An animal that lived 245 million years ago normally leaves nothing but bone. In a fossil from Yunnan province in southern China, however, palaeontologists found a dark discolouration between the ribs – and inside it the remains of an almost complete digestive system. It is the oldest known case of its kind in a reptile.
The animal is called Austronaga minuta and is described in the journal Science Advances by a team led by Wei Wang of the Chinese Academy of Sciences. Among the co-authors are Stephan Spiekman of the State Museum of Natural History Stuttgart and the University of Hohenheim, and Nicholas Fraser of National Museums Scotland. The region has already yielded numerous exceptionally well-preserved fossils from the period following the great mass extinction at the Permian-Triassic boundary.
At about 60 centimetres, Austronaga was small, with a strikingly long neck, an even longer tail and a small head with a narrow snout. Its front limbs had been reshaped into large flippers, while the hind limbs were much reduced, and the firm connection between sacrum and pelvis was missing. The reptile appears to have propelled itself with its tail, using the front flippers to steer and stabilise.
A stomach with only one chamber
What makes the find remarkable is less the skeleton than what sits inside it. A sac-like stomach is followed by a distinctly red liver in which traces of the blood pigment haemoglobin can still be detected, and behind that a long, simple tube formed by the small and large intestine. The stomach had only a single chamber – unlike that of today's birds and crocodiles, which is divided in two. The two-chambered design, Spiekman concludes, is therefore likely to have appeared later, closer to the origin of birds and crocodiles.
The animal's last meal could be reconstructed as well: the stomach held fish scales. Together with a set of slender, fang-like teeth, that suggests Austronaga actively hunted small fish, seizing them with its long neck. Nostrils set well back on the skull would have made breathing at the water's surface easier.
For specialists, the find matters above all because Austronaga is neither an ichthyosaur nor a mosasaur but a relative of the archosaurs – the lineage that produced crocodiles, pterosaurs, dinosaurs and birds. That group was long thought to have adapted only to a limited degree to life at sea. Austronaga shows the opposite: early on, individual members developed a body plan that could match the better-known marine reptiles.