EARLY COLONIZATION OF THE GREATER ANTILLES
Surrounded by ocean, the islands of the Greater Antilles presented an interesting question even for pre-Darwin biologists: How did non-flying animals - not bats, birds or winged insects - from the mainland colonize these islands? Two principal hypotheses offer explanations. The first is the “over-water dispersal” hypothesis, and the second is the “GAARlandia” hypothesis.
Over-water dispersal, or the “rafting” hypothesis. describes a series of accidental excursions from the South American mainland. You might be surprised that this idea is taken seriously by the scientific community. How do animals raft across the Caribbean Sea? If you imagine extinct primates cutting wood and building a raft - stop right there; this is not how it would have happened. Consider instead a simpler scenario.
If you have ever sat beside a river, you might have seen small accumulations of twigs and other debris floating by, and perhaps even saw a bird perched on a floating branch, or insects on a matte of moss. Now, consider the huge amounts of branches, tree trunks, vines and leaves that pile up at major river deltas, where a rushing river spreads out in the shallows just before it reaches the sea.
Massive piles of debris form in this way every year, all around the world. In tropical regions, these clumps of debris might occasionally be occupied by curious or hungry animals, looking through the mess for food to eat. In these same regions, the onset of the wet season can bring fast, violent floods that are known to wash huge amounts of debris into deeper waters, clearing the shallow river deltas again. This is the idea of how a “raft” might form and make its way out to sea – and if the conditions are right and an unlucky clutch of stowaways was caught on the debris when the floods came, you may have an epic journey at hand. But how did the animals end up on Hispaniola or other Greater Antilles islands? This is actually one of the simpler problems. An experiment was conducted to model the effect of ocean currents passing by the Northeastern coast of mainland South America. It showed that flotsam from South America would end up on the Greater Antilles, carried by a Southeast to Northwest current.
The second hypothesis for the colonization of the islands is the GAARlandia land bridge hypothesis. GAARlandia refers to an ancient landspan that bridged the many small gaps between the islands of the Lesser and Greater Antilles, connecting them with Northeastern South America. It may have provided a path for terrestrial animals and plants to colonize the rich, largely uninhabited landscapes of the Greater Antilles. This landspan still exists today, but much of it is submerged; it is called Aves Ridge. The Aves Ridge was above sea level near the beginning of the geological epoch known as the Oligocene, approximately 33 million years ago, when tectonic compression elevated land in the Caribbean and global ocean levels were lower.
More research on extinct and living Caribbean species can help refine these hypotheses and give priority to one or the other. But neither can explain all the colonizations by non-flying mammals. For example, a very early dispersal of North American hooved mammals and insectivores, possible around 55 million years ago or more, may have reached the Greater Antilles from the west, departing from a large continental shelf that is now submerged, the Nicaraguan Rise. Like many island chains, the Carribean islands have a complex history, which we are only now beginning to reconstruct using fossil evidence, for the first time. The fossils housed at the Museo del Hombre Dominicano are sure to shed light on crucially important parts of that story.
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