A 60-centimeter marine reptile fossil from China kept its stomach, liver and intestines

A fossil about 60 centimeters long from southwest China has yielded what its describers call the oldest largely complete digestive system known in a reptile: a stomach, a liver and intestines, preserved as dark and reddish traces between the ribs. The animal is Austronaga minuta, a long-necked swimmer from the Middle Triassic, roughly 245 million years ago.

Lead author Wei Wang of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing put the claim plainly in the team’s release: “This fossil is the oldest known example of a largely complete digestive system in a reptile.” The wording matters, because the organs are identified from position, shape and colour rather than from original tissue, and the paper is careful about that line.

Austronaga minuta: the Luoping skeleton and its 60 cm length

The work, led by Wang with co-authors including Nicholas Fraser of National Museums Scotland and Stephan Spiekman of the State Museum of Natural History Stuttgart, appeared in Science Advances in a paper called “Highly aquatic marine stem archosaur with soft tissue preservation.” A summary of the Stuttgart museum release gives the animal’s length as about 60 cm and describes a nearly complete skeleton plus large portions of the digestive system.

The full paper identifies the specimen as IVPP V18579, a complete articulated skeleton on two slabs, collected at Waina Village in Luoping County, Yunnan Province, from Member II of the Guanling Formation, which is Middle Anisian (Pelsonian) in age. The paper’s text does not itself report a total length; the figures carry scale bars, so the 60 cm figure rests on the release.

How a dark patch became a stomach, a liver and a gut

Observation and interpretation separate cleanly here. What can be seen is a set of dark masses lying between the ribs. The full paper assigns the front mass to the stomach “on the basis of its relative position, size, and shape,” describes it as a single chamber, and says a second mass “represents the liver” because of its position and distinctive colour. The intestinal material is hedged more heavily: the authors write that “it is suggested” the remains are small intestine and large colon.

That hedging is the reason the reptile’s record claim reads as it does. Most of the identifications rest on anatomy and appearance, and the authors add that the liver may continue forward beneath the stomach, so it may be larger than the exposed part.

Wang described the findings in the release as “a large, sac-like stomach at the front,” “a strikingly red liver in which traces of haemoglobin can still be detected,” and “a long, simple tube forming the small and large intestines.” The paper is more cautious about the red colour, calling it assumed to be a derivative of hemoglobin, with iron enrichment as support. In other words, the detectable trace is a chemical residue interpreted as blood-derived, not intact liver.

Imaging, chemistry and the Luoping seafloor

The team photographed the slabs under ultraviolet light and examined samples under a scanning electron microscope with elemental analysis. Under UV-A light the liver layer appears dark black, which the authors link to hematite, and the electron microscope work found iron enrichment relative to the surrounding sediment, along with iron-rich framboidal material in the liver region. The Stuttgart museum’s press release also lists mass spectrometry among the methods, a technique the paper’s text does not describe, so that detail comes from the release alone.

The gut region adds a second line of chemical evidence. The authors describe the intestinal material as dark brown coprolites, meaning fossilised droppings or gut contents, and report high phosphate concentrations in the rear gut along with possible fish scales. That combination fits an animal that ate fish, and Fraser said in the release that the arrangement is very similar to that of today’s fish-eating reptiles.

On depositional conditions, the paper is modest. The skeleton came from marine sediments in Luoping that also hold marine invertebrates, fishes and other marine reptiles, and the authors give no specific mechanism for why this individual kept its viscera. They note only that the stomach of a different Early Cretaceous theropod was not preserved, possibly because of acidic gastric secretions. The release and the sci.news summary likewise leave the burial history undescribed, and the latter dates the animal to about 244 million years ago.

The anatomy also carries a lesson about the animal itself. The paper calls Austronaga the earliest stem archosaur with clear aquatic specialisations, citing extra finger bones, paddle-like forelimbs and a loss of the connection between sacrum and pelvis. Spiekman said in the release that early members of this lineage had already developed complex adaptations to life in the oceans. Wang noted that, compared with the highly specialised body shape, the internal organs seem less modified, and that the overall arrangement shows a relatively simple digestive system.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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