This Tiny Beast Is Not Half Mammal Half Reptile

November 19, 2018

A furry little creature with a blunt snout and sharp eyes traversed what is now eastern Utah 130 million years ago. While this little beast was certainly unusual and fascinating, there was one thing it definitely wasn’t: a half-mammal and half-reptile.

Headlines about the recent discovery have described it as a bizarre mix of reptile and mammal. But while it might be fun to imagine a beast with a lizard front end and a mouse back end, it’s not exactly scientific.

Originally only 3 inches (7.6 centimeters) tall and weighing about 2.5 pounds (1.1 kilograms), the tiny creatures belonged to a group known as haramids, which appeared in the late Triassic period (251 million to 199 million years ago) and are known primarily from fossilized teeth. Scientists have debated whether haramiyidans were early mammals or a sister group – closely related to mammals – but lacked some of the features that paleontologists use to decide who is a mammal and who isn’t.

In a new study describing the tiny skull – which represents a new genus and species of haramiyidan called Cifelliodon wahkarmoosuch and is thought to be between 139 million and 124 million years old – researchers determined that haramiyidans are relatives of mammals, though not true mammals. Scientists report that while haramiyidans look like mammals, they retain more “non-mammalian” structures from their ancient ancestors rather than the first true mammals.

The origins of both haramiyidans and mammals can be traced back to a group known as synapsids. The last time all syapsids shared a common ancestor with reptiles was about 330 million years ago, “so the split between reptiles and mammals is very deep,” paleontologist Elsa Panciroli, who was not involved in the new study, told Live Science in an email.

In other words – mammals don’t have a “reptilian ancestor.” Instead, both mammalian and reptilian ancestors branched off from a common ancestor hundreds of millions of years ago, Panciroli explained.

In the past, scientists might have said that early synapsids shared more features with reptiles than with mammals.” But in reality, we need to look at it as which group is more like their common ancestor,” says Panciroli, a PhD student at the University of Edinburgh and National Museums Scotland who studies the origins of mammals.

One trait found in reptiles and preserved in some modern mammals is egg laying, seen in platypus and spiny anteaters. Egg laying can be traced back to a very distant ancestor of the reptilian and mammalian lineages. Mammals with placentas, on the other hand, are thought to have originated from a shrew-like animal called Juramaia sinensis, which lived about 160 million years ago.

A transitional skull
In life, the newly described Cifelliodon certainly resembles a mammal. It has a furry body and a long tail, teeth that cut and crush vegetation, and a small eye socket, suggesting it has small eyes and poor vision. However, according to the study, its olfactory bulb was unusually large, suggesting that it relied heavily on its sense of smell.

The high-resolution X-ray scans also revealed the shape of the inner cranium, which is “transitional between early stem and crown mammals,” the researchers wrote. This means that Cifelliodon – and other haramiyidans – have some mammal-like features, but not as many as the group of defining mammals alive today.

The particular condition of the Cifelliodon skull – particularly its three-dimensional shape – provides clues about haramiyidan groups that were previously only speculated upon, when the only available fossils were rare or flattened, lead study author Adam Huttenlocker, assistant professor of clinical integrated anatomical sciences at the Keck School of Medicine of USC told Southern California News.

“The three-dimensional preservation of Cifelliodon highlights the primitive brain, palate and feeding structures of this particular group and reinforces their location near the base of the mammalian family tree,” Huttenlocker said.