New Study Sheds Light on Mammal Group that Puzzled Darwin

by AMNH on

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Toxodon platensis
Analysis of fossil collagen shows that unusual South American ungulates (like Toxodon platensis, seen here) were more closely related to horses and their allies than to other living placental mammals.
Illustration © Peter Schouten

Charles Darwin called them the “strangest animals ever discovered” when he found these mammals’ fossilized remains nearly 200 years ago. Now, new research led by the Museum, the Natural History Museum in London, and the University of York shows that South America’s so-called “native ungulates”—the last of which disappeared only 10,000 years ago—are related to mammals like horses rather than elephants and other species with ancient evolutionary ties to Africa.

“Fitting South American ungulates to the mammalian family tree has always been a major challenge for paleontologists, because anatomically they were these weird mosaics, exhibiting features found in a huge variety of quite unrelated species living all over the place,” said Ross MacPhee, curator in the Museum’s Department of Mammalogy and one of the authors of the study, which is published today in the journal Nature. “This is what puzzled Darwin and his collaborator Richard Owen so much in the early 19th century.”

When the researchers turned to genetic analysis to map evolutionary relationships, they were confronted with a new problem: ancient DNA extracted from fossils did not survive in these specimens. That’s because the warm, wet conditions typical of South America destroy the DNA molecule. Instead, the team drew on another source of genetic information: collagen, a structural protein found in all animal bones that can survive for a million years or more in a wide range of conditions. 

The chemical structure of the amino acids that make up proteins like collagen is dictated by specific coding sequences in the organism’s DNA. Thanks to that relationship, amino acid compositions of the same protein in different species can be compared, providing insight into how closely the species are related. The authors analyzed collagen samples from 48 fossils of Toxodon platensis and Macrauchenia patachonica, the same species whose remains mystified Darwin when he discovered them in Uruguay and Argentina.

Macrauchenia patachonica
The South American native ungulate Macrauchenia patachonica may have had a mobile proboscis, as pictured here.
Illustration © Peter Schouten

Using this data, researchers were able to show that the closest living relatives of these species were the perissodactyls, the group that includes horses, rhinos, and tapirs, which makes ungulates part of Laurasiatheria, one of the major groups of placental mammals.

The molecular evidence corroborates a view held by some leading paleontologists that the ancestors of these South American ungulates came from North America more than 60 million years ago, probably just after the mass extinction that killed off non-avian dinosaurs and many other vertebrates. 

 For more information, see the Museum’s press release.