A 245-Million-Year-Old Marine Reptile Fossil Still Has Its Stomach, Liver and Intestines

Quick Answer: Scientists have described an exceptionally preserved fossil of Austronaga minuta, a roughly 245-million-year-old marine stem archosaur from what is now Yunnan, China. The fossil preserves the animal's skeleton along with a largely complete digestive system, including a stomach, liver and intestines. The discovery shows that an early relative of the archosaur lineage had already evolved extensive adaptations for life in water.

An illustrative depiction of Austronaga minuta, a highly aquatic marine stem archosaur from about 245 million years ago, inspired by the documented fossil discovery and scientific research. Illustration: The Pader.

Introduction

Fossils can reveal the bones of animals that disappeared hundreds of millions of years ago. Much rarer are fossils that preserve traces of what was inside those animals.

That is what makes a remarkably preserved specimen of Austronaga minuta so important. The small, long-necked marine reptile lived about 245 million years ago in the Triassic Period, yet its fossil preserves not only an almost complete skeleton but also recognizable remains of its stomach, liver and intestines.

The specimen gives scientists an unusual look at two sides of the animal's biology at the same time: a body that was highly specialized for swimming and an internal digestive system that remained comparatively simple.

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SPECIES PROFILE

Austronaga minuta

Austronaga minuta

GROUP Marine stem archosaur

AGE About 245 million years

LENGTH About 60 centimetres

HABITAT Triassic marine environment in what is now Yunnan, China

DIET Fish; the fossil preserves fish scales in the stomach contents

AQUATIC ADAPTATIONS Long tail, paddle-like forelimbs and greatly reduced hind limbs

FOSSIL STATUS Extinct Triassic reptile

The species belongs to the tanysaurian group within Archosauromorpha, the broader reptile lineage that includes modern crocodilians and birds as well as dinosaurs.

A Small Reptile That Was Already Built for Life in the Water

Austronaga minuta was not a giant marine predator. The described specimen measures about 60 centimetres long, but its anatomy reveals an animal that had undergone substantial changes for aquatic life.

Its neck and tail were elongated, while the forelimbs were extended and shaped like paddles. Researchers interpret the long tail as an important source of propulsion, with the forelimbs helping with steering and stabilization. The hind limbs were greatly reduced and appear to have played little role in swimming.

That combination is particularly interesting because Austronaga belonged to the archosaur lineage rather than to the better-known groups of marine reptiles such as ichthyosaurs or mosasaurs.

In other words, a reptile lineage associated largely with terrestrial animals had produced an extensively aquatic form remarkably early in its history.

Its Fossil Preserved More Than Bones

The most extraordinary part of the specimen is found between its ribs.

Researchers identified a dark region as preserved remains of the digestive system. They used several analytical approaches, including ultraviolet photography and mass spectrometry, to investigate the fossilized soft tissues.

The resulting evidence allowed them to identify a large, sac-like stomach, a liver and a long intestinal tract. The study describes the specimen as the oldest known example of a largely complete digestive system preserved in a reptile.

What Scientists Found Inside the Animal

The preserved digestive system provides a rare anatomical snapshot from the Triassic seas.

The stomach was large and apparently consisted of a single chamber. Behind it was a liver that retained a striking reddish coloration in the fossil. Chemical analyses detected iron-rich mineral traces associated with the preserved liver tissue, supporting the interpretation that the coloration was linked to hemoglobin-derived material.

Farther back, the intestines formed a relatively long but simple tube without the strongly differentiated structure seen in more specialized digestive systems.

The researchers describe the overall digestive arrangement as comparatively conservative when placed beside the animal's highly specialized swimming anatomy.

The Animal Was Adapted to the Sea, but Its Digestive System Was Simpler

This contrast is one of the most interesting findings of the study.

The skeleton shows multiple features associated with extensive aquatic adaptation, including modified limbs, reduced pelvic structures and a body plan suited to swimming. Yet the gastrointestinal system remained relatively uncomplicated.

The researchers interpret the single-chambered stomach as evidence that the more derived two-part stomach organization seen in living crown-group archosaurs, including birds and crocodilians, evolved later.

That does not mean Austronaga was a direct ancestor of modern birds or crocodiles. Rather, it preserves an early stage in the broader evolutionary history of the archosaur lineage.

Fish Were Part of Its Diet

The animal's teeth and preserved stomach contents provide another clue to how it lived.

Its jaws carried prominent conical teeth suited to catching prey, and fish scales were preserved among the stomach contents. Together, these features support the interpretation that Austronaga was a fish-eating aquatic predator.

The finding is especially useful because fossilized stomach contents can connect anatomy with actual feeding evidence. Instead of inferring diet only from teeth, researchers can compare the animal's feeding structures with material preserved inside its digestive system.

A Different Route Into the Water

One reason the fossil is scientifically significant is its position within the reptile family tree.

Many famous marine reptiles evolved specialized bodies for life in the ocean, but they came from different reptile lineages. Austronaga represents an aquatic branch of the archosauromorph lineage, which otherwise includes many terrestrial forms.

The research therefore provides an unusually clear example of a lineage containing terrestrial relatives and an extensively aquatic member. It adds evidence that the transition from land toward life in the sea occurred more than once among reptiles and could produce highly specialized swimming anatomy relatively early in the Mesozoic.

How Does Austronaga Compare With Better-Known Marine Reptiles?

The comparison is important because similar aquatic body plans do not necessarily mean that the animals were closely related. Evolution can produce comparable adaptations in different lineages when animals face similar environmental demands.

Why This Fossil Matters

The importance of the Austronaga fossil is not simply that it is old or that it preserves unusual tissues. Its value comes from the combination of information preserved in one specimen.

The skeleton reveals how an early archosauromorph moved through water. The digestive system reveals what its internal anatomy looked like. The stomach contents provide evidence about its diet. Together, these details allow researchers to reconstruct aspects of its ecology that would normally be difficult to recover from a fossilized skeleton alone.

The study also adds to the scientific picture of how reptiles diversified during the Triassic, when marine ecosystems were recovering and reorganizing after the end-Permian mass extinction.

When Was the Discovery Published?

The scientific paper describing the fossil was published in Science Advances on July 29, 2026. The State Museum of Natural History Stuttgart and the University of Hohenheim issued their research press release the following day.

Later coverage in September 2026 brought the fossil back into public attention, but September was not the original publication or discovery date of the research.

Takeaway

Austronaga minuta offers an unusually detailed glimpse into a reptile that lived about 245 million years ago. Its fossil preserves not only an almost complete skeleton but also a largely complete digestive system, including the stomach, liver and intestines.

Most strikingly, the animal's body had already undergone extensive changes for life in the sea even though its digestive system remained comparatively simple. The specimen therefore gives scientists a rare opportunity to study anatomy, diet and aquatic adaptation together in an early member of the archosaur lineage.

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