A shallow sea in Patagonia
About 135 million years ago, during the Early Cretaceous Period, a shallow sea penetrated deep into the South American continent. This epicontinental embayment, known as the Neuquén Basin, created a marine environment in what is now the arid region of northern Patagonia. The geological unit that captures this time, the Agrio Formation, consists of layers of fossiliferous marlstone and shale. These rocks preserve a detailed record of the life that inhabited this ancient sea. The basin's connection to the Pacific Ocean allowed for the migration of large marine reptiles, whose fossils now record the region's history.
The ecosystem was complex and supported a wide variety of life. The seafloor hosted communities of bivalves and ammonites, which are now found as common fossils in the layers of the Agrio Formation. Small calcareous sponges also formed meadows on the sea bottom. Swimming in the waters above were numerous species of fish and squid-like belemnites. These animals formed the lower and middle tiers of a food web that was ruled by large predatory marine reptiles. The exceptional preservation at sites like Bajada del Agrio allows paleontologists to reconstruct these predator-prey relationships with unusual clarity.
Predators and their fossilized prey
The top predators of the Agrio Formation's sea were ichthyosaurs and plesiosaurs. The ichthyosaur Caypullisaurus bonapartei, a large dolphin-like reptile that could reach lengths of around 7 meters (23 feet), was a common inhabitant. Plesiosaurs, with their long necks and paddle-like limbs, were also present. Fossils of these animals are significant because some have been discovered with their stomach contents intact. The undigested remains of an animal's last meal, known as a bromalite, offer direct evidence of its diet.
In plesiosaurs, another clue comes from gastroliths, or stomach stones. These reptiles swallowed rocks, which were held in their digestive tracts. Scientists propose two main functions for these stones: they may have helped grind up hard-shelled prey like ammonites, or they could have acted as ballast to help control buoyancy and stability in the water. The presence of both bromalites and gastroliths in fossils from Bajada del Agrio allows for detailed reconstructions of what these animals ate, and how they lived and hunted. Analysis shows their diets included fish and cephalopods, establishing them as high-level consumers in the Cretaceous food web.