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article imageScientists solve the puzzle of 'Darwin's strangest animals'

By Megan Hamilton     Mar 27, 2015 in Science
When Charles Darwin sailed the HMS Beagle to South America in the 1830's, he discovered fossils of bizarre mammals that defied classification and scientists have never been able to figure out where these creatures fit in the mammalian family tree.
Until now.
In South America, Darwin discovered fossils of several huge and strange beasts that just couldn't be easily classified. Two such fossils were Macrauchenia, which had an extremely long snout and otherwise looked like a humpless camel and Toxodon, which had teeth similar to a rodent's, and a hippo's head on a rhino's body. Darwin described the creature as "perhaps one of the strangest animals ever discovered."
Toxodon illustration by Robert Bruce Horsfall  1913.
Toxodon illustration by Robert Bruce Horsfall, 1913.
By Robert Bruce Horsfall [Public domain], via Wikimedia Commons
By studying the ancient collagen protein from the 12,000-year-old fossils, researchers now say they have solved the mystery, and they also believe that studying ancient proteins may well revolutionize the study of long-extinct species. It may help to reveal the secrets of fossils millions of years older than can be studied using DNA, Scientific American reports.
In the study, published in the journal Nature, researchers from the American Museum of Natural History (AMNH) studied fossil protein sequences, and they were able to look back in time 10 times further than possible with DNA, The International Business Times reports.
"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, one of the study's authors.
"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."
Ancient DNA from the extinct critters wouldn't have been of much use because it was destroyed in South America's warm, wet conditions.
As part of a group consisting of more than 250 mammals known as the South American ungulates, the animals lived on the continent for some 60 million years before becoming extinct about 12.000 years ago. Their fossil history is fragmentary, and that's added to some of the confusion over their ancestry. Researchers' inability to isolate DNA molecules from the ungulate fossils has also added to the problem, Scientific American reports.
Ian Barnes, a molecular evolutionary biologist at the Natural History Museum in London and bioarchaeologist Matthew Collins of the University of York, UK, teamed up with an international team of researchers to try another tactic by extracting collagen. Along with the fact that it survives well when DNA doesn't, the protein is a major structural component of bone.
"Compared to DNA, there's absolutely tons of it," Barnes says.
So the team started out by building a collagen family tree, and that laid out the collagen sequences of different mammals based on their familial relationship, Scientific American reports. The researchers extracted and sequenced collagen from tapirs, hippos, and aardvarks to enhance their picture. Having done so, they sequenced collagen from four extinct ungulate specimens from two museums in Argentina. Of these, two were Toxodon specimens that were about 12,000 years old and two Macrauchenia that couldn't be carbon-dated. Then the scientists compared the ancient proteins against their family tree.
The researchers discovered that Macrauchenia and Toxodon are descended from the condylarths, an ancient group of placental mammals. They are related to Perissodactyla — which includes tapirs, rhinos, and horses, among other creatures, IFL Science reports. It was previously thought that they were related to the Afrotheria, a group which includes Elephants, aardvarks, and manatees, but this isn't the case. In fact, the molecular evidence suggests that their ancestors came from North America more than 60 million years ago, after the mass extinction at the end of the Cretaceous period.
Proteins may very well be useful for studying recently extinct species that lived in hotter environments where DNA studies are difficult, Collins noted, per The International Business Times. They could be used to study what he called the "weird and wonderful" animals that lived during the late Pleistocene — from the dwarf elephants and enormous rodents on the Indonesian Island of Flores to Australia's giant lizards and kangaroos.
Proteomics, the study's authors write, may revolutionize systematics like that achieved by genomics, IFL Science reports. In a statement, Collins writes: "We now have the potential to address many more of these challenges and to explore the evolutionary process much further back in prehistory."
Note: In the video above, you can see a pride of hungry Smilodon put the moves on a herd of Macrauchenia.
More about Darwin's strangest animals, South america, Argentina, Charles Darwin, HMS Beagle
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