Researchers from China, Europe and the United States, including paleontologist Dr. Stephan Speakman of the Stuttgart State Museum of Natural History and the University of Hohenheim, have studied an extraordinary specimen of the long-necked marine reptile Austronnaga minuta.

Although its ancestors lived on land, Austronnaga had already developed advanced adaptations for aquatic life by the Middle Triassic. The nearly complete fossil preserves most of the animal’s skeleton, as well as much of its digestive system.

Using modern analytical techniques, scientists identified several internal organs, including the stomach, liver and intestines. The specimen represents the oldest known example of a nearly complete digestive system preserved in a reptile.

A Small Marine Reptile Built for Life in the Sea

Long-necked marine reptiles were common in the Triassic seas that covered parts of present-day China, and scientists have discovered fossils from several different species. However, the Austronnaga minuta specimen is especially significant because it preserves both the animal’s external anatomy and internal organs.

The reptile was only about 60 centimeters long, but it had an unusually elongated neck and tail. Its anatomy indicates that it was already highly specialized for swimming. The long tail likely provided propulsion, while the elongated, fin-like front limbs may have helped with maneuvering and stabilization. The much smaller hind limbs appear to have played a limited role in swimming.

Its body plan is particularly remarkable. Although Austronnaga used swimming strategies similar to those of other marine reptiles, it was not closely related to famous groups such as ichthyosaurs and giant mosasaurs. Instead, it was related to land-dwelling archosaurs, the broader reptile group that includes crocodiles and dinosaurs.

“Archosaurs and their ancestors were the most diverse group of reptiles on land during the dinosaur era, but we always thought they had limited adaptations to life in the sea,” says Dr. Stephan Speakman, an expert on long-necked marine reptiles and a co-author of the study. “This shows that early representatives of this lineage had already developed complex adaptations to marine life on par with other well-known groups of fossil marine reptiles.”

The Oldest Known Nearly Complete Reptile Digestive System

The fossil became even more unusual when researchers examined dark areas preserved between the ribs. Additional analysis revealed that the material represented the remains of a mostly intact digestive tract.

Scientists used several techniques, including ultraviolet photography and mass spectrometry, to study the fossil and identify multiple internal organs.

“Compared with highly specialized body types, the internal organs of Austronnaga do not seem to have changed much,” said Dr. Wei Wang, lead author of the study and a marine reptile expert at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.

The preserved organs include a large, pouch-like stomach at the front of the body, followed by a distinctively red liver with traces of hemoglobin still detectable. Behind these organs are long, simple tubes forming the small and large intestines.

“The overall arrangement of the digestive system in Austronnaga was relatively simple,” Dr. Wang said. “This fossil is the oldest known example of a nearly complete digestive system in a reptile.”

Despite the reptile’s highly specialized swimming anatomy, its digestive system appears to have retained a relatively simple arrangement.

“Today’s birds and crocodiles have two stomachs, while Austronnaga had only one stomach chamber,” explains study co-author Dr. Nick Fraser of the National Museums of Scotland. “This shows that archosaur stomachs were initially very simple and evolved later in their evolutionary history. Austronnaga had a fairly short and uncomplicated digestive tract, very similar to that of modern fish-eating reptiles.”

New Clues About Marine Reptile Evolution

The discovery provides researchers with an unusually complete view of both the external and internal anatomy of an early marine reptile. It also offers new evidence of how some relatives of land-dwelling archosaurs rapidly developed complex adaptations for life in the ocean.

The study was carried out by scientists from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, the National Museum of Scotland, the State Museum of Natural History Stuttgart, the University of Hohenheim, the Field Museum in Chicago, Luoping Biota National Geological Park, and the University of Zurich.

The research forms part of a broader investigation into the early history of modern reptile groups and the diversity of reptiles that lived in Triassic seas. Researchers hope future discoveries will provide additional clues about the evolution of marine reptiles and their surviving land-based relatives.

The fossil is housed in Beijing.