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Jamaican Monkey - Xenothrix mcgregori
Jamaican Monkey - Xenothrix mcgregori
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Temporal range: Pleistocene-Holocene

Scientific Classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Pitheciidae
Subfamily: Callicebinae
Genus: †Xenothrix
Species: †X. mcgregori

The Jamaican monkey (Xenothrix mcgregori) is an extinct species of New World monkey first uncovered at Long Mile Cave in Jamaica by Harold Anthony in 1920. Anthony is responsible for many species descriptions of Caribbean taxa during this period and his field notes record the discovery of the monkey material:

“January 17 – Spent all day digging in the long mile cave and secured some good bones. The most important find was the lower jaw and femur of a small monkey, found in the yellow limestone detritus. It was not associated with the human remains but not so far from them that the animal must not be strongly suspected as an introduced species. It was deeper than any of the human bones by at least 10” to 1’…” (reproduced in Williams and Koopman, 1952)

The eventual species description was not completed until 1952 when two graduate students, Ernest Williams and Karl Koopman, found the associated femur and mandibular fragment forgotten in a drawer at the American Museum of Natural History. They remained circumspect in placing this primate taxonomically as it had shared characteristics with a number of platyrrhine taxa.

The small mandible has a dental formula of 2 incisors, 1 canine, 3 premolars and 2 molars – a departure from the vast majority of living platyrrhines, with the notable exception of the callitrichines. It is significantly larger than the living callitrichines, and work by Rosenberger has largely eliminated the possibility that these taxa share a close phylogenetic relationship. Rosenberger suggested that the absence of the third molar in Xenothrix was not homologous with this character state in callitrichines. He based his assessment on the length of the molars relative to the molar row, and the inferred retention of hypocones on M1-2, which have been greatly reduced in the marmosets and tamarins. He further suggested that Xenothrix shared a close phylogenetic affinity with Callicebus or Aotus. His conclusions were tentative due to the fragmentary nature of the material.
The postcranial remains discovered by Anthony in the 1920s were eventually described by MacPhee and Fleagle who attributed the femur, os coxae, and tibia to the order Primates. MacPhee and Fleagle stated that the primate postcrania bore little resemblance to modern forms, but they interpreted the femur as being indicative of slow climbing. The femur also shares some similarities with Potos flavus, the kinkajou. They provisionally accepted Hershkovitz’s family Xenotrichidae until further analysis could fully elucidate the relationships of Xenothrix.

Further research
In the 1990s, several expeditions to Jamaican cave sites resulted in the recovery of additional cranial and postcranial material attributed to Xenothrix, including a partial lower face containing the palate with left and right P4-M2, most of the maxilla and parts of the sphenoid. This discovery confirmed that the dental formula of this taxon is With the new partial face, Horovitz and MacPhee were able to further develop the hypothesis, first proposed by MacPhee et al., that all the Antillean monkeys (the others being the two Cuban monkey species of genus Paralouatta and Antillothrix bernensis of Hispaniola) belonged to a monophyletic group linked most closely with modern Callicebus.

Rosenberger has objected to this hypothesis and has suggested that Xenothrix was a Jamaican owl monkey, thus modifying his earlier view. He based his conclusions on the fairly large orbit size as inferred from the preserved orbital rim, large inferior orbital fissure, and the large I1 alveolus as compared to the I2 alveolus. These characters are shared with Aotus. MacPhee and Horovitz tested this alternative phylogeny with extensive anatomical comparisons and by extending their parsimony analysis using PAUP*. They maintained that the monophyly of the Antillean monkeys was still supported in the most parsimonious trees, but in slightly less parsimonious trees, Aotus does appear to be linked with Xenothrix. MacPhee and Horovitz assigned the Antillean monkeys to the tribe Xenotrichini – the sister group of the tribe Callicebini.
DNA analysis indicates that the species is a type of titi monkey, sister to the recently recognized northern South American genus Cheracebus, that colonized Jamaica around 11 million years ago.

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An extinct monkey evolved to live like a sloth in the Caribbean

By Michael Marshall

About 11 million years ago, monkeys somehow crossed the sea from South America to the island of Jamaica in the Caribbean. There they evolved into a new species that was unlike any other known monkey. It’s a striking example of how living on an island can transform a species. The details have been revealed by preserved DNA.

The first remains of Xenothrix mcgregori were discovered in Long Mile Cave, Jamaica in 1920. The few bones found reveal a highly unusual monkey, with relatively few teeth and leg bones similar to those of a rodent.

“What they suggest is a very slow-moving, perhaps even sloth-like lifestyle, which is perhaps not unexpected in an animal living on an island with few predators other than large birds,” says Ross MacPhee of the American Museum of Natural History in New York. Some extinct Madagascan lemurs also evolved a similar slow-moving lifestyle.

Ever since it was described in 1952, X. mcgregori has been an enigma. It was related to South American monkeys, but it was unclear which group it belonged to or when its ancestors reached Jamaica. Several suggestions had been made based on the bones, but the monkey was so unusual it was impossible to be sure. “It’s been all over the place,” says MacPhee.

To clear up the mystery, MacPhee and his colleagues obtained DNA from two preserved X. mcgregori bones. They recovered the entire mitochondrial genome – which animals only inherit from their mothers – and seven chunks of the nuclear genome.

The team compared these samples of DNA with the equivalent sequences from 15 different groups of South American primate. They found that X. mcgregori belonged to a group called the titi monkeys. These monkeys live in forests, eat fruit and do not have prehensile tails.

X. mcgregori does not look like a typical titi monkey, though, so arriving on the island evidently forced its ancestors to evolve. “The selective pressures on them must have been just extreme,” says MacPhee. “It looks like it got thrown into the mixer.” Island species often evolve rapidly, because there are few large predators but also little fresh water.

Xenothrix split from its closest South American relatives roughly 11 million years ago, suggesting that is when its ancestors reached Jamaica. They must have somehow crossed the sea, perhaps on a raft of vegetation.

Other primates were present in the Caribbean rather earlier, from about 18 million years ago. It seems several groups made the crossing at the different times, establishing a unique ecosystem. Unfortunately, most of the Caribbean’s native species died out when humans first arrived on the islands, so are only known from preserved remains.

X. mcgregori died out about 900 years ago. There is no hard evidence as to why. “What we think but cannot demonstrate is that Xenothrix, like hundreds of other species, was a victim of either direct or indirect impacts by the first humans who got there,” says MacPhee.

Journal reference: PNAS, DOI: 10.1073/pnas.1808603115
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