Athletic, crocodile-like reptiles with bladed teeth made their last stand in the Caribbean as recently as 4.5 million years ago.
New fossils unearthed in the Dominican Republic suggest the reptile group went extinct millions of years later than previously thought, researchers report April 30 in Proceedings B of the Royal Society. The findings also help paint an unexpected picture of ancient Caribbean ecosystems.
The Age of Reptiles came to a dramatic close 66 million years ago with the abrupt extinction of non-avian dinosaurs and other major reptile groups at the end of the Cretaceous Period. Mammals came into their own in the aftermath. In South America, things were more complicated, at least at the top of the food chain. There, reptiles called sebecids, relatives of crocodilians, survived the mass extinction and were top predators on the continent for tens of millions of years after dinosaurs went kaput, well into the current Cenozoic Era.
Unlike their modern alligator and crocodile cousins, sebecids had relatively long limbs positioned directly under their bodies and likely prowled for prey on land. Like carnivorous therapod dinosaurs, their heads were tall and narrow and their teeth were compressed and serrated. After an era of dominance alongside the towering terror birds, they vanished from South America around 10 million years ago.
“I really am sorry they’re gone, because they must have been totally awesome to see,” says Christopher Brochu, a vertebrate paleontologist at the University of Iowa in Iowa City who was not involved with the research.
Not surprisingly, sebecid fossils from after the end of the Cretaceous extinction have been found in South America and more limitedly in Europe. But mysterious, serrated teeth also kept popping up at fossil sites in the Caribbean: first in Cuba, then in Puerto Rico. Multiple crocodile-like groups have teeth like that, so their identity was unclear.
When Lazaro Viñola López, a vertebrate paleontologist then at the University of Florida in Gainesville, and his colleagues visited the Dominican Republic, they found fossil vertebrae along with another tooth in an exposed road cut.
“The vertebrae are the thing that really brings home that they’re sebecids,” says López, who is relocating to the Field Museum in Chicago. All crocodile relatives with such serrated teeth would also have had ball-and-socket joints between vertebrae. In sebecids, that part of the spinal anatomy is flattened.
It’s the first record of sebecids in the Caribbean, and the researchers think the teeth found on other islands could be from sebecids as well. Those teeth date back as far as 29 million years, by which point — according to the fossil record — other reptiles on nearby continents with similar teeth may have gone extinct.
The vertebrae and tooth from the Dominican Republic are much younger than all those fossils —between roughly 4 million and 7 million years old. The findings suggest that sebecids held on in the Caribbean long after their South American counterparts died out. These mysterious predators would also be the last of the notosuchians, a broader group of reptiles that first appeared in the Jurassic Period, says Jonathan Bloch, a vertebrate paleontologist at the University of Florida and research team member.
“It’s kind of mind-blowing, really,” says Bloch, “and in the fossil record on the islands of the Caribbean, after a glorious record extending back the whole Age of Dinosaurs into the Cenozoic.”
It’s possible that sebecids were incrementally squeezed into tropical latitudes by a changing, cooling climate over millions of years, says Bloch, their range eventually contracting to just the islands.
The sebecids could have reached the Caribbean through ancient, now-submerged land bridges or by rafting on vegetation. Back then, the islands hosted many animals not found there today, such as species of gharials — crocodilians found today only in South Asia — large rodents, sloths and even monkeys.
The findings have implications for how researchers understand the evolutionary history of the Caribbean. For instance, there are many independent instances of various now-extinct birds evolving to be more ground dwelling or even fully flightless. Many researchers thought that there must not have been land predators on the region’s islands at the time if the birds could safely give up flight, says López.
The sebecid from the Dominican Republic may have measured about two meters long, and other species were known to be much larger — like Barinasuchus, one of the last sebecids from South America, which could have been 10 meters long and weighed nearly two metric tons. These powerful carnivores were probably extinct on the islands by the time the birds turned to flightlessness.
“[The findings] show that these [animals] were still important elements of past ecosystems until relatively recently,” says Pedro Godoy, a vertebrate paleontologist at the University of São Paulo in Brazil who was not involved with the study. “Which is always surprising because we naturally associate them with much older ages, before the late Cretaceous mass extinction event.”
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