Fossil shows reptile species' aquatic transition

Chinese scientists have identified a 244-million-year-old reptile fossil from Southwest China's Yunnan province as the earliest known archosauromorph that was highly adapted to life in water, providing the first clear evidence that some early members of the evolutionary group had become almost completely aquatic.

The newly identified reptile, named Austronaga, challenges the long-held view that early archosauromorphs — a broad group that later gave rise to crocodiles, dinosaurs, pterosaurs and birds — were predominantly land-dwelling, with previously known aquatic members only partly adapted to life in water.

The study, led by Li Chun, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, was recently published in Science Advances.

The findings shed new light on when archosauromorphs began moving into aquatic environments and suggest that the return of reptiles to the water after the end-Permian mass extinction was more widespread among different reptile groups than previously known.

Researchers studied exceptionally well-preserved Austronaga fossils discovered in the Luoping biota in Yunnan. A detailed examination of the skeleton revealed a series of features indicating that the animal was highly adapted to life in water.

Its forelimbs were considerably longer than its hindlimbs, with the upper arm bone more than twice the length of the thigh bone. Its greatly reduced hindlimbs would have made movement on land difficult, said Wang Wei, first author of the study.

Its hands and feet were also enlarged and had more toe and finger bones than expected. The outermost bones were reduced and lacked claws, forming flipper-like limbs similar to those seen in other marine reptiles, including ichthyosaurs, plesiosaurs and sea turtles.

By comparing the proportions of its limbs with those of a wide range of living and extinct aquatic animals, the researchers found that Austronaga fell within the range of animals that are fully dependent on aquatic environments.

"This suggests that it had almost completely abandoned life on land, making it the earliest known highly aquatic archosauromorph," Wang said.

The fossil also preserves exceptionally rare traces of soft tissues, including the stomach, liver and intestines. By examining the fossil under both natural and ultraviolet light and conducting elemental analysis, researchers were able to identify the locations of these organs.

Partially digested fish scales were found in the large intestine, providing direct evidence that Austronaga fed on fish. Together with its highly specialized limbs, the finding suggests that the reptile was an active aquatic predator well adapted to swimming, according to the study.

The discovery fills a major gap in the early evolutionary history of archosauromorphs. No fully aquatic members of the group had previously been confirmed, and the fossil pushes the group's known adaptation to aquatic environments back to about 244 million years ago.

It also adds an important piece to the evolutionary history of Tanysauria, the reptile group to which Austronaga belongs. According to Wang, Tanysauria is now the only known reptile lineage to include land-dwelling, semi-aquatic and highly aquatic members.

This provides an unusually complete sequence for scientists to study how reptiles gradually adapted to life in water.

The findings also suggest that archosauromorphs played a role in the recovery and restructuring of aquatic ecosystems following the end-Permian mass extinction.

The research team said the discovery raises broader questions about whether different reptile groups that moved into aquatic environments during this period were responding to similar environmental pressures and developed similar adaptations for living in water.

Contact the writers at yanyj@chinadaily.com.cn