After the demise of the dinosaurs following an asteroid strike 66 million years ago, mammals became Earth's dominant land animals. But that does not mean they went unchallenged. In South America, for instance, nightmarish land-living crocs - cousins of today's crocodiles and alligators - became apex predators.
This lineage of terrestrial crocs, called sebecids, lasted longer than previously believed, according to researchers who described fossils recently unearthed in the Dominican Republic that reveal that the islands of the Caribbean served as an unexpected last refuge for these ferocious predators.
Until now, the most recent fossils of sebecids were found in Colombia and dated to about 10.5-12.5 million years ago. The Dominican Republic fossils date to about 5-7 million years ago. The largest of the sebecids reached roughly 20 feet (6 meters) long, though the partial remains from the Dominican Republic indicate an animal up to about 7 feet (2 meters) long.
"These were the type of predators that one thinks were from the dinosaur times," said Lazaro Vinola Lopez, a graduate student in paleontology at the University of Florida and lead author of the research published this week in the journal Proceedings of the Royal Society B.
Instead, Vinola Lopez said, sebecids were at the top of the food chain in South America during the age of mammals alongside terror birds, giant flightless birds up to about 10 feet (3 meters) tall with massive hooked beaks, and saber-toothed marsupials, counterparts to the saber-toothed cats of North America and elsewhere.
Various types of crocs have inhabited Earth dating back to the Triassic Period more than 200 million years ago. Most, like the ones alive today, lived a semiaquatic lifestyle. But some conquered the marine realm and others lived exclusively on land, like the sebecids.
The sebecids were built differently than the usual semiaquatic crocs.
They had longer legs and a more upright stance, capable of running quickly to chase down prey. They had a narrow and deep skull - superficially resembling that of a meat-eating dinosaur and much different from modern crocs that have a wider and shallower skull. And the teeth of sebecids were tall and narrow with fine serrations running along the edges for cutting through meat, also similar to carnivorous dinosaurs.
Like many other crocs, they had protective armor made of bony plates called scutes embedded in their skin.
The fossils found in the Dominican Republic in 2023 were a single tooth that closely resembled those of South American sebecids and two vertebrae with characteristics that enabled the researchers to definitively conclude that these remains belonged to a sebecid.
"It is amazing to think that these fast-moving, dinosaur-like terrestrial crocs with serrated teeth specialized for cutting meat survived in the Caribbean hunting sloths, rodents and whatever else was around up until just a few million years ago," said study co-author Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History at the University of Florida.
The researchers said that fossils of two teeth apparently from a sebecid dating to about 18 million years ago that were previously discovered in Cuba and a similar one dating to around 29 million years previously found in Puerto Rico suggest that this lineage was widespread in the islands of the West Indies.
But how did land-living crocs from South America manage to get there? The researchers said their presence on the islands is another clue indicating there may have been a pathway of temporary land bridges or a chain of islands that permitted land animals to travel from South America to the Caribbean around 32-35 million years ago.
"The distance between the islands and northern South America was significantly shorter than what it is today. This likely facilitated the dispersal of sebecids from South America," Vinola Lopez said. "This shows how important islands can be as a biodiversity museum, preserving the last members of some groups that have gone extinct everywhere."
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