An exquisitely preserved specimen of Austronaga minuta, a small, long-necked marine reptile from the Triassic of China, contains a fossilized stomach, liver and intestines, offering paleontologists their oldest detailed glimpse yet of how early reptiles digested food.
Life reconstruction of Austronaga minuta. Image credit: Gabriel Ugueto.
Austronaga minuta inhabited the Tethys Ocean during the Middle Triassic period, some 244 million years ago.
The species was a type of tanysaur, a distant cousin of the lineage that eventually gave rise to dinosaurs, crocodiles and birds.
“Despite being only 60 cm long, Austronaga minuta had a strikingly long neck and tail,” said Dr. Wei Wang, a researcher with the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, and colleagues.
“Impressively, the animal 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 minuta is not at all closely related to the other famous fossil sea reptiles, including ichthyosaurs or giant mosasaurs that share this method of swimming.”
In their study, the researchers re-examined a well-preserved skeleton of Austronaga minuta first described two years ago.
Not only is the skeleton essentially complete, but the fossil also retains traces of internal organs: a stomach, liver and intestines.
Photographs taken under ultraviolet light helped the scientists map out the ghostly outlines of these organs, including a liver whose reddish tint is likely due to iron-rich compounds left behind by decayed blood.
The animal’s digestive tract was surprisingly unspecialized: a simple, single-chambered stomach and a short, loosely coiled intestine, more reminiscent of a fish-eating monitor lizard than of a fully marine specialist.
The authors suggest this reflects weaker evolutionary pressure on internal anatomy compared with the dramatic remodeling of the limbs and pelvis.
The holotype specimen of Austronaga minuta. Image credit: Wei Wang.
“Compared with the highly specialized body shape of Austronaga minuta, its internal organs seem less modified,” Dr. Wang said.
“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 minuta 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 minuta had only a single stomach chamber,” said Dr. Nick Fraser, a researcher at the National Museums Scotland.
“This shows that the stomachs of archosaurs were initially quite simple and only evolved later in the evolutionary process.”
“It is also striking that Austronaga minuta had a fairly short and uncomplicated digestive tract, very similar to that of today’s fish-eating reptiles.”
Because Austronaga minuta’s close relatives within their family range from largely terrestrial species to the highly aquatic, long-necked Dinocephalosaurus, the paleontologists argue the group offers a rare, clear-cut example of a reptile lineage caught in the act of transitioning from land to sea.
“The discovery of Austronaga minuta reshapes our understanding of early archosauromorph ecology, demonstrating that marine colonization by stem archosaurs was earlier and more extensive than previously recognized,” they concluded.
“The land-to-sea transition occurred in this major reptilian clade at the beginning of the Mesozoic, similar to the more widely known and longer lasting marine reptile clades, such as Sauropterygia and Ichthyosauria, but also other exclusively Triassic clades like Thalattosauria.”
“Collectively, these findings suggest a broader adaptive strategy among reptiles to survive in arid zones such as those in southern China during the Triassic.”
The results appear in the journal Science Advances.
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Wei Wang et al. 2026. Highly aquatic marine stem archosaur with soft tissue preservation. Science Advances 12 (31); doi: 10.1126/sciadv.aeb7528
