Unique fossil reveals the internal organs of a 245-million-year-old marine reptile
Scientists from China, Europe and the U.S., including Dr. Stephan Spiekman, a paleontologist at the State Museum of Natural History Stuttgart and the University of Hohenheim, have described a fossil of the long-necked marine reptile Austronaga minuta from China. Although this reptile descended from land-dwelling reptiles, it was already well adapted to life in the water around 245 million years ago.
This almost completely preserved fossil provides the first detailed view of this prehistoric animal's entire skeleton and large parts of its digestive system. Various internal organs, such as the stomach, liver and intestines, have been clearly identified using modern analytical methods. This is the oldest known example of a largely complete digestive system in a reptile.
The research results have been published in the journal Science Advances.
Early adaptations to life in water
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 centimeters (24 inches) long, the animal had a strikingly long neck and tail.
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 stabilization. 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 closely related to other well-known fossil sea reptiles, including ichthyosaurs and giant mosasaurs, which 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 Spiekman, a specialist in long-necked marine reptiles and coauthor of the study.
The oldest known digestive tract of any reptile
Between the ribs of the fossil, the researchers came across a dark region that, 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," says Dr. Wei Wang, lead author of the study and marine reptile expert at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
"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. 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 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 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 discoveries that will shed light on the evolutionary history of marine reptiles and their terrestrial relatives.
The fossil is located in Beijing.
Publication details
Wei Wang et al, Highly aquatic marine stem archosaur with soft tissue preservation, Science Advances (2026). DOI: 10.1126/sciadv.aeb7528. www.science.org/doi/10.1126/sciadv.aeb7528
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Citation: Unique fossil reveals the internal organs of a 245-million-year-old marine reptile (2026, July 29) retrieved 29 July 2026 from https://phys.org/news/2026-07-unique-fossil-reveals-internal-million.html
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