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66-Million-Year-Old ‘Crazy Beast’ Finds a Taxonomical Home

The opossum-sized mammal lived in Madagascar at the end of the age of the dinosaurs

Adalatherium hui's strange teeth made it especially difficult to classify. (Image courtesy Denver Museum of Nature & Science/Andrey Atuchin)
smithsonianmag.com

In 1999, researchers uncovered a 66-million-year-old fossil of a bizarre mammal in Madagascar. The creature was about the size of an opossum and may have looked more like a badger or a beaver—but it's the ancestor of none of them. The animal’s anatomy was so confounding, researchers named it Adalatherium hui, using a Malagasy word for “crazy” and the Greek word for “beast.”

New research, published on April 29 in the journal Nature, takes a closer look at the crazy beast’s skull and teeth to finally group it among its closest relatives, the gondwanatherians.

“It is so strange compared to any other mammal, living or extinct,” Denver Museum of Nature and Science paleontologist David Krause tells Science NewsMaria Temming. To the BBC, Krause adds that the creature "bends and even breaks lots of rules."

Adalatherium’s teeth are the strangest part of the fossil. Its front teeth are long and curved like a rodent’s, but otherwise its teeth are unlike any modern animal’s, paleontologist Guillermo Rougier says in a statement. Rougier specializes in using teeth to classify mammals, so the strange fossil presented a challenge.

The key to classifying Adalatherium came when the researchers compared its skull to a known gondwanatherian fossil also found in Madagascar, but in 2014. Their snout bones were a match, linking them as relatives.

Gondwanatherians are an obscure group of mammals that died out about 45 million years ago, leaving no modern descendants, per Reuters’ Will Dunham. Their fossils are relatively rare, making the crazy beast’s addition to their ranks valuable for paleontologists. The 1999 fossil of Adalatherium hui is not only mostly complete, but also preserved in three dimensions.

“When you have an animal that dies and is preserved in the rocks, the weight of the rocks on top of it flattens it out. Often it looks like a steam roller ran over it,” Rougier says in the statement. “This animal was preserved in 3D, so this gives us a wealth of detail that we very rarely have in other specimens."

Its legs also defied usual expectations for mammals. While its front legs go straight down, like most mammals, its back legs are splayed out to either side like a reptile. A large hole at the top of its snout is also odd, as is its size.

“We suspect some of this bizarreness might be due to evolution in isolation on an island,” New York Institute of Technology paleontologist Simone Hoffmann says to Reuters.

The fossilized creature wasn’t fully grown, but was about 20 inches long, making it “a giant in its time,” Krause tells Reuters. Most mammals at the time were closer to mouse-sized, while dinosaurs dominated the landscape.

The fossil dates to a time when Madagascar was almost unreachable from larger landmasses as it moved between India and Africa. Isolation leads to strange and specific products of evolution. As Reuters reports, a 16-inch frog called Beelzebufo and a short-snouted, plant-eating crocodile called Simosuchus lived in Madagascar alongside the crazy beast.

But Adalatherium’s anatomy suggests a connection with another group of mammals called multituberculates, which lived in the northern hemisphere. Given their similarities, the crazy beast and its gondwanatherian cousins may be like “southern counterparts” to multituberculates, Krause tells Science News.

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