Chinese fossil shows internal organs of prehistoric marine reptile
The rare fossil offers new clues about reptile evolution and digestion shortly after Earth's largest mass extinction.
Scientists have discovered a fossil in China of a long-necked marine reptile from 245 million years ago that is so well preserved that it reveals the animal's stomach, liver and intestines, providing an unparalleled look at reptilian internal anatomy before the age of dinosaurs.
Researchers said the fossil of the Triassic Period creature Austronaga minuta was unearthed in Yunnan Province in southern China in a region that has yielded many nicely preserved fossils dating to a time of tremendous evolutionary innovation in the aftermath of Earth's worst mass extinction of species.
The fossil shows that this Austronaga individual was around 2 feet (60 cm) long, with a relatively small head and a narrow snout filled with fang-like teeth ideal for snaring slippery fish, a long neck and torso, and an even longer tail, with large front flippers and small hind flippers.
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The animal, descended from land reptiles, appears to have been an agile swimmer, mainly using its tail for propulsion, its front flippers for steering and its long neck to catch fish.
Austronaga was closely related to a reptile lineage called archosaurs that includes crocodiles, dinosaurs, the flying reptiles called pterosaurs and birds. Very few animals related to this lineage evolved a marine lifestyle — Austronaga displayed the highest degree of aquatic adaptation of any of them — though there were many other kinds of marine reptiles.
Most of its skeleton was preserved with the bones in the position they would have been in life. But what makes this fossil particularly revelatory is the preservation of internal organs, a rarity, showing the complete gastrointestinal system.
"To our knowledge, this represents the oldest well-preserved digestive tract of any reptile," said paleontologist Stephan Spiekman of the State Museum of Natural History Stuttgart and the University of Hohenheim in Germany, one of the authors of the study published in the journal Science Advances.
The fossil showed that Austronaga had a sac-like stomach with a single chamber, unlike the double chambers present in birds, crocodiles and certain dinosaurs. That means Austronaga had a simpler arrangement, with one compartment for digestion.
"It tells us that the double-chambered stomach likely did not evolve at the beginning of stem-archosaur evolution, but a later stage, closer to the origin of birds and crocs," Spiekman said.
Behind the stomach was the animal's liver, the organ that processes nutrients, balances energy and filters waste, still retaining its red color.
"Its positioning posterior to the stomach is no surprise. The most surprising thing to me was the preservational aspect — the fact that residues of hemoglobin, with elevated iron contents, could still be distinguished in a fossil that is 245 million years old," Spiekman said.
Hemoglobin is a protein present in red blood cells that carries oxygen from the lungs to the rest of the body and carries carbon dioxide back. It provides blood its red color due to the iron inside the protein.
Behind the liver was a long tube forming the small and large intestines, showing that its digestive system was structurally simple.
"The shape of the intestine in particular corresponds to what we would expect from a predatory reptile, which has a much simpler intestine than a herbivore, since meat — including fish — is easier to digest than plants," Spiekman said. "We see more complex intestines that are longer and more folded up on itself in later archosaurs, including dinosaurs, whereas in Austronaga we primarily see a straight tube."
Dinosaurs first appeared roughly 230 million years ago.
A FISH DINNER
There were identifiable food remains — fish scales — preserved in the Austronaga specimen's gut.
Life bounced back with aplomb following the mass extinction that ended the Permian Period around 252 million years ago, and multiple lineages of land reptiles adapted to the seas. Several of these displayed long necks, like Austronaga, showing it was an effective fish-hunting adaptation.
Austronaga inhabited a warm, shallow equatorial sea teeming with life, with numerous fish and marine reptiles. Among the top predators were nothosaurs, known for their menacing jaws, that reached 16 feet (5 meters) long.
The researchers are not certain whether the Austronaga individual was an adult.
"We are not sure if this animal was fully grown at the time of death, so it's possible that the species could get bigger. But it was likely a small marine reptile and did not grow to become many meters long," Spiekman said.
