Ancient “Oddball” Mammal Reshuffles Family Tree?

By Linda Qiu

A mysterious mammal that waded through South Asian swamps 48 million years ago is a distant cousin of modern rhinoceroses and tapirs, a new study says.

The discovery, based on fossils unearthed in Pakistan, has prompted a reshuffling of the mammalian family tree.

That’s because these ancient animals, known as Anthracobunids, were previously thought to be primitive relatives of elephants and manatees, which have similar skeletons. (Also see “Chisel-Toothed Beasts Push Back Origin of Mammals.”)

An Anthracobunid wades in a swamp in an artist’s impression. Illustration by Emily M. Eng, NG Staff. Source: Lisa Noelle Cooper, Northeast Ohio Medical University

But that knowledge was based on a scarce, incomplete fossil record of Anthracobunids, four-foot (meter-long) beasts that resembled hornless rhinos with flat foreheads, long snouts, and golf ball-size eyes, according to the research, published October 8 in the journal PLOS ONE.

“The fragmentary fossils … looked like a lot of different animals. They were all over the place and gave unclear pictures of … where these animals came from,” said study leader Lisa Cooper, a paleontologist at Northeast Ohio Medical University in Rootstown, Ohio.

Ancient Oddballs

Tribal instabilities in the regions where Anthracobunid fossils are found have restricted scientific research and thus the number of fossils.

However, a collaborative agreement between Howard University and the Geological Survey of Pakistan permitted scientists to uncover the new fossils at a site near Islamabad.

The newfound fossils, which include a crushed skull and a full set of teeth, double the known collection of Anthracobunid bones, which previously numbered from about 15 to 20 jaw and tooth pieces that were unearthed in the 1930s.

Though these limited fossils had pointed to elephants and manatees as kin, there had always been a geographical problem looming over that theory.

A photo of the crushed skull of a relative of the rhino.
The crushed skull recently found in Pakistan. Photograph by Cooper Lab, NEOMED

Elephants and manatees are part of a group of mammals that originated in Africa, not Asia.

“[Anthracobunid] was this oddball group in Pakistan that no one knew what to do with,” said Gregg Gunnell, a paleontologist at Duke University in Durham, North Carolina, who was not involved with the study.

After comparing the remains with the molecular and anatomical features of modern species, Cooper and colleagues concluded that Anthracobunids were more likely related to odd-toed ungulates such as rhinos and tapirs. (Also see “Volcano Eruption Baked Rare Rhino Fossil.”)

Since rhinos and tapirs evolved on the Asian continent, where all Anthracobunid fossils have been found, the new research makes more sense, said Gunnell.

Like modern rhinos, which live in Asia and Africa, Anthracobunids fed on land plants and were large and heavy. But whereas rhinos wallow in mud, Anthracobunids liked to cool off in streams during the hot middle Eocene epoch (47.8 million to 38 million years ago), the study says. (See “Ancient Elephant Ancestor Lived in Water, Study Finds.”)

Still Mysterious

Anthracobunid and modern rhinos share a common ancestor that lived around 60 million years ago in the Paleocene epoch (66 to 56 million years ago). That’s around the same time that primitive elephant relatives were roaming Africa, said study co-author Erik Seiffert, a paleontologist at Stony Brook University in Stony Brook, New York.

A photo of the jaw of a relative of the rhino.
A newfound jaw of an Anthracobunid. Photograph by Cooper Lab, NEOMED

So how did Anthracobunids and early elephants come to resemble one another two continents and genetic branches apart?

It’s a classic case of convergent evolution, in which two genetically dissimilar animals adapt to similar environments and end up looking and acting like each other.

Despite finding its distant cousin, the rhino-like mammal is still not entirely understood, and its origin story needs more evidence, the study authors say.

“Their fossils raise more questions than answers.”

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