The Caribbean is typically thought of as a lovely spring break destination. If you are an animal lover, the area is great for diving and birding, but there are not many land mammals to be found. Sure, you will find bats, some endemic rodents, and of course invasive cats and rats, but besides that you will not find anything larger, like, oh, a sloth.
That’s right, up until a few thousand years ago, sloths weren’t just a South American thing. There were also primates, specifically New World monkeys like the Jamaican monkey, on islands throughout the Greater Antilles. The Greater Antilles consist of Cuba, Hispaniola, Puerto Rico, Cayman Islands, and Jamaica.
This now extinct terrestrial fauna of the Greater Antilles still remains very mysterious for a variety of reasons. As previously mentioned, only a few species of endemic rodents remain on Caribbean islands. The two biggest questions in this evolutionary conundrum are how did the mammals arrive? And then, how did they go extinct? The arrival debate is probably the most contentious- if we know the ancestors of extinct Antillean sloths, primates, and rodents originated in South America, how did the end up on islands in the middle of the Caribbean Sea? Did they all come from South America? It is possible, but seemingly unlikely, these different groups of mammals came in a series of overwater dispersals. A sloth floating its way to Jamaica may seem unbelievable, but overwater dispersals can be viable migration methods.
There is another theory though—perhaps there was a late Eocene- early Oligocene land bridge that connected the Greater Antillean Ridge and South America via the Aves Ridge. Welcome to:
GAARlandia was proposed by Iturralde-Vincent and MacPhee in 1999 as a way to explain the dispersal of land mammals from South America to the Caribbean islands in one continuous event, rather than a series of random overwater dispersals. Phylogenetic evidence has both supported and refuted the potential for the existence of GAARlandia, so the debate can still continue.
This month, in the Journal of Vertebrate Paleotology, Vélez-Juarbe et al. describe rodent incisors from Puerto Rico that date to the Oligocene- making them the earliest rodent fossils in the Caribbean. They are just incisors, so we cannot learn too much about the actual animal they belonged to (besides the fact their enamel structure indicates they belonged to a caviomorph rodent), but the existence of this fossil pushes back the date of rodent arrival in the Greater Antilles by approximately 9 million years. This new date is consistent with other molecular divergence estimations of caviomorph rodent groups in South America. In addition, a dated molecular phylogeny indicates there was a split between a Greater Antillean toad and its sister taxon in South America during the late Eocene- early Oligocene. The coincidence between molecular divergence dates, paleontological finds, and the hypothesized date of the land-bridge adds support to the idea there may have been a short lived sub-aerial land bridge bringing non-flying mammals to the Greater Antilles. Of course, this new evidence still does not preclude the possibility of overwater dispersal! These biogeographic hypotheses are very difficult to disprove so the debate rages on.
So where have the Caribbean monkeys and sloths gone? The Caribbean mammalian fauna experienced extremely high rates of extinction during the Quaternary, and these extinctions seemed to occur both before and after humans were a factor in the environments. During the Holocene, climate change could have potentially altered the environments these mammals were living in and this caused their extinction. But with the arrival of humans, overhunting, habitat destruction, disease, and introduction of invasive predators could also have been extremely damaging to biodiversity. The zooarchaeological and paleontological records on these islands can be spotty, but much more remains to be done to figure out the complex history of the ancient life of the Caribbean. Ancient DNA, stable isotopes, and re-examination of the taxonomy of fossils and sub-fossils from the Caribbean will continue to inform biogeographic theory and help us understand what drove such a unique mammalian biota to extinction.
Iturralde-Vincent, M. A., R. MacPhee. 1999. Paleogeography of the Caribbean region: implications for Cenozoic biogeography. Bulletin of the American Museum of Natural History 238:1-95.
Velez-Juarbe, J., T. Martin, R. D. E. MacPhee, D. Ortega-Ariza. 2014. The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico. Journal of Vertebrate Paleontology 34:157-163.