After Crocs and Lemurs Went Extinct on the Mainland, Many Survived on Islands for Millions of Years
Isolation allows creatures to thrive as their relatives perish due to the threats present on much larger landmasses
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The crocodiles were the last of their lineage. Unlike their cousins that lurked in the swamps, these terrestrial carnivores trotted around on land, where they snapped at prey with deep skulls full of blade-like teeth. And while they had evolved from earlier crocs that trotted across prehistoric South America, such reptilian carnivores had gone extinct there. The island-dwelling crocodiles wandering around the shores of what’s now the Dominican Republic were the last remainders of a carnivorous legacy stretching back more than 60 million years, able to hang on in an island refuge buffered from changes on the mainland.
The crocs wandering the prehistoric Dominican Republic five million years ago were descended from carnivores that evolved back when dinosaurs like Tyrannosaurus rex still roamed the planet. Called sebecids, the reptiles were active hunters, evolving to be dino-like predators after the mass extinction that wiped out all dinosaurs except birds. During this time, South America was an island continent, and sebecids thrived there. As sea levels shifted with climate, though, some sebecids began to wander, walking to landmasses that then became isolated as sea levels rose.
Islands have often been celebrated as laboratories of evolution. Many birds that settled on islands lacking large predators, for example, have evolved to be larger and flightless. The lemurs and tenrecs of Madagascar, found nowhere else on earth, have proliferated into an array of different species among the island’s forests. But the supposed “splendid isolation” of islands is only part of the story. Islands can also act as refuges for forms of life that have gone extinct elsewhere, respites for creatures that swim, raft and even walk to the landmasses. The surprise crocs of the ancient Dominican Republic underscore how the relative seclusion of islands can be a life preserver for species that take unusual routes to get there.
Fun fact: Where did crocodiles come from?
Today’s alligators and crocodiles are the remaining members of a broader group of reptiles called pseudosuchians, which evolved over 235 million years ago, as the first dinosaurs were making their mark.The unexpected Caribbean land crocs are just the latest examples of ancient organisms finding refuge on islands. Studying fossils found in a roadcut along the Juan Pablo II highway in the Dominican Republic, University of Florida paleontologist Lázaro Viñola López and colleagues found two vertebrae and a knife-like tooth. The fossils were described this April in the journal Proceedings of the Royal Society B. Similar fossils had been found before from different time periods on Cuba and Puerto Rico, but they had not received much attention. “There was a doubt, because the teeth are not diagnostic enough on their own,” Viñola López says. But the discovery of identifiable bones and teeth in the Dominican Republic provided the missing connection. The additional fossils seemed likely to belong to sebecids, too, indicating that the crocs had left prehistoric South America at some point and ventured to what are now islands in the Caribbean.
Precisely how the sebecids made it from South America to the islands is still murky. But, based upon the ages of the island fossils and how the world was changing, the crocodiles may have walked. The croc fossil from Puerto Rico, for example, is about 33 million years old. At that time, geologists have proposed, there was a land connection or string of islands linking what are now Caribbean islands with what is now the coast of Venezuela. One of the critical connections, nicknamed GrANoLA, formed a link between today’s Greater and Lesser Antilles, Viñola López says. And while sebecids could have gone another way, the fact that they were terrestrial crocs suggests that they walked along these island connections. “There is always the possibility of overwater dispersal,” Viñola López says, “but it seems more unlikely given the terrestrial adaptation of the sebecids.”
However the crocodiles arrived, though, they settled in and survived for millions of years. As sea levels changed, however, the terrestrial connections to South America were cut off, and sebecid populations became isolated on ancient islands. When sebecids eventually became extinct on South America about ten million years ago, the new find from the Dominican Republic demonstrates, the island crocs persisted for at least another five million years. “I believe further fieldwork in the Caribbean will shed light into how sebecids and other groups arrived,” Viñola López says.
The wandering crocs are not the only examples of animals that were able to survive on islands far longer than on the mainland. The lemurs and tenrecs of Madagascar, for example, are descended from primates and insectivores that used to be much more widespread. The earliest lemurs evolved about 62 million years ago on mainland Africa, when Madagascar was already an island separated by the Mozambique Channel. Primates living along the western coast must have been swept up with rafts of vegetation and sent across the channel to the island, where they began to evolve in unique ways. Experts call this the “sweepstakes” hypothesis, in which repeated events like storms that send vegetation rafts out to sea lead species to disperse to new places where they start to proliferate. When lemurs went extinct elsewhere, they still thrived on Madagascar.
Rafts of vegetation may very well be the secret to how terrestrial creatures, at least, reach islands that would otherwise seem out of reach. Fossils indicate that monkeys living on prehistoric Africa’s western coast were inadvertently swept across the Atlantic at least three times during prehistory, including the ancestors of today’s capuchin, howler and spider monkeys. Reptiles, too, benefit from such strange fortune. Different anole lizard species have dispersed through Caribbean islands thanks to clinging to storm-tossed rafts, and paleontologists have recently uncovered evidence of a 5,000-mile journey that brought ancient iguanas from North America to Fiji.
Four different species of iguana live on the Pacific islands of Fiji and Tonga today. Zoologists have long been puzzled how they got there, so far from other iguana species. Perhaps, experts speculated, ancient iguanas were more widespread, and the ancestors of the Fiji iguanas died out, making the island species be the odd lizards out. But the new analysis published in PNAS by University of California, Berkeley, herpetologist Simon Scarpetta and colleagues suggests a different story.
The project grew out of Scarpetta’s broader research, which included looking at genomes of over 200 lizard species from around the world. “One of the first results that stuck out to me from my initial analyses was the Fijian iguanas were most closely related to North American desert iguanas,” Scarpetta says.
The closest relatives of the Fijian iguanas turned out to be desert species of the genus Dipsosaurus that live in the southwestern United States and Mexico. The two iguana groups last shared a common ancestor about 34 million years ago, around the same time the sebecids were walking to ancient Puerto Rico—and about the time that land was beginning to rise out of the sea where Fiji is today. The iguanas reached the island as soon as it existed, indicating that reptiles had likely been sent out for long sea journeys before. All that was missing was a shore for them to land on.
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The connection between the island and continental reptiles indicates that the ancestors of Fijian iguanas evolved on North America. This time, no vanished land bridge existed that could explain their journey. The prehistoric Pacific yawned wide at the time, meaning dispersal over the water is the only route consistent with the evidence. Much like the lemurs headed toward Madagascar or the anoles bobbing around the Caribbean, the iguanas must have rafted—but over a record-setting journey.
The trip would probably have taken months. Even today, the journey from California to Fiji can take a month by boat. But iguanas were likely pre-adapted to withstand such a trip. Rafts of vegetation often include entire trees, sometimes with fruit on them. The iguanas would have been surfing on their food source. The reptiles are also ectothermic and don’t have the stringent caloric requirements of a warm-blooded creature like a mammal, allowing them to get by on less. Their scales, too, would have added some protection from the sun and salt water. In fact, if the ancestral iguanas were anything like their desert cousins, Scarpetta notes, the iguanas would have already been used to harsh, sunbeaten conditions. “If you had to pick a vertebrate group to survive a rafting event across hundreds or thousands of miles of ocean and establish on arrival, iguanas are a great choice,” Scarpetta says.
The Fijian iguanas are not the only members of their family to have undergone such fantastic journeys. Of 45 iguana species, Scarpetta says, 34 are found on islands, and 28 show evidence of having evolved on the islands they live on. The lizards have traveled far and wide over water, landing on new beaches and evolving in new ways. The journeys can only happen by chance, events that must be harrowing for the lizards as the lucky ones reach unfamiliar shores and make themselves at home for millions of years.
