Darwin's 'Strangest Animals' Finally Placed on Family Tree

Macrauchenia was once thought to be related to a camel

(Image credit: Illustration by Peter Schouten from the forthcoming book "Biggest, Fiercest, Strangest" (W. Norton Publishers, in production))

The origins of two wacky beasts that Darwin dubbed the "strangest animals ever discovered" had remained a mystery for some 180 years. But now, researchers say they've pinned down the evolutionary history of the two odd creatures — an ancient horselike animal with a long snout, and a rhinoceros-shaped animal with a head like that of a hippopotamus.

The new study reveals that these ungulates (hooved animals) native to South America descended from an ancient group of mammals called the condylarths — a sister group to the perissodactyls, which includes horses, tapirs and rhinos.

Charles Darwin first collected the two species, in the genera Macrauchenia and Toxodon, during his South American voyage on a ship called the Beagle. He purchased fossils of the rhinolike Toxodon from a rancher in Uruguay for a few shillings, and collected fossils of Macrauchenia, which sported a muzzle that looked like an anteater's snout, in a sandy channel on the coast of southern Patagonia, said study co-author Duncan Porter, a professor emeritus of biological sciences at Virginia Tech. [What the Heck?! Images of Evolution's Extreme Oddities]

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"The problem is not a lack of fossils — there are thousands of South American-native ungulate fossils in museums in many countries — nor is it a lack of ideas and possible explanations," said Ross MacPhee, a curator in the Department of Mammalogy at the American Museum of Natural History in New York. "Rather, the fundamental problem is that South American-native ungulates display detailed resemblances to a whole range of non-South American groups."

So the researchers screened bone samples. Long after DNA has degraded, collagen — a particularly durable protein — can be recovered from bone. But it's still difficult to glean information from it. Of the 45 specimens the team sampled for collagen, only five revealed any protein sequence information. The researchers then compared this with collagen DNA from a wide range of living mammals and a few extinct ones.