Long-necked marine reptiles were not uncommon in the seas of what is now China during the Triassic period, and several species have been found as fossils. However, the
Austronaga minuta fossil is unique in many ways. Despite being only 60 centimetres long, the animal had a strikingly long neck and tail.
Impressively,
Austronaga was highly adapted to life in the water, using its long tail for propulsion and its elongated, fin-like front legs for steering and stabilisation. Its hind legs, on the other hand, were greatly reduced and played hardly any role in swimming.
This unusual body shape is surprising given that
Austronaga is not at all closely related to the other famous fossil sea reptiles, including ichthyosaurs or giant mosasaurs that share this method of swimming. Instead,
Austronaga is closely related to land-dwelling archosaurs, a group that includes crocodiles and dinosaurs.
“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.
The oldest known digestive track of any reptile
Between the ribs of the fossil, the researchers came across a dark region which, upon closer inspection, turned out to be remnants of a largely intact digestive tract. Using various examination methods, including UV light photography and mass spectrometry, the scientists were able to clearly identify several internal organs for the first time.
“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 haemoglobin 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 Palaeontology and Palaeoanthropology in Beijing. “This fossil is the oldest known example of a largely complete digestive system in a reptile.”
“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.”
Further research on the evolution and ecology of marine reptiles
This study was conducted by researchers, from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing (China), the National Museums Scotland (UK), the State Museum of Natural History Stuttgart (Germany), the University of Hohenheim (Germany), The Field Museum in Chicago (USA), the Luoping Biota National Geopark (China) and the University of Zurich (Switzerland).
It is part of a comprehensive analysis of the early history of modern reptile groups and their diversity in the Triassic seas. The researchers hope to make further significant discoveries in the future that will shed light on the evolutionary history of marine reptiles and their terrestrial relatives.
The fossil is located in Beijing.
ORIGINAL PUBLICATION:
Wei Wang, Nicholas C. Fraser, Stephan N. F. Spiekman, Zhiheng Li, Olivier Rieppel, Hong Lei, Torsten M. Scheyer and Chun Li: Highly aquatic marine stem archosaur with soft tissue preservation. Science Advances. Publication date: 29.07.2026.
www.science.org/doi/10.1126/sciadv.aeb7528