What were these mysterious mammals: one looking like a hippo-rhino-rodent hybrid and another resembling a humpless camel with an elephant’s trunk?

Ever since British naturalist Charles Darwin found their fossils in South America and brought them back to England about 180 years ago, scientists have been scratching their heads just exactly where to place these odd beasts that went extinct just 10,000 years ago on the mammal family tree.

Thanks to 21st century technology, the mystery is now solved.

Researchers say a sophisticated biochemical analysis of bone collagen extracted from fossils of the two mammals, Toxodon platensis and Macrauchenia patachonica, show that they were related to the group that includes horses, tapirs and rhinos.

An artist rendering shows the unusual native ungulates of South America Toxodon platensis, which were more closely related to perissodactyls (horses and their allies) than to other living placental mammals. – Reuters

Some scientists previously thought the two herbivorous mammals, which were apparently the last of a successful group called South American ungulates, had been related to mammals of African origin like elephants and aardvarks, or perhaps other South American mammals like armadillos and sloths.

“We have resolved one of the last unresolved major problems in mammalian evolution: the origins of the South American native ungulates,” says molecular evolutionary biologist Ian Barnes of London’s Natural History Museum, whose research appears in the journal Nature on March 19.

Toxodon, about 2.75m, possessed a body like a rhinoceros, head like a hippopotamus and ever-growing molars like a rodent. Macrauchenia, just as long but more lightly built, had long legs, an extended neck and apparently a small trunk.

An artist's rendering shows the South American native ungulate Macrauchenia patachonica which had a number of remarkable adaptations, including the positioning of its nostrils high on its head. – Reuters

“Some of Darwin’s earliest thoughts about evolution by means of natural selection were engendered by contemplating the remains of Toxodon and Macrauchenia, which resembled so confusingly the features of a number of other groups, but had died out so recently,” says paleomammalogist Ross MacPhee of New York’s American Museum of Natural History.

The researchers tried but failed to get DNA from the fossils, but were able to coax the longer-lasting collagen from the remains. Collagen is the main structural protein in various types of tissues, including bone and skin. The scientists compared the collagen to a wide range of living and a few extinct mammals to properly place the creatures on the mammal family tree.

MacPhee says this group most likely entered South America from North America at about the time the dinosaurs were wiped out 65 million years ago in a calamity that enabled mammals to become Earth’s dominant land animals. 

An eclectic array of mammals including elephant-sized ground sloths and saber-toothed marsupials arose in South America. – Reuters