The mystery of the evolutionary history of a highly unusual Caribbean monkey has finally been solved. Xenothrix, a long-extinct primate, was so unlike any other living monkey that no-one could agree what it was related to or how it evolved. Thanks to DNA analysis we now know that it is most closely related to South America's titi monkeys (Callicebinae).
The breakthrough came from of a team of experts from international conservation charity The Zoological Society of London (ZSL), London's Natural History Museum, and the American Museum of Natural History in New York. They were able to obtain a DNA sample from fossils uncovered in a Jamaican cave. This is the first time that DNA has been successfully extracted from an extinct Caribbean primate, giving important insights into the evolution of this creature. They were also able to demonstrate that monkeys must have colonized the Caribbean more than once given the date of the split between Xenothrix and other titi monkeys.
"Recovering DNA from the bones of extinct animals has become increasingly commonplace in the last few years," says Professor Ian Barnes, who runs the National History Museum's ancient DNA lab and who co-authored the paper. "However, it's still difficult with tropical specimens, where the temperature and humidity destroy DNA very quickly. I'm delighted that we've been able to extract DNA from these samples and resolve the complex history of the primates of the Caribbean."
Xenothrix bones were first found in Long Mile Cave, Jamaica, in 1920. The leg bones were almost rodent-like. It was thought to have been a slow moving tree-dweller with relatively few teeth. With no other monkey sharing these characteristics, there has been much dispute in the intervening years as to where exactly it belongs in the evolutionary tree.
"Ancient DNA indicates that the Jamaican monkey is really just a titi monkey with some unusual morphological features, not a wholly distinct branch of New World monkey," says Ross MacPhee of the American Museum of Natural History's Mammalogy Department, a co-author of the study.
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It is believed that Xenothrix arrived in the Caribbean around 11 million years ago. Their journey from South America is thought to have happened on rafts of vegetation drifting across the ocean. Once there, with few predators, they were able to evolve in a unique way.
"This new understanding of the evolutionary history of Xenothrix shows that evolution can take unexpected paths when animals colonize islands and are exposed to new environments," Professor Samuel Turvey from ZSL, a co-author on the paper, adds. "However, the extinction of Xenothrix, which evolved on an island without any native mammal predators, highlights the great vulnerability of unique island biodiversity in the face of human impacts."
One mystery that remains unsolved is what the Xenothrix would have looked like. If South America's titi monkeys of today are anything to go by, they might have been small tree-dwelling monkeys, with a long furry coat of brown or black, or even red or gray. Wild titi monkeys have a lifespan of around 12 years.
Radiocarbon dating from the last known Xenothrix in the fossil record suggests that it died out about 900 years ago, the precise cause of which is up for debate. One theory is that it could be related to the fact that the Caribbean has experienced an unparalleled rate of mammal extinction, most likely caused by human impact, and by invasive predators introduced by early settlers.
A paper detailing the research has been published in the journal Proceedings of the National Academy of Sciences.
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