaurine metriorhynchid currently under study suggests the presence of an additional large predator, although available evidence indicates that this taxon was less abundant and probably occupied a secondary apex-predator role.

Intermediate trophic levels were considerably more diverse. This component of the ecosystem included Thunnosauria ichthyosaurs, Metriorhynchinae crocodyliforms, predatory hybodontid sharks, and predatory actinopterygians such as Kaykay lafken, the sister taxon of macropredatory pachycormids (Gouiric-Cavalli and Arratia, 2022), and Jonoichthys challwa, an aspidorhynchid with specialized adaptations for piscivory. These predators likely partitioned available resources through differences in body size, feeding strategies, and habitat preferences. However, dietary specialization in actinopterygians should be interpreted cautiously, as ontogenetic changes in prey preferences cannot be excluded (Cooper, 2024). It is known that carnivorous pachycormids feed on fishes, a variety of cephalopods and some are almost exclusively teutophagous (Cooper and Maxwell, 2022).

Among ichthyosaurs, ecological differentiation is suggested by differences in body size, rostral proportions, dentition, and sclerotic ring morphology. Caypullisaurus bonapartei is the largest form from the Vaca Muerta Formation. The holotype of this species, found in this locality, consists of an almost complete skeleton measuring 4.8 m in body length (Campos, 2022). Given the lack of preserved teeth in all known Caypullisaurus specimens, specific dietary habits are difficult to infer; however, an analysis of the Ophthalmosauridae disparity and morphospace occupation patterns placed C. bonapartei close to “top predator” forms, a position supported by its wide postorbital region. Additionally, Campos et al. (2025) reported high jaw opening mechanical advantage values for Caypullisaurus, indicating a slower but more forceful jaw action compared with other ichthyosaurs. The other nominal ichthyosaur that inhabited this area, Catutosaurus gaspariniae, has a slender and delicate rostrum, needle-like teeth with crowns with subtle striations, and no noticeable wear, indicating that this taxon was probably a soft-prey-eater (Fernández et al., 2021). Furthermore, differences in sclerotic ring aperture among the ichthyosaurs recovered from the Neuquén Basin likely suggest a visual system adapted to exploiting different parts of the water column (Campos et al., 2025). The medium-sized metriorhynchid Cricosaurus araucanensis—estimated body length 2.5 m (Herrera, 2012)—was also characterized by a slender rostrum and a relatively high number of small, smooth bicarinate teeth. Based on tooth morphology it can be defined as a piercer of fish and squid (e.g. Foffa et al., 2018). Many of the undescribed metriorhynchid and ichthyosaur specimens found in this locality (see Supplementary Material S2) are not completely prepared; some consist only of partially preserved cranial fragments or are represented solely by postcranial material. However, none of these specimens stands out in terms of exceptional body size or a specialized dentition, suggesting that they also represented mesopredatory reptiles within the ecosystem.

The trophic structure of the ecosystem at Cerro Lotena also included the large-bodied pachycormid suspension feeders Leedsichthys (Gouiric-Cavalli, 2017). This actinopterygian occupied a distinct pelagic filter-feeding guild and likely represented an important component of ecosystem biomass. Although traditionally interpreted as a suspension feeder, evidence suggests some ecological flexibility during ontogeny including possible shifts between benthic and suspension-feeding strategies (Liston, 2010, 2013).

Overall, the vertebrate assemblage from Cerro Lotena reflects a structurally complex marine ecosystem in which multiple predators coexisted through ecological differentiation. Morphological disparities among these meso-predators, including variation in body size, mandibular and tooth morphology, sclerotic ring aperture, and other functional traits, indicate that differences in feeding strategies, sensory capabilities, and habitat preferences likely facilitated niche partitioning within the shallow marine environment. Although some taxa may have exploited similar prey resources, these ecological differences would have reduced direct competition and allowed the coexistence of diverse predatory guilds.

5.4 Vaca Muerta Formation paleoecological overview

5.4.1 Integrated paleoecological overview

Within the water column, ammonites experienced shifts in dominant mode of life that broadly reflect the changes in sea-level along the succession. The replacement in dominance from a demersal/nektonic-nektonic habit in the Va–Ap Interval to a mostly planktonic mode of life observed in the three remaining intervals is expected given the general shallowing-upwards trend of the facies associations. The basal portion of the Vaca Muerta Formation is generally characterized by oxygen-deficient bottoms with restricted circulation, whereas the water column was well oxygenated and supported a varied association of nektonic and planktonic fauna (Spalletti et al., 1999, 2014). In this context, streamlined variably compressed ammonites could actively swim in search of food sources or to elude potential predators through brief spurts of faster acceleration (Jacobs, 1992; Westermann, 1996). The predominance of nektobenthic (= demersal/nektonic) forms in high sea-level contexts and of evolute, variably depressed nektopelagic (= demersal/plantonik) forms during transgressive phases has already been noted elsewhere (Neige et al., 1997).

Ammonites with compressed whorl sections were more stable during turbulent flows, as those resulting from the occurrence of storm-events in HCS-dominated middle ramp settings, and thus could occupy shallower environments than those with more inflated whorl sections. This is more evident in the Sk–An Interval, where large conch size and fast growth rates combined with the development of a strong sculpture aided in the stabilization of the shells in high-energy waters (Elmi and Benshili, 1987), while also offering an enhanced resistance to potential predation pressure (Ward, 1981; Wilsem and Mosavinia, 2011).

Despite the estimated depth is around 50 m for middle ramp and up to 200 m for outer ramp and basinal settings (Kietzmann and Palma, 2014), there is a poor representation of vertical migrants all along the section, contrary to the trend observed in other carbonate-siliciclastic systems (e.g. Burgos and López-Horgue, 2019). This may indicate that during the Vaca Muerta Formation deposition, ammonites mostly preferred surficial waters where epipelagic food-sources were more abundant.

For different species of ataxioceratids (e.g. Indansites, Choicensisphinctes, Virgatosphinctes), himalayitids (e.g. Windhauseniceras, Corongoceras), spiticeratids (e.g. Spiticeras) and neocomitids (e.g. Substeueroceras), sexually mature lappeted microconchs are recorded in the same beds along with their corresponding macroconchs, which can be up to five times bigger. The members of the ammonite dimorphic pairs usually inhabited separate areas (Westermann, 1996; Lukeneder, 2015), but although some degree of transport cannot be neglected, the joint occurrence of both antidimorphs in middle to proximal outer ramp facies could indicate a preference for mating in shallower and more oxygenated settings (Lukeneder, 2015; Mapes et al., 2018).

The Westermann Morphospace is a powerful tool to quantify and visualize changes in the occupation of the ecospace along different environmental conditions or across evolutionary turnovers (Ritterbush and Bottjer, 2012; Smith et al., 2014; Whalen et al., 2020). Nevertheless, large morphologically variable species might plot in different areas of the ternary diagrams even when they inhabited the same environment due to the influence of developmental heterochronies (Bersac and Bert, 2012; Monnet et al., 2015), or as a result of different drivers constraining shell ornamentation and general form (Yacobucci, 2004). In addition, ammonites could occupy different depths along their ontogenies according to sea-parameters changing cycles or mating/spawning seasons. Habitat change is best estimated by complementing morphology with stable isotope analyses (Lukeneder et al., 2010; Ritterbush et al., 2014; Lukeneder, 2015).

Much like ammonites, vertebrates inhabiting the water column showed a rather stable paleoecological configuration. The vertebrate assemblage of the Vaca Muerta Formation is abundant, taxonomically diverse, and morphologically disparate, ranging from small fishes and turtles to large-bodied apex predators and giant suspension-feeding actinopterygians. This assemblage is recorded from the Tithonian–Berriasian in several localities of the Neuquén Basin (Supplementary Material S2), indicating that the main groups coexisted for over nine million years (Lena et al., 2019; Aguirre Urreta et al., 2023). Overall, this assemblage highlights a complex and vertically structured marine ecosystem in which disparate lineages successfully partitioned trophic resources through differences in body size, locomotor capabilities, and feeding strategies during the Jurassic–Cretaceous transition.

Recent taxonomic revisions and ecomorphological analyses indicate that osteichthyans occupied a broad range of ecological roles, from pelagic predators to suspension feeders and demersal consumers. The pachycormids Kaykay and Notodectes occupied active pelagic predator niches, combining fusiform bodies and efficient locomotor morphologies associated with sustained swimming and pursuit of mobile prey which might include small fishes but also ammonites and coleoids. In contrast, the giant pachycormid Leedsichthys occupied a specialized suspension-feeding guild, representing a high-biomass pelagic consumer that exploited planktonic resources and occupied a trophic position fundamentally different from predatory pachycormids. The elongated rostra and streamlined bodies of aspidorhynchids such as Jonoichthys challwa and Belonostomus indicate specialization toward active predatory lifestyles, particularly involving small fishes and other nektonic prey (Gouiric-Cavalli, 2015). Although these taxa shared some ecological space with predatory pachycormids, differences in body architecture and feeding morphology likely reduced direct competition. Pachycormiforms appear to have exhibited a range of feeding strategies, from specialized suspension-feeding to more generalized trophic habits. Additional actinopterygian groups occupied more generalist or demersal niches. Amiiform fishes likely represented versatile predators potentially exploiting fishes and invertebrate prey in near-bottom or structurally complex environments rather than representing a strictly benthic invertivorous guild. Similarly, lepisosteiforms probably contributed to lower trophic components of the ecosystem, although dietary interpretations remain tentative because recent studies suggest small fishes and invertebrates as their prey, but the group likely occupied broader ecological roles than previously assumed (Gouiric-Cavalli et al., 2023). Hybodontiform sharks were likely opportunistic predators capable of exploiting a wide range of prey, including fishes, belemnites, ammonites, and other invertebrates. Batoid records are scarce and lack diagnostic dental material; therefore, ecological interpretations should remain conservative, although comparisons with extant rhinobatid-like forms suggest potential consumption of invertebrates (ammonite and belemnite) and small fishes.

Although both pliosaurs and geosaurin metriorhynchids within the Vaca Muerta Formation could have acted as top or apex predators—animals that fed on a wide range of prey; including other vertebrates (Fischer et al., 2016)—morphological traits indicate that pliosaurs occupied a higher trophic level upon which no other animal preyed (e.g. Fernández et al., 2019). For example, bite marks on the fin of the giant Leedsichthys, and on the skull of an elamosaur plesiosaur and a geosaurini metriorhynchid have been attributed to them (Thulborn and Turner, 1993; Johanson et al., 2022; Herrera et al., 2026). Gut contents mainly represented by cephalopod hooklets suggest a broad diet for these predators (e.g. Martill, 1992; Wahl et al., 2007). The preservation of chondrichthyan remains (likely a hybodontiform shark) was reported within the abdominal cavity of the geosaurin Dakosaurus maximus (Herrera et al., 2026). The mesopredatory niche was occupied by Thunnosauria ichthyosaurs and metriorhynchine metriorhynchids. Dietary preferences in both groups are directly supported by fossils preserving gut contents: actinopterygian fish, a hatchling-sized marine turtle, and an enantiornithine bird remains were found in the Cretaceous ichthyosaur Platypterygius longmani (Kear et al., 2003), while cephalopods were found in the ichthyosaur Stenopterygius quadriscissus (Dick et al., 2016). Meanwhile, in metriorhynchine metriorhynchids, cephalopod hooklets, a fragmented belemnite, pterosaur bones (Martill, 1986), the pachycormid Leedsichthys, and invertebrate remains (Hua et al., 2024) have been reported. Post-mortem scavenging is the likely explanation for the Leedsichthys gut content in metriorhynchine specimens, suggesting that they were piscivorous but also opportunistic (Johanson et al., 2022; Hua et al., 2024). Furthermore, scavenging on a long-necked plesiosaur cervical vertebrae by a metriorhynchid has also been reported (Forrest, 2003).

Intra- and interclade differences in body size and morphology (including dentition) among ophthalmosaurid and metriorhynchine suggest that they were able to feed in different ways and on different prey (e.g. cephalopods and small- to medium-sized vertebrates). This trophic differentiation should have enabled coeval species to coexist within the same ecosystem (e.g. Fernández et al., 2019; Herrera et al., 2021; Campos et al., 2025). Similarly, while the dietary preferences of Jurassic Thalassochelydia remain largely unknown, the mandibular morphology of Neusticemys suggests a specialization for soft prey (Fernández et al., 2019).

The benthic invertebrate fauna is quite decoupled from the inhabitants of the water column. This is consistent with rare earth geochemical proxies that indicate a stratified water column, with oxygenated surface and intermediate waters coexisting with more reducing conditions at depth (Olivero et al., 2026). Overall, along the section, richness and ecospace occupation tend to increase among benthic invertebrates, although with oscillations. For the first three highly impoverished intervals, bottom oxygenation was likely the strongest limiting factor. From interval 4 onwards, the interplay between substrate consistency, water energy, and sedimentation rate likely dictated benthic community dynamics. Interval 5 represents the maximum richness and tiering diversity, marking the optimal conditions for development of benthic fauna in the studied section. Benthic Intervals 6 and 7 represent a gradually decreasing richness (though never nearly as low as in intervals 1–3) and a slightly more limited exploration of the ecospace as the environment becomes progressively shallower, culminating in the regression marked with the Intravalanginian Discontinuity, which triggered a reorganization of the benthic fauna (see Toscano et al., 2025).

At times, ammonite faunas reflect the same constraints that affected the benthos. For example, the Va–Pz interval of ammonites coincides with the lower Interval 1, and comprises forms that mainly inhabited the well-oxygenated water column, avoiding the dysoxic bottom waters. Another partial coupling of benthos and the water column takes place much upwards in the section, in the Sk–An ammonite interval, in which ammonite morphologies indicate adaptations to more agitated waters; the upper part of this interval coincides with benthic Interval 5, in which oyster autoparabiostromes are firstly recorded in the measured section and also reflect increased water energy. Water energy diminishes towards Interval 7, which lacks oyster biostromes; the Sd–Nw interval is also suggestive of calmer waters, since ammonites are predominantly planktonic and their ornamentation is milder and more variable.

Considering the sequence stratigraphy scheme for this unit (Gulisano et al., 1984), the lowermost intervals fall within the TST, while the remaining ones are included within the HST which records a progressive shallowing of the basin, culminating in the Intravalanginian Discontinuity (Gulisano et al., 1984). This is consistent with a stronger oxygen-controlled facies towards the base of the section and an overall paleoenvironmental improvement towards the top, which has also been recorded by geochemical tools such as δ¹³C excursions (Rodríguez Blanco et al., 2022). In addition, at a smaller scale, the boundaries between consecutive intervals for intervals 1–4 are also approximately coincident with high-frequency maximum flooding surfaces recorded by Kietzmann et al. (2014); boundaries between consecutive intervals 4–7 coincide with high-frequency transgressive surfaces. Some of these surfaces are also coincident with (larger-scale) regional flooding and transgressive surfaces (see Figure 4). Therefore, the benthic intervals could be tied to high-frequency and regional oscillations of the sea level, which reinforces that benthic faunas are more sensitive to sea-level changes than water-column dwellers.

5.4.2 Indirect evidence of ecospace occupation and biotic interactions

Besides body fossils, there is indirect evidence of biotic interactions recorded as drilling traces in several oyster taxa in the Sierra de la Cara Cura section. These traces were identified as Oichnus, and presented absence of stereotypy and selectivity. These features, combined with a low frequency of attacks, indicate the action of an unidentified generalist or facultative predator (Toscano et al., 2023).

In addition, Aetostreon species presented dwelling traces attributed to polychaetes, acrothoracic balanids and boring bivalves (see Toscano et al., 2021). A dwelling behavior has been interpreted for all these traces. Interestingly, both dwelling and predation traces are recorded only within the Cretaceous interval of the section, coinciding with (and adding to) the higher richness and space occupation of the benthic fauna at the improved environmental setting.

Regarding ammonites, no paleopathological specimens were recorded at the Sierra de la Cara Cura section, but lateral and latero/ventral sublethal injuries have been previously reported in specimens from other sections of the Vaca Muerta Formation (Vennari, 2011). They included “V-shaped” ventral marks located over the posterior portion of the body chamber that are usually interpreted as failed cephalopod (nautilid/coleoids) predation attempts (Hoffmann and Keupp, 2015) and lineal ornamentation disruptions over one of the flanks resulting from injuries to the peristome that might have been inflicted by decapod crustaceans (Hölder, 1956).

Although no body fossils of decapod crustaceans have been recorded from the Sierra de la Cara Cura section, decapod microcoprolites have been documented from Jurassic–Cretaceous levels of the Vaca Muerta Formation. The earliest records include Palaxius caracuraensis Kietzmann in Kietzmann et al. (2010), reported from the “Corongoceras alternans” and Substeueroceras koeneni zones (Kietzmann and Palma, 2010, 2014; Kietzmann et al., 2010). Additionally, in the Río Seco del Altar section, north of the Sierra de la Cara Cura, microcoprolites of the same genus have been recorded at levels from the upper Berriasian to the lower Valanginian, as well as others referred to the genus Favreina (Kietzmann and Palma, 2014).

All the microcoprolite genera recorded in the Sierra de la Cara Cura section have been attributed to callianassid ghost shrimps in both the fossil and extant record (Kietzmann et al., 2010). Callianassid body fossils have been recorded in other sections of the Vaca Muerta Formation, including the Tithonian Zapalianassa magna, from the Los Catutos (Leanza and Zeiss, 1990; Schweitzer et al., 2022), Arroyo del Cajón Grande (Figure 1A), and Cañada de Leiva (Rusconi, 1948; Andrada et al., 2022; Andrada, 2024; Figure 1A).

Additional microcoprolite taxa have also been documented from other localities of the Vaca Muerta Formation, including the genus Helicerina from beds assigned to the lower Valanginian Lissonia riveroi and Olcostephanus atherstoni zones (Kietzmann et al., 2010). First, this genus was tentatively attributed to producers belonging to the Mecochiridae, Erymidae, or Nephropidae, based on reports of decapods from other coeval units of the Neuquén Basin (Aguirre-Urreta, 2003; Kietzmann et al., 2010). However, carcass remains of the mecochirid lobster Atherfieldastacus rapax have recently been reported from the “Neocomites wichmanni” Zone at the Vaca Muerta Formation in Portada Quintuco (Andrada, 2024).

Both inferred producers (callianassid ghost shrimps and mecochirid lobsters) are commonly associated with a burrow-dwelling habit. Extant callianassid ghost shrimps are well known as burrow-dwellers, showing great diversity in burrow morphology and depth (Dworschak et al., 2012). Therefore, the presence of their microcoprolites provides indirect evidence that the substrate was exploited by burrowing decapods throughout part of the succession. Their occurrence is consistent with the relatively diverse benthic communities recognized in Intervals 4 and 7, where shallow endobenthic organisms are well represented. In addition, the occurrence of callianassid microcoprolites within levels assigned to the “Corongoceras alternans” Zone suggests that burrowing decapods could also have occurred during Interval 3, indicating that the infauna may have been slightly more diverse than inferred solely from the body-fossil record.

Furthermore, both callianassid ghost shrimps and mecochirid lobsters have been reported preserved within their burrows in the fossil record (e.g. Hyžný, 2011; Neto De Carvalho, 2016; Andrada, 2024). Atherfieldastacus rapax has been recorded within Thalassinoides suevicus burrows from the lower Barremian of Portugal (Neto De Carvalho, 2016). Although Thalassinoides isp. was recorded in the Sierra de la Cara Cura section (Figure 2), no in situ records of decapods are known. Because these Thalassinoides isp. systems are 1–3 cm wide, it is unlikely that the dweller was A. rapax, as even its carapace alone exceeds these dimensions (2–5 cm). A more likely dweller could have been the callianassid Zapalianassa magna, given its smaller size.

Callianassid ghost shrimps are generally considered deposit feeders, manipulating sediment with their mouthparts as they burrow (Dworschak et al., 2012). In contrast, mecochirid lobsters are interpreted as scavengers or opportunistic benthic feeders, based on the lack of true chelae and their elongate first pereiopods, which may have been used to search for food on the seafloor (Simpson and Middleton, 1985; Feldmann et al., 2002).

An interesting biotic interaction between invertebrate benthic communities and nektonic reptiles takes place when large vertebrate carcasses fall into the sea-floor, suddenly introducing a significant amount of nutrients from the water column. These deadfall communities have been reported from many Mesozoic sites involving remains of ichthyosaurs, plesiosaurs, thalatosuchians and testudines (e.g. Maxwell et al., 2022; Talevi et al., 2026). A typical ecological succession in deadfall communities involves firstly mobile scavengers (e.g. sharks, fishes, cephalopods), then enrichment opportunists (e.g. ophiuroids), followed by sulfophilic taxa (e.g. bacterial and fungal microborings and localized Chondrites burrows), and lastly reef species encrusting the hard bone-substrate (e.g. cementing mollusks or serpulids) (Smith and Baco, 2003; Maxwell et al., 2022). In Mesozoic examples, not every stage is always recognizable (e.g. Danise et al., 2014). In the Vaca Muerta Formation, a case involving a partially articulated ichthyosaur was reported by Lazo et al. (2022) from the Tithonian–lower Berriasian of the Arroyo Paulino section (Figure 1A). There, only the two last stages were recognized: the sulfophilic stage with bacterial microboring over the bones, and the reefal stage, with epibyssate bivalves colonizing the bones as benthic islands over a mostly dysoxic sediment-water interface. Faunal composition around the carcass was not different from that assessed elsewhere in the same strata, including epibyssate and shallow-burrowing bivalves, gastropods and mostly imprints of serpenticonic to platyconic ammonites. Nevertheless, abundance of the benthic fauna around the skeleton was significantly higher, reinforcing the observed pattern that both oxygenation and substrate consistency might have exerted an important control over the benthic community distribution.

Unreported bromalites containing fish scales and scarce bones have been found at upper Tithonian beds of the Vaca Muerta Formation in Las Alcantarillas, and thus they are indirect evidence of predation on fishes. The term bromalite involves any oral, anal, or digestive material sensu Hunt (1992), and although the producers cannot be inferred with certainty, comparisons with Northern Hemisphere localities point to ichthyosaurs, pliosaurs, and thalatosuchians as potential consumers (e.g. Hofmann et al., 2020).

Finally, ichnofossils have been reported to be rather scarce for the Vaca Muerta Formation, which is considered a largely unbioturbated unit (Spalletti et al., 1999; Leanza et al., 2011; Kietzmann et al., 2014; Paz et al., 2023). As mentioned above, very few traces were recognized in Cara Cura. This is unfortunate since trace fossil analysis is excellent for paleoecological and paleoenvironmental reconstructions; however, other authors have analyzed the ichnological associations in other sections of the Vaca Muerta Formation.

One of the earliest and more comprehensive studies was carried out by Doyle et al. (2005) on the upper portion of the Vaca Muerta Formation at Arroyo Salado (Figure 1). Unfortunately, no ammonite zonation was provided so it is impossible to precisely correlate their section to the one studied here. Doyle et al. (2005) found a low diversity ichnological assemblage, established in a background of fine-grained substrates. They described 12 ichnotaxa, organized into four associations; laminated facies associations (non-bioturbated), Chondrites association, Thalassinoides association and Arenicolites association. The first two are commonest in the Vaca Muerta Formation, with minor occurrences of small-sized Thalassinoides and rare Arenicolites. A slight increase in trace size is recorded upwards for Thalassinoides, suggesting improved oxygenation. Overall, trace analyses indicate a lowered oxygenation at the basal portion of the section, improving upwards; this is in agreement with our overall findings.

An analysis of tapho- and biofacies was carried out by Kietzmann and Palma (2009) on three localities of southern Mendoza. They reported several ichnotaxa for a few taphofacies, including Chondrites, Thalassinoides, Palaeophycus and Rhizocorallium. Traces are found most commonly at the top of wackestones, mudstones and some packstones; some of them were exhumed, and interpreted as indicative of pauses in sedimentation. However, while the authors placed each tapho- and biofacies along the carbonate ramp, they did not present their vertical distribution. They mention an absence of bioturbation in shales intercalated with Chondrites for the Virgatosphinctes mendozanus (= V. andesensis), Pseudolissoceras zitelli, and Substeueroceras koeneni zones. Thalassinoides is reported to be abundant in nodular limestones in the Aulacosphinctes proximus and Windhauseniceras internispinosum zones, and it is mentioned also for the V. mendozanus, S. koeneni and Spiticeras damesi zones, sometimes along with Rhizocorallium or Planolites. Thus, Thalassinoides was found along the section, being occasionally abundant. Kietzmann and Palma (2009), while they did not interpret the fossil traces specifically, did interpret the changes in biofacies and taphofacies as caused by the interplay of several environmental parameters, with anaerobic, dysaerobic and aerobic zones, and substrate energy and consistency as other key factors in the shaping of benthic communities.

More recently, Paz et al. (2023) conducted a thorough study of the ichnotaxa of the Vaca Muerta Formation. Their analysis was based on nine well core samples and one outcrop from central Neuquén (thus, far from Sierra de la Cara Cura), covering mostly the V. andesensis–W. internispinosum zones, that is, mostly the lowermost portion of the unit. Most traces were recorded on tuff levels. They recognized an atypical oxygen-deficient ichnocoenoses, characterized by a lack of Chondrites and scarce Zoophycos. They identified dysoxic to anoxic conditions, and defined nine ichnocoenoses corresponding to different combinations of oxygenation levels, substrate consistency and water energy at the sea bottom. For the studied portion of the Vaca Muerta Formation a predominance of anoxic conditions was found, with dysoxic and oxic conditions in second place.

Wetzel et al. (2023) studied a single volcaniclastic bed, interbedded among organic-rich mudrocks typical of the Vaca Muerta Formation in Yesera del Tromen (Figure 1). This bed bore an unusual and abundant Teichichnus, at the base of the Vaca Muerta Formation (placed just a few meters above its base). This stratum was interpreted as a dysoxic setting, with its erosive base suggesting the action of storm-induced currents under a high sedimentation rate. According to Wetzel et al. (2023), these findings are in agreement with the broad setting along the eastern margin of the Neuquén Basin as the transgression of the Vaca Muerta Formation took place; the overall dysoxic to anoxic conditions were repeatedly interrupted by oxygenated phases induced by storms, hyperpycnal flows, and freshwater input from the Palaeo-Pacific that caused bottom currents of different intensities.

This brief overview shows that, while some variations across the basin took place, the ichnofossil record agrees with the main findings presented here regarding the benthic fauna of the Sierra de la Cara Cura locality. Trace fossils were also controlled by oxygenation, substrate consistency, and water energy as the main drivers. Anoxic to dysoxic waters were occasionally punctuated by more oxygenated intervals, and an overall improvement towards the top of the section is recorded by Doyle et al. (2005) in southern Mendoza.

Through these traces/indirect evidence, ecological interactions that would not be recorded otherwise are preserved, revealing a wider ecospace occupation than previously thought, based on body fossils only. Ignoring this line of evidence would result in an underestimation of both richness, ecospace utilization, and biotic interactions.

5.4.3 Upper Jurassic–Lower Cretaceous hypothetical trophic structure

The variety of feeding styles, life-habit strategies and ecospace occupation shown by the pelagic and benthic marine communities of the Neuquén Basin during the Tithonian–lower Valanginian are a reflection of the complex trophic relations between them. In this subsection, we suggest a potential food web structure from the direct and indirect evidence of interactions among high-rank taxa (Figure 6). Also, although invertebrate and vertebrate associations have been studied here from different paleogeographic areas in the basin, there are sites with conjoined records (e.g. Fernández et al., 2019 for Arroyo Paulino and Arroyo Durazno sections) where predator/prey interactions were most likely.

While the reconstructed trophic web likely incorporates all the major trophic roles that existed during the deposition of the Vaca Muerta Formation, it is possible that many of the organisms that fulfilled several of these roles were not incorporated into the fossil record. The different taphonomic routes involved likely favored the preservation of some organisms over others. For example, among benthic organisms, all semi-infaunal and infaunal taxa are more likely to be preserved than epifaunal ones. Among ammonites, shell morphology is strongly determinant of the taphonomic route (Luci et al., 2016; Yacobucci, 2018), so some forms may have been transported out of their habitat. Likewise, vertebrates can float as they decay and size differences may influence which taxa are more commonly preserved (e.g. Reisdorf et al., 2012). Thus, the reconstructed trophic web is likely a simplified glimpse of the original one, that emphasizes those organisms with a higher preservation potential.

The lower levels of the food web included organic and mineralized-walled microplanktonic forms such as prasinophytes, dinoflagellates, acritarchs, radiolarians, calpionellids, and coccolithophorids (Lescano and Concheyro, 2014; Leanza et al., 2020; Agüero et al., 2025). Microscopical planktonic pellets containing coccoliths and other undifferentiated carbonate grains (Paz et al., 2023) point to the presence of some undetermined planktonic first-order consumer in the water column. Meanwhile, microbenthic associations were represented by foraminifers and ostracods both in oxygen-deficient and well oxygenated areas (Ballent et al., 2004; Hiriart et al., 2021). In the latter, calcareous algae proliferated (Kietzmann et al., 2014).

Saccocomid microcrinoids were pelagic first-order consumers, either as passive or active suspension feeders (Hess and Etter, 2011). Presence of their ossicles in Lumbricaria type coprolites (Kietzmann and Bressan, 2019; Knaust and Hoffmann, 2021) and ammonite stomach/crop content in German lagerstätten (e.g. Schweigert and Dietl, 1999) indicate that saccocomids were most likely preyed upon by ammonites.

In general, aptychi-bearing ammonites are interpreted as second-order omnivorous consumers with a planktotrophic to microphagous habit (e.g. Klug and Lehmann, 2015 and references therein). Other pelagic second-order consumers were represented by suspension-feeding pachycormids and by benthic and demersal durophagous chondrichthyans and lepisosteiforms.

In turn, first order consumers in benthic communities involved filter feeder bivalves, gastropods, brachiopods, serpulids, and hexactinellid sponges; deposit feeder bivalves, holothurians and callianasids; and deposit feeder/browser gastropods. In addition, ellipsoidal to irregularly-shaped benthic fecal pellets with abundant microcrystalline silica have been interpreted as produced by polychaetes, though other possible producers are bivalves, echiurans, and enteropneust (Paz et al., 2023). In addition to being targeted by durophagous fishes, hard-shelled benthic organisms could also be predated by carnivorous gastropods and ophiuroids, or be consumed by scavengers/opportunistic mecochirid lobsters and scavenger/predatory nautilids (Tanabe et al., 2015).

In the water column, belemnites were streamlined, fast, and efficient swimmers that preyed upon other cephalopods, crustaceans, and fishes. Their hatchlings, as well as those from ammonites, were probably consumed by suspension-feeders pachycormid fishes (Hoffmann and Stevens, 2020; Vennari et al., 2023). In addition to belemnites, the third-order consumers level was disputed by an array of fishes and reptiles. Among the first, macropredatory pachycormids and aspidorhynchids were active predators in open waters (Cooper, 2024), whilst amiiform fishes had more maneuverability in restricted environments. Regarding reptiles, the apex predator guild was occupied by pliosaurs, closely followed by hypercarnivorous geosaurines. The mesopredatory level was represented by generalist ophthalmosaurid ichthyosaurs and metriorhynchine metriorhynchids with distinct feeding preferences over nektonic mollusks and fishes. Finally, it has been suggested that thalassochelydians turtles might have preyed upon soft-bodied faunas.

6 Conclusions

This study provides the first comprehensive, integrated paleoecological reconstruction of both macroinvertebrate and vertebrate marine associations across the Jurassic–Cretaceous transition in Western Gondwana. Through multi-taxonomic datasets from the Neuquén Basin, we contributed to bridging the traditional gap between compartmentalized benthic and pelagic studies in the Southern Hemisphere.

The main conclusions of this contribution are as follows:

• The Vaca Muerta Formation preserves a highly complex, vertically tiered marine ecosystem that remained functionally stable for over nine million years during the Tithonian–early Valanginian interval. Within the pelagic realm, clearly differentiated trophic and locomotor strategies allowed the coexistence of multiple marine reptile lineages and diverse actinopterygian fishes, minimizing niche overlap through variations in body size, dentition, and visual adaptations.
• Quantitative analysis using the Westermann Morphospace, for the first time in the Vaca Muerta Formation ammonite fauna, revealed a significant shift in ammonite life habits, broadly tracking the shallowing-upward trend of the carbonate ramp. The lower Tithonian was dominated by streamlined demersal/nektonic active swimmers probably adapted to oxygenated surface waters above oxygen-deficient bottoms. Conversely, the younger intervals saw a major turnover toward heavily sculptured planktonic/demersal forms. This heavy ornamentation and larger sizes provided mechanical stabilization in higher-energy, storm-dominated middle ramp settings while protecting against predation pressure.
• In contrast to the stratified water column, where ecological roles were occupied by the same high-rank elements, benthic faunas were more subject to variations. In the lower portion of the section, bottom-water dysoxia acted as the primary limiting factor, restricting benthic life to highly impoverished opportunistic taxa or epibionts directly attached to ammonite shells. Albeit with some variations, from there on, an improvement in oxygen availability shifted the main environmental constraints toward water energy, sedimentation rates, and substrate consistency. This culminated in the upper Berriasian with maximum taxonomic richness, tiering complexity, and the onset of extensive oyster biostromes. Finally, a trend toward lower-energy and shallower waters near the top of the succession is mirrored by a moderate decrease in specific richness leading up to the major ecological reorganization triggered by the Intravalanginian Discontinuity.
• Through the analysis of indirect evidence of biotic interactions in the Neuquén Basin and in other Mesozoic depocenters, taxonomic uniformitarianism, tooth/buccal masses morphologies, body size and dietary preferences, we explored possible interactions between macroinvertebrate and vertebrate high-rank taxa. This has revealed a most likely complex and well-structured food web involving the marine communities of the Vaca Muerta Formation.

While the morphological and ecological proxies used here proved robust for capturing major ecosystem turnovers, this study faced limitations due to the historical collection of vertebrate data lacking precise bed-by-bed biostratigraphic control at certain benchmark localities. To achieve a higher-resolution temporal framework across all trophic levels, future research should focus on obtaining high-accuracy stratigraphic data for fishes and marine reptiles, and detailed samplings of benthic fauna, microfossils, and nannofossils associations, matching the Andean ammonite zonation. Additionally, supplementing the paleoecological and morphological analyses with stable isotope studies of δ¹⁸O and δ¹³C over sediments and well-preserved shells and teeth will be critical to test depth-stratification, possible ontogenetic trajectories, and predator/prey interactions.

All in all, this pioneer study puts forward the Neuquén Basin as a critical window into the paleoecological dynamics of the Jurassic–Cretaceous transition in Western Gondwana.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2026.1923277/full#supplementary-material

Ammonite taxonomic distribution in the Cara Cura Section, representative species for Westermann Morphospace construction, shell measurements, description and interpreted mode of life. Supplementary material at the source

Taxonomic distribution of revised Tithonian–Berriasian vertebrates from age/stratigraphically constrained localities of the Vaca Muerta Formation. Supplementary material at the source

Some comments and abridged taxonomy of ammonoid taxa in open nomenclature and unnamed taxa. Supplementary material at the source

Table summarizing type of data, information source, and localities of provenance for the analyzed major fossil groups. Supplementary material at the source

Footnotes

Edited by: Abdelouahed Lagnaoui, Hassan Premier University, Morocco

Reviewed by: Zekun Wang, Natural History Museum, United Kingdom

Yael Afriat, American Museum of Natural History, United States