Mammal-like reptiles lived much longer than previously thought

The last known family of mammal-like reptiles, a group of herbivores called tritylodontids, lived much longer than originally thought and co-existed with early mammals for several million years, according to a new study by researchers from Kyoto University in Japan.

Writing in a recent edition of the Journal of Vertebrate Paleontology, Hiroshige Matsuoka and colleagues revealed that they had found dozens of fossilized teeth in the Kuwajima Formation in Japan which they used to identify a never-before-seen new species of tritylodontid.

Their discovery suggests that these mammalian reptiles, which serve as the evolutionary bridge from reptiles to mammals, actually lived alongside mammals for millions of years, which runs contrary to the belief that such creatures were wiped out shortly after mammals emerged.

“Tritylodontids were herbivores with unique sets of teeth which intersect when they bite,” lead author Matsuoka explained Monday in a statement. “They had pretty much the same features as mammals – for instance they were most likely warm-blooded – but taxonomically speaking they were reptiles, because in their jaws they still had a bone that in mammals is used for hearing.”

Species was identified using only fossilized teeth

The researchers were excavating a Cretaceous era geologic layer at the Kuwajima Formation when they discovered more than 250 tritylodontid teeth, the first ever found in that part of the world. The tritylodontids lived primarily during the Jurassic era, and were thought to have died out prior to the start of the Cretaceous, but the newfound fossils suggest otherwise.

While Matsuoka said that the notion that they died out in the late Jurassic “made sense, because otherwise tritylodontids and the herbivorous mammals would have competed for the same niche” the new research suggests that the mammal-like reptiles seemed to have survived more than 30 million years longer than paleontologists had originally believed.

The discovery is “raises new questions about how tritylodontids and their mammalian neighbors shared or separated ecological roles,” Matsuoka said. It is also the first study of its kind to utilize only details from fossilized teeth to determine whether or not a species is new, as well as to find its place on the evolutionary tree, the study authors noted. Typically, scientists need to find more complex structures, such as a jawbone, to identify fossils belonging to a new species.

“Tritylodontid teeth have three rows of 2-3 cusps. This time we paid attention to fine details like the size and shape of each cusp. By using this method it should be possible to characterize other species on the evolutionary tree as well,” said Matsuoka. “Because fossils of so many diverse families of animals are to be found in Kuwajima, we’d like to keep investigating the site to uncover things not just about individual species, but also about entire ecological dynamics.”

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Image credit: Seishi Yamamoto/Hiroshige Matsuok

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