![]() ![]() On the plus side, there are powerful east-to-west currents in the equatorial Atlantic and trade winds going in the same direction, thanks to the Coriolis effect and ultimately Earth’s rotation. For starters, why only two groups of animals? Or, could that imply a one-off event carrying only ancestral rodents and monkeys? It would need to be a special kind of raft: large enough to provide security against storm waves immune to waterlogging, and carrying substantial food. To suggest that the > 1500 km migration to the Americas of ancestral platyrrhine primates, or rodents for that matter, involved their being carried on drifting vegetation rafts obviously invites scepticism. But it is too deep to have emerged as a result of falls in sea level. The Walvis-Rio Grande Rise – a hotspot track – that spans the South Atlantic Ocean floor from Namibia to São Paulo in Brazil, has been the subject of some speculation since it is dotted with sea mounts and in places has micro-continental fragments. Unless, that is, there is a hitherto undiscovered land bridge. The only alternative is a sea trip across the mighty Atlantic. Island hopping across the far northern, narrowest part of the North Atlantic during the Eocene may have been possible, although many islands there could have been subject to intense volcanic activity, as is Iceland today. In any case the earliest known primate fossils from China are just 55 Ma old. Since 60 Ma years ago it would have been impossible for the ancestors of ‘New World’ rodents and primates simply to have walked there. The site of a recent fossil primate discovery in eastern Peru is marked by the yellow dot. World palaeogeography at the Eocene-Oligocene boundary. The South Atlantic had become a wide ocean long before that, beginning in the far south during the early Cretaceous Period (138 Ma), with the mid-Atlantic Ridge steadily propagating northwards thereafter. Complete isolation of the Americas dates from around 60 Ma ago, when the northernmost part of the North Atlantic began to open. ![]() By the late-Eocene the Americas were completely separated from Eurasia and Africa by the actively spreading Atlantic Ocean, then between 1500 to 2000 km wide. Both primates and rodents had been inhabiting other continents long before this, so it is certain that, somehow, members of the two groups must have migrated to become isolated in the Americas. ![]() Interestingly, they are predated by the earliest rodent remains by only a few million years (41 Ma). The earliest platyrrhine primates of the Americas date to around the Eocene-Oligocene boundary (34 Ma). The two monkey groups are genetically related, but their last common ancestor is estimated, using the ‘molecular clock’ approach, to have lived at least 31 Ma ago, in the Oligocene. There are other differences, such as the unique prehensile tails of many ‘New World’ monkeys. They are members of five families, collectively known as platyrrhine (‘flat-nosed’) primates, all having wide-spaced nostrils compared with the primates of the ‘Old World’. There are plenty of monkey species in South and Central America and in Mexico. This is one of the great mysteries of palaeontology.
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