Artist’s impression of a 20-foot-long sebecid. Credit: Illustration by Jorge Machuky. CC BY.

The fossils of a giant carnivorous crocodile-like sebecid have been found in the Caribbean helping to explain a decades-old mystery.

Imagine a crocodile built like a greyhound — that’s a sebecid. Standing tall, with some species reaching 20 feet in length, they dominated South American landscapes after the extinction of dinosaurs until about 11 million years ago.

Or at least, that’s what palaeontologists thought, until they began finding strange, fossilised teeth in the Caribbean.

Jonathan Bloch, curator of vertebrate palaeontology at the Florida Museum of Natural History, said: “The first question that we had when these teeth were found in the Dominican Republic and on other islands in the Caribbean was: What are they?”

This initial confusion was warranted. Three decades ago, researchers uncovered two roughly 18-million-year-old teeth in Cuba.

With a tapered shape and small, sharp serrations specialised for tearing into meat, it unmistakenly belonged to a predator at the top of the food chain.

But for the longest time, scientists didn’t think such large, land-based predators ever existed in the Caribbean.

The mystery deepened when another tooth turned up in Puerto Rico, this one 29 million years old. The teeth alone weren’t enough to identify a specific animal, and the matter went unresolved.

That changed in early 2023, when a research team unearthed another fossilised tooth in the Dominican Republic — but this time, it was accompanied by two vertebrae. It wasn’t much to go on, but it was enough.

The fossils belonged to a sebecid, and the Caribbean, far from never having large, terrestrial predators, was a refuge for the last sebecid populations at least five million years after they went extinct everywhere else.

Last surviving members

The study’s lead author, Lazaro Viñola Lopez, conducted the research as a graduate student at the University of Florida.

He knew his team members had come upon something exceptional when they unearthed the fossils: “That emotion of finding the fossil and realising what it is, it’s indescribable.”

Sebecids were the last surviving members of the Notosuchia, a large and diverse group of extinct crocodilians with a fossil record that extends back into the age of dinosaurs.

They represented a wide range in size, diet and habitat and were notably different from their crocodile relatives, as most of them lived entirely on land.

The sebecids acted like carnivorous dinosaurs, sprinting after prey on their four long, agile limbs and tearing through flesh with their notorious teeth.

Some species could reach 20 feet in length and had protective armour made of bony plates embedded in their skin.

The mass extinction event 66 million years ago that wiped out non-avian dinosaurs nearly destroyed notosuchians as well.

In South America, only the sebecids endured and, with the dinosaurs gone, they quickly rose to be the apex predator.

Land bridges

The open sea separating the Caribbean islands and mainland South America would have posed a serious challenge for a terrestrial sebecid to swim across.

In finding the fossils, the research team revealed possible evidence in support of the GAARlandia hypothesis, which suggests a pathway of temporary land bridges or a chain of islands once allowed land animals to travel from South America to the Caribbean.

If, as scientists hypothesise, the serrated teeth discovered on other Caribbean islands also belonged to a sebecid, the history of these giant reptiles extends beyond the Dominican Republic.

They would have occupied and shaped the region’s ecosystems for millions of years, yet today you’d be hard-pressed to find evidence of the large terrestrial predators.

In their absence, smaller endemic predators like birds, snakes and crocodiles have evolved to fill the gap in the food chain.

Bloch said: “You wouldn’t have been able to predict this looking at the modern ecosystem.

“The presence of a large predator is really different than we imagined before, and it’s exciting to think about what might be discovered next in the Caribbean fossil record as we explore back further in time.”

Museums of biodiversity

This revelation aligns with similar observations ecologists have described worldwide.

Islands are known to act as ‘museums of biodiversity’, providing a haven that allows plants and animals to survive even after their related species have gone extinct on the mainland.

Although the tropics are among the most biodiverse places on Earth, much of their natural history remains a mystery.

That’s why, according to Bloch, they’re the most important — albeit challenging — regions for palaeontologists to study.

Historically, many palaeontologists in the Caribbean have excavated fossils from caves and blue holes, where large accumulations of remains are often found.

Caves can serve as shelter against harsh conditions for animals, and predatory birds like owls and hawks frequently bring their prey inside to eat, leaving behind pellets or discarded bones.

Blue holes preserve fossils exceptionally well, as they lack the oxygen that fuels decay.

But these locations only provide a narrow snapshot of past biodiversity because most of the fossils are relatively young. While these sites provide valuable insight into recent history, they have their limitations when it comes to older, less well-known fossils.

Renaissance

Today, Caribbean palaeontologists are taking a new approach.

Finding deep-time fossils often requires more effort and fortunate circumstances, but they’re willing to face the obstacles.

“This is like a renaissance,” said Viñola-Lopez, describing the renewed interest and excitement in the region.

Local scientists have the advantage of being able to react quickly when a potential fossil bed is discovered.

The dry, rocky landscapes that contain fossils are hard to come by in the Caribbean, where wind and rain erode outcrops and today’s forests cover fossil beds.

Finding sebecid fossils in the Dominican Republic site was possible because local work crews happened to be cutting a road directly through it.

Viñola-Lopez said: “Outcrops don’t last too long, so you go there when you can.

“When they’re cutting the road or a few months after that, you find the fossils.

“If you’re looking in a few years, it will be gone.”

The study is published in the Proceedings of the Royal Society B.