When the father of evolution found the first fossilized example of what would become the genus Toxodon in South America, he called it the "strangest animal ever discovered." Charles Darwin's find had the curved tooth of a rodent, but also seemed to share features with hippos and sloths, among other things. To hedge their bets, he and his colleagues suggested that it could presumably be a relative of any number of these animals.
In a new study published Wednesday in Nature, the secrets of the group that included this and another of Darwin's strange finds are finally revealed.
According to analysis of the protein in the bones of Toxodon and Macrauchenia bones, these strange creatures -- one a bit like a toothy hippo, the other something of an elephant-nosed, flat-backed camel -- are most closely related to Perissodactyla, a group of animals that includes horses, tapirs, and rhinos.
"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," Ross MacPhee, one of the paper's authors and a curator in the American Museum of Natural History's Department of Mammalogy, said in a statement. "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."
Modern scientists have been similarly puzzled, because the bones of these creatures have lost too much DNA in the hot, wet conditions of South America.
"Over the past 180 years, people have tried to place them pretty much everywhere in the mammal family tree based on their physical features," study author Ian Barnes of the Natural History Museum in London told The Post.
Ancient DNA is Barnes's speciality, but he and his colleagues quickly found that they'd have more luck by analyzing proteins from the animals' bones instead. They looked specifically at collagen, a structural protein that's much heartier than DNA. Because proteins are made up of chemical structures dictated by DNA, protein analysis can show how different animals are related. Even with the bones far beyond the reach of DNA analysis, Barnes said, he and his team were able to recover 90 percent of the collagen's make-up.
After analyzing 48 fossils of Toxodon platensis and Macrauchenia patachonica, they finally gave the creatures a place in the mammalian family tree. While they're most closely related to Perissodactyla, they're quite distinct, having branched off from the lineage that produced animals we know today some 18 million years ago. The two species are also quite distinct from each other, and probably hadn't had a common ancestor for around 60 million years.
"It was pretty clear going in that no one had a really convincing idea of where these animals had come from, what they'd evolved from, or how they'd gotten to South America," Barnes said. But the new evidence supports theories that ancestors of the creatures came to North America around 60 million years ago, just before the dinosaurs died out. They must have made their way down to South America not long after.
Barnes hopes that this protein evidence will give scientists a solid starting point for studying the animals in-depth. Before, researchers blindly assigned attributes of the bones to one known group of animals or another. Now, he said, they can work in the opposite direction.
"Working from what we're reasonably certain about and going back into all that morphology could be really interesting," he said. "I'm hoping people will take this new information and use it to pinpoint what features really do group these animals together, and use similar living animals to reconstruct what the ancestral forms may have looked like."