The finding comes from the most complete evolutionary tree of the genus Crocodylus, featuring all but one of the living crocodile species. Evon Hekkala of Fordham University in New York and colleagues sequenced mitochondrial genomes of all 11 species, eight of which had not had their mitochondrial DNA sequenced before.
This revealed that all four American species are most closely related to the Nile crocodiles of east Africa, and must have split away roughly 7 million years ago, long after Africa and South America began drifting apart 130 million years ago. By 7 million years ago, over 2800 kilometres of ocean lay between the two continents.
Palaeontologists have long suspected that crocodiles swam the Atlantic, but Hekkala’s finding is “strong evidence in support of that scenario”, says Christopher Brochu of the University of Iowa in Iowa City.
Crocodiles are salt-tolerant and can go up to six months without eating. What’s more, females can carry viable sperm for several months after mating, which means a single female could have crossed the Atlantic and produced a litter on the other side. It’s unlikely that a single such event would have given rise to all American crocodiles. But Hekkala points out that animals that got lost at sea off the coast of Africa may well have been carried across on the westward-flowing equatorial currents.