Among thousands of fossils from an Early Jurassic lake, researchers discovered the most complete skeleton of an ancient dipnoan fish.

In eastern China, within the Sichuan Basin, formations from the transition between the Triassic (251.9-201.3 million years ago) and Jurassic (201.3-145 million years ago) periods are located: the Late Triassic Xujiahe and the Early Jurassic Zhenzhuchong, among others. Recently, Chinese paleontologists discovered a significant accumulation of fossils in these areas, characteristic of deep freshwater lakes. This fossil complex has been named the Yuzhou biota.

In a new article, published in the journal Scientific Reports, researchers discussed the exceptionally well-preserved fauna from the Sinemurian stage (199.5 (±0.3)-192.9 (±0.3) million years ago) of the Jurassic period. Thousands of fossil records captured one of the oldest food chains, through which paleontologists reconstructed a complex lake ecosystem. It appears that this ecosystem survived after a mass extinction that occurred between geological periods.

The biota is represented by invertebrates (bivalves and gastropods), crustaceans, ray-finned and diploid fish, extinct hybodonts (close relatives of modern sharks), and pliosaurids (plesiosaurs with short necks). Flora was also preserved (more than 20 species) — ferns, ginkgos, conifers, and others.

The most numerous class was the ray-finned fish (Actinopterygii), with paleontologists recording several thousand specimens — among them, six new species that have not yet been described. The largest of these, a member of the subclass Ptycholepiformes, reached 70 centimeters in length. Interestingly, the smallest fish from this biota was speculated to be a specimen measuring 13 centimeters, which might belong to the Early Jurassic taxon Dorsetichthys, found only in Europe.

Among the diploid fish, scientists identified a surviving skeleton of a half-meter-long specimen — this specimen includes details of the skull, axial skeleton, fins, and scales. According to the authors of the article, this is the most complete fossil of this ancient group known to date. From the hybodonts, teeth and dorsal fins remain, through which paleontologists determined the fish's maximum length to be nearly one meter. The size of the pliosaur, represented by several vertebrae and fragments of the skull, is estimated to be four times larger. It has been classified as species Bishanopliosaurus youngi, which remains poorly understood.

Notably, there is an almost complete femur of a theropod dinosaur. Researchers could not determine its owner, but it seems to belong to a species closely related to eocursor. This Late Triassic lizard inhabited what is now South Africa and likely ran quickly. However, the dinosaur bone ended up there by chance — the species is terrestrial after all. The native inhabitants of the ancient water body showcased a complex food chain.

Mollusks and crustaceans likely fed on algae and plankton. In turn, they were preyed upon by jawed fish. Hybodonts hunted them, and at the top of the chain were pliosaurs. In their bromalites, fossilized stomach and intestinal contents, specialists found remains of diploid fish. No one preyed on the pliosaurs in the studied freshwater lake, so this environment could have served as a refuge for these reptiles. Nevertheless, not all samples of the Yuzhou biota have been prepared and described yet, so new details may expand the diversity of the ecosystem.