A small marine reptile called Austronaga minuta has given paleontologists the oldest known example of a largely complete digestive system preserved in a reptile, according to a study in the journal Science Advances. The nearly complete fossil from southwestern China, about 245 million years old, still shows traces of the animal's stomach, liver and intestines between its ribs.

The specimen also reveals a creature far better adapted to life at sea than scientists expected for its lineage. Austronagabelonged to the broader group that includes the ancestors of crocodiles, birds and dinosaurs, a group whose early members lived mostly on land.

The find offers a rare look inside an ancient animal rather than only at its bones. Soft tissue usually decays long before fossilization, so a preserved gut offers clues about diet and evolution that skeletons alone cannot provide.

A Specimen First Named in 2023

Austronaga minuta was formally named in 2023 from this same specimen, based mainly on its skull, neck and tail, as an earlier report on the naming describes. The new paper, published in late July, describes the complete skeleton and its soft tissues; it is not the announcement of a new species. The fossil came from Waina Village in Luoping County, Yunnan Province, and is housed at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.

The animal was small, about 60 centimeters (roughly 2 feet) long, with an unusually long neck and tail. Its tail likely provided propulsion, while elongated, fin-like front limbs helped with steering and stability, according to a museum summary of the work from the State Museum of Natural History Stuttgart. Its hind limbs were much smaller and appear to have contributed little to swimming.

The paper reports aquatic adaptations previously unknown among its relatives, including extra bones in the digits of its paddle-like forelimbs and a pelvis no longer firmly joined to the spine. The authors describe it as the earliest stem archosaur known to evolve unequivocal specializations for life in water.

Stephan Spiekman of the Stuttgart museum, a co-author, said the archosaurs and their ancestors were the most diverse land reptiles in the time of the dinosaurs, but "we always thought that their adaptation to life in the sea was limited."The new fossil, he said, shows early members of the lineage had already developed complex ocean adaptations.

Reading a 245-Million-Year-Old Gut

Researchers noticed a dark area preserved between the fossil's ribs. Using ultraviolet photography and mass spectrometry, they identified it as the remains of a largely intact digestive tract.

Lead author Wei Wang of the Institute of Vertebrate Paleontology and Paleoanthropology described a large, sac-like stomach at the front, followed by "a strikingly red liver in which traces of hemoglobin can still be detected." Behind that, he said, a long, simple tube formed the small and large intestines.

The stomach had a single chamber, unlike the two-part stomachs of today's birds and crocodiles, said co-author Nick Fraser of National Museums Scotland. That suggests archosaur stomachs began simple and became more complex later in evolution. Fraser added that the short, uncomplicated digestive tract is "very similar to that of today's fish-eating reptiles."

The researchers also reported partially digested fish scales in the large intestine, according to China Daily's report on the study, which provides direct evidence that Austronaga ate fish.

The Evidence and Its Limits

The study is peer-reviewed, but it rests on a single specimen. That makes the fossil extraordinary and also limits how broadly its details can be applied. Scientists cannot yet say whether every individual of the species had the same internal arrangement.

The reported detection of hemoglobin traces in the liver is the kind of finding other researchers typically scrutinize closely, because molecular remnants in very old fossils are difficult to confirm. The claim comes from the study team, and independent work would strengthen it.

Preservation itself also shapes what can be seen. Soft tissue survives only under unusual chemical conditions, and some structures may have shifted, shrunk or been lost before burial. The researchers identified the organs by their position, color and chemistry, an approach that still involves interpretation. Additional specimens from the same rock layers would let scientists check whether the arrangement seen here is typical.

A Chicago Connection and the Bigger Picture

The international team included researchers from China, Scotland, Germany, Switzerland and the United States, among them a scientist from the Field Museum in Chicago, along with the Luoping Biota National Geopark. The work forms part of a broader effort to understand how reptiles diversified in Triassic seas after Earth's largest known mass extinction, at the end of the Permian.

Austronaga was not closely related to better-known ocean reptiles such as ichthyosaurs or the much later mosasaurs, yet it arrived at a similar swimming strategy, an example of separate lineages solving the same problem. It belongs to Tanysauria, a reptile group that now includes known land-dwelling, semi-aquatic, and highly aquatic members, as Earth.com's account of the fossil notes.

That makes the fossil useful for studying how reptiles made the transition from land to sea, one of the major evolutionary shifts in vertebrate history. Future finds from the same Chinese fossil beds could show whether Austronaga's relatives shared its simple gut and specialized body.

What Readers Want to Know

What is Austronaga minuta?

It is a small, long-necked marine reptile from the Middle Triassic of China, about 60 centimeters (roughly 2 feet) long, that lived around 245 million years ago.

What makes this fossil special?

It preserves traces of the stomach, liver, and intestines, making it the oldest known largely complete digestive system found in a reptile.

Is Austronaga a newly discovered species?

No. It was named in 2023 from the same specimen. The new study describes the complete skeleton, its soft tissues, and its swimming adaptations.

What did Austronaga eat?

Researchers reported partially digested fish scales in its large intestine, and its short, simple gut resembles those of modern fish-eating reptiles.

How is it related to dinosaurs?

Austronaga belonged to the broader lineage that includes the ancestors of crocodiles, birds, and dinosaurs, a group whose early members lived mostly on land.

Where is the fossil kept?

The specimen is housed at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.