Life on the Ancient Seafloor: Vaca Muerta Fossils Reveal Nine Million Years of Ocean Stability

Roughly 145 million years ago, as the Jurassic period gave way to the Cretaceous, the waters covering what is now western Argentina teemed with ammonites drifting through the water column, ray-finned fishes hunting at every depth, and marine reptiles ruling the food chain from above. A new study published in Frontiers in Ecology and Evolution has now reconstructed this vanished world in unprecedented detail, offering the first integrated, multi-trophic paleoecological analysis of both marine invertebrates and vertebrates from Western Gondwana across the Jurassic-Cretaceous transition. The findings reveal a pelagic ecosystem that remained functionally stable for more than nine million years, even as the seafloor beneath it underwent dramatic ecological upheaval.

The research addresses a long-standing imbalance in how scientists understand Mesozoic marine ecosystems. Many of the most influential paleoecological models have been built on Northern Hemisphere fossil sites with exceptional preservation, while the extraordinarily fossil-rich rocks of Western Gondwana received comparatively little quantitative attention. The Vaca Muerta Formation, deposited on a broad carbonate ramp in the Neuquén Basin of Argentina, is one of the most important exceptions. Its layered limestones and mudrocks preserve an abundance of ammonites, bivalves, fish, and marine reptiles, yet the ecosystem as a whole, how its species related to one another, occupied space, and shared resources, had never been synthesized in a single framework until now.

To build that framework, the research team assembled two taxonomic datasets from stratigraphically constrained sections at different paleogeographic positions on the ancient carbonate ramp: the Sierra de la Cara Cura and Cerro Lotena sections. Because the strata are carefully dated, the researchers could track ecological change through time from the early Tithonian stage of the Late Jurassic into the Berriasian and up to the Intravalanginian Discontinuity in the Early Cretaceous. Rather than simply listing which species were present, the study applied quantitative and qualitative analytical tools to understand how the ecosystem actually functioned.

Three complementary methods formed the core of the analysis. Westermann Morphospace modeling, a technique originally developed for ammonites, classifies cephalopods by shell shape into functional categories that reflect swimming ability, habitat preference, and lifestyle. Multi-level benthic tiering analysis examines how far above and below the seafloor surface different invertebrates lived, from infaunal burrowers to elevated epifaunal forms attached to the substrate. Vertebrate trophic-guild analysis, meanwhile, sorts fish and marine reptiles by body form, locomotion, and feeding strategy. Combining all three allowed the team to compare the dynamics of the water column with those of the seafloor, and the contrast between the two realms turned out to be one of the study’s most striking results.

The pelagic realm, the open water above the seabed, emerges as a complex, vertically stratified ecosystem of remarkable resilience. The vertebrate assemblage is both abundant and taxonomically diverse, including apex predators at the top of the food web alongside more specialized forms below them. Actinopterygian, or ray-finned, fishes displayed an exceptionally wide range of morphologies, locomotor modes, and feeding strategies. Some were pelagic cruisers of the open water while others occupied demersal and benthic habitats near the bottom. Their diets spanned suspension feeding on tiny particles, invertebrate eating, microphagy on the smallest food items, piscivory on other fish, and durophagy, the crushing of hard-shelled prey. This functional breadth indicates that fish had partitioned the water column into a finely subdivided array of ecological niches.

Mesopredatory marine reptiles show a parallel pattern of ecological specialization. Although their morphological disparity suggests that different reptile groups preyed upon broadly similar food sources, their body forms imply strict niche partitioning, with each lineage exploiting those shared resources in its own way, likely at different depths, speeds, or hunting styles. Such partitioning is a hallmark of mature, resource-limited ecosystems, and its persistence across the study interval helps explain why the pelagic community remained functionally stable for over nine million years even as individual species came and went and sea levels rose and fell around the ramp.

The ammonites, the iconic coiled cephalopods of the Mesozoic, tell a story of their own. Their communities tracked a shallowing-upward trend across the succession. In the early Tithonian, when waters were deeper, the assemblages were dominated by streamlined forms interpreted as demersal active swimmers, capable of maneuvering near the seabed. As the ramp shallowed, the ammonite fauna shifted toward heavily sculptured, planktonic drifters whose robust ornamentation represents a mechanical adaptation to the turbulent, storm-dominated conditions of shallow water. In other words, the changing shapes of these cephalopod shells record the changing physics of the water column above them, making ammonites a sensitive barometer of environmental evolution on the ramp.

The benthic realm, by contrast, marched to a different drummer. Its dynamics remained decoupled from the water column and were governed instead by three local factors: oxygenation, water energy, and substrate consistency. In the early Tithonian, the seafloor communities were highly impoverished, dominated by opportunistic species or even pseudoplanktonic organisms that drifted in attached to floating debris rather than living on the bottom itself. As oxygenation improved toward the late Berriasian, the benthos blossomed into highly diversified, multi-tiered assemblages, punctuated by extensive oyster biostromes, dense seafloor pavements of oysters that created new hard substrates and structural complexity for other organisms. Finally, a renewed shallowing trend culminated in a major ecological reorganization at the Intravalanginian Discontinuity, a marked stratigraphic boundary that reset the benthic ecosystem.

Evidence for direct and indirect biological interactions, such as predation traces and ecological associations among the fossils, allowed the team to sketch the outline of a highly interconnected trophic network. The web extended from microplanktonic producers at its base, through grazing and suspension-feeding invertebrates, to fish at multiple trophic levels, up to the hypercarnivorous reptiles that crowned the system as apex predators. The picture that emerges is not a simple chain but a coordinated eco-spatial and trophic architecture, in which vertical stratification of the water column, niche partitioning among predators, and substrate-controlled structure on the seafloor together sustained a functioning ecosystem across a interval of global environmental change.

Beyond its portrait of an ancient sea, the study carries a broader significance for how scientists model Mesozoic ecosystems. By demonstrating that Western Gondwanan faunas can be analyzed with the same quantitative rigor applied to the classic Northern Hemisphere sites, the work helps correct a geographic bias that has shaped paleoecological theory for decades. It also establishes the Neuquén Basin as a critical natural laboratory for understanding global marine dynamics during the Jurassic-Cretaceous transition, a span that included major sea-level fluctuations, biotic turnovers, and the gradual assembly of the modern marine fauna. The Vaca Muerta Formation, already famous as one of the world’s most important source rocks for petroleum, now takes on a second role as one of the clearest windows into how ocean ecosystems endure, adapt, and reorganize through deep time.

Subject of Research: Paleoecological reconstruction of Jurassic-Cretaceous marine ecosystems in the Vaca Muerta Formation, Neuquén Basin, Argentina

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

Article References: Vennari, V. V., Luci, L., Herrera, Y., Cataldo, C. S., Toscano, A. G., Marin, L. S., Andrada, A. M., Gouiric-Cavalli, S., & Aguirre-Urreta, B. (2026). Paleoecological dynamics of the Jurassic-Cretaceous transition: insights from the Vaca Muerta Formation marine biota. Frontiers in Ecology and Evolution, 14, Article 1923277. https://doi.org/10.3389/fevo.2026.1923277

Image Credits: AI Generated

DOI: 10.3389/fevo.2026.1923277

Keywords: paleoecology, Vaca Muerta Formation, Jurassic-Cretaceous transition, Neuquén Basin, ammonites, marine reptiles, actinopterygian fishes, benthic tiering, Westermann Morphospace, Gondwana, trophic networks, carbonate ramp