In South America during the 1830s, Darwin stumbled upon the fossil remains of what he described as the “strangest animal[s] ever discovered.” Now, almost two centuries later, researchers studying ancient proteins have finally figured out the evolutionary backstory of this strange group collectively known as South American native ungulates. Their findings are published in Nature this week.
The South American native ungulates were a diverse group of recently extinct mammals with approximately 280 different genera. These evolutionarily cryptic animals included Macrauchenia (pictured above), a leggy, long-necked, long-snouted, humpless camel-looking animal, and Toxodon (pictured below), who had rodent-like teeth in a hippo-like head on rhino-like body. Most went extinct a few million years ago, and the remainders held on until the Late Pleistocene, which ended just 11,700 years ago. So, are they closer to elephants or horses or cows? Did they have one or multiple origins, and how long ago did they emerge? Previous studies based on morphology and DNA have been unconvincing and unsuccessful.
“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 from the American Museum of Natural History says in a news release. “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.”
Now, a huge international collaboration led by University of York’s Frido Welker turned to proteins—specifically, collagen. This common, structural protein found in connective tissue (like bones and cartilage) is particularly durable—surviving 10 times longer than DNA, which degrades quickly in warm, wet conditions. “Compared to DNA, there’s absolutely tons of it,” Ian Barnes from the Natural History Museum in London tells Nature News.
They screened 48 bone samples from Toxodon and Macrauchenia for phylogenetically informative protein sequences. Four of these specimens, unearthed in the 1800s, yielded 90 percent of the collagen sequence for both animals.
Macrauchenia and Toxodon, they found, are descended from an ancient group of placental mammals called the condylarths, and they’re a sister group to Perissodactyla—horses, tapirs, rhinos, and friends. They’re not, as it turns out, related to the group of animals with an African origin called Afrotheria, which includes elephants, aardvarks, and manatees. Furthermore, the molecular evidence indicates that their ancestors came from North America more than 60 million years ago, likely just after the mass extinction near the K-Pg (formerly K-T) boundary.
Proteomics, the authors write, may produce a revolution in systematics like that achieved by genomics. University of York’s Matthew Collins says in a statement: “We now have the potential to address many more of these challenges and to explore the evolutionary process much further back in prehistory.”
Illustrations by Peter Schouten from the forthcoming book "Biggest, Fiercest, Strangest" W. Norton Publishers (in production)