An international research team examined a unique fossil from China that, for the first time, preserves the digestive system of a marine reptile with a long neck that lived in the oceans 245 million years ago.
Inside an almost complete fossil from China, researchers identified the stomach, liver, and intestines of a marine reptile that lived about 245 million years ago. The specimen belongs to Austronaga minuta, a small, long-necked reptile descended from land animals but already highly specialized for life in the water.
Scientists from China, Europe, and the United States, including Dr. Stephan Spiekman of the State Museum of Natural History Stuttgart and the University of Hohenheim, examined the fossil using modern analytical techniques. It provides the first detailed view of the animal’s complete skeleton and much of its digestive tract.
The preserved organs represent the oldest-known example of a largely complete digestive system in a reptile. The findings were published in Science Advances.
A land lineage mastered the sea
Several species of long-necked marine reptiles inhabited the seas covering what is now China during the Triassic period. Even among these animals, however, the 60-centimeter-long Austronaga minuta stands out for its unusual anatomy.
Austronaga had an exceptionally long neck and tail. Its tail supplied the main propulsive force in the water, while elongated, fin-like front limbs helped it steer and remain stable. Its greatly reduced hind limbs contributed little to swimming.
This body plan is especially surprising because Austronaga was only distantly related to marine reptiles such as ichthyosaurs and giant mosasaurs that evolved a similar swimming style. Instead, it belonged close to the land-dwelling archosaur lineage, which includes dinosaurs and crocodiles.
“The archosaurs and their ancestors were the most diverse group of reptiles on land during the time of the dinosaurs, but we always thought that their adaptation to life in the sea was limited. However, the discovery of Austronaga shows that early representatives of this lineage had already developed complex adaptations to life in the oceans, comparable to those of other, better-known groups of fossil marine reptiles,” says Dr Stephan Spiekman, a specialist in long-necked marine reptiles, who is also the co-author of the study.
Soft tissues reveal ancient anatomy
A dark area preserved between the fossil’s ribs drew the researchers’ attention. Closer analysis showed that it contained remnants of a digestive tract that had survived largely intact.
The scientists used several methods, including ultraviolet light photography and mass spectrometry, to distinguish and identify individual organs.
“Compared with the highly specialized body shape of Austronaga, its internal organs seem less modified. The preserved organs include a large, sac-like stomach at the front, followed by a strikingly red liver in which traces of hemoglobin can still be detected. Behind this is a long, simple tube forming the small and large intestines. The overall arrangement shows that the digestive system of Austronaga was relatively simple in structure,” says Dr Wei Wang, lead author of the study and marine reptile expert at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. “This fossil is the oldest known example of a largely complete digestive system in a reptile.”
Archosaur stomachs began relatively simple
“While today’s birds and crocodiles have a two-part stomach, Austronaga had only a single stomach chamber. This shows that the stomachs of archosaurs were initially quite simple and only evolved later in the evolutionary process,” explains Dr Nick Fraser of the National Museums Scotland, one of the study’s authors. “It is also striking that Austronaga had a fairly short and uncomplicated digestive tract, very similar to that of today’s fish-eating reptiles.”
The work forms part of a broader investigation into the early history of modern reptile groups and their diversity in Triassic seas. Future discoveries could provide further evidence about the evolutionary relationships between marine reptiles and their relatives on land.
The fossil is located in Beijing.
Reference: “Highly aquatic marine stem archosaur with soft tissue preservation” by Wei Wang, Nicholas C. Fraser, Stephan N. F. Spiekman, Zhiheng Li, Olivier Rieppel, Hong Lei, Torsten M. Scheyer and Chun Li, 29 July 2026, Science Advances.
DOI: 10.1126/sciadv.aeb7528
This research was supported by National Natural Science Foundation of China (42002019) and Youth Innovation Promotion Association of the Chinese Academy of Sciences.
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