Stomachs, livers and intestines are soft, and soft things rot. That is why fossils are nearly always bones, teeth and shells, and why a preserved gut is such a rare treat for palaeontologists.
So imagine the surprise when researchers noticed a dark patch between the ribs of a tiny reptile from China and realised they were looking at the remains of its digestive system, still there after about 245 million years.
The animal is Austronaga minuta, and according to a study published in July in the journal Science Advances, it is the oldest known reptile with a largely complete digestive system.
A little swimmer with a very long neck
Austronaga lived in the Middle Triassic. The Triassic period began roughly 252 million years ago, after a huge mass extinction, and saw the rise of the first dinosaurs. Seas covered parts of what is now China, and long-necked marine reptiles were not uncommon there.
Even so, this one stands out. According to a release from the Stuttgart State Museum of Natural History, the animal measured only about 60 centimetres, roughly two feet, yet had a strikingly long neck and tail.
The tail likely drove it through the water, while long, fin-like front legs helped it steer and stay stable. Its hind legs were much smaller and appear to have done little of the swimming.
The real surprise is its family tree. Austronaga was not closely related to ichthyosaurs or the other famous sea reptiles that swam in a similar way. It belongs instead with the archosaurs, the 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," said Dr Stephan Spiekman, a co-author based at the Stuttgart museum and the University of Hohenheim. "However, the discovery of Austronaga shows that early representatives of this lineage had already developed complex adaptations to life in the oceans."
What the gut reveals
To work out what the dark material was, the team used techniques including UV light photography and mass spectrometry. They identified several organs: a large stomach at the front, a liver, and a long, simple tube forming the small and large intestines.
"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," said lead author Dr Wei Wang, of the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing.
Haemoglobin is the molecule that carries oxygen in the blood, so detecting traces of it after so many millions of years is a striking result.
Wang added that, compared with the animal's highly specialised swimming body, its internal organs seem "less modified".
The Scottish connection
One of the study's authors is Dr Nick Fraser of National Museums Scotland in Edinburgh.
Fraser pointed to a difference from the animal's living relatives. "While today's birds and crocodiles have a two-part stomach, Austronaga had only a single stomach chamber," he said. "This shows that the stomachs of archosaurs were initially quite simple and only evolved later in the evolutionary process."
The gut also offers a hint about how the animal lived. "It is also striking that Austronaga had a fairly short and uncomplicated digestive tract," Fraser said, "very similar to that of today's fish-eating reptiles."
In plain terms, its plumbing resembles that of modern reptiles that eat fish, though the researchers' release stops short of saying exactly what was on this particular reptile's menu.
Why it matters
The fossil gives scientists a rare snapshot of an early marine reptile, inside and out. It also suggests that relatives of the land-living archosaurs took to the sea surprisingly early and, in some ways, surprisingly thoroughly.
The research involved scientists from institutions in China, Scotland, Germany, the United States and Switzerland, including the Luoping Biota National Geopark in China's Yunnan province. The team hopes future finds will shed more light on the early history of marine reptiles and their cousins who stayed on land.
The fossil itself is held in Beijing.
Sources: the Science Advances paper "Highly aquatic marine stem archosaur with soft tissue preservation" (DOI 10.1126/sciadv.aeb7528), published 29 July 2026, and the release from the Staatliches Museum für Naturkunde Stuttgart, via ScienceDaily and EurekAlert.
