Mystery of ‘strange’ South American mammals solved

Brett Smith for – @ParkstBrett

Based on remains found by Charles Darwin and others, we knew that a group of mammals known as South American ungulates had a body that resembled a camel, nostrils high on their heads and even short elephant-like trunks.

Now, a new DNA analysis has just revealed that these strange animals are closely related to horses, not elephants and other animals with ancient ties to Africa, according to a new study in the journal Nature.

“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 study author Ross MacPhee, a curator in the American Museum of Natural History’s Department of Mammalogy. “This is what puzzled Darwin and his collaborator Richard Owen so much in the early 19th century. With all of these conflicting signals, they couldn’t say whether these ungulates were related to giant rodents, or elephants, or camels–or what have you.”

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To reach their conclusion, the study team analyzed DNA from collagen, which can survive for a million years or more in the hot ecosystems of South America. The chemical framework of the amino acids that makeup collagen is determined by particular coding sequences in the organism’s DNA. Due to this crucial relationship, amino acid compositions of the same protein in various species can be analyzed, offering clues about how tightly species are related.

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“People have been successful in retrieving collagen sequences from specimens dating up to 4 million years old, and this is just the start,” said co-author Matthew Collins, a biomolecular archaeology expert at the University of York in the United Kingdom.

The genetic analyses were conducted on 48 fossils of Toxodon platensis and Macrauchenia patachonica, both of which were discovered by Darwin 180 years ago in South America. The scientists were able to find that the closest living relatives of these species were the perissodactyls, the taxonomic group that includes horses and rhinos.

Toxodon platensis

Analysis of collagen preserved in fossils reveals that the unusual native ungulates of South America (such as Toxodon platensis, seen here) were more closely related to perissodactyls (horses and their allies) than to other living placental mammals. Rhino-sized Toxodon is estimated to have weighed about 1,500 kilograms (3,300 pounds). (Credit: © Peter Schouten)

This molecular data confirms a view held by some paleontologists: The ancestors of these animals came from North America over 60 million years ago, possibly after the mass extinction that wiped out the dinosaurs and many other vertebrates.

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“This is a definite possibility,” MacPhee said, “and we are now working with our South American colleagues to sample fossils that might settle once and for all where these magnificent beasts came from.”


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