On the Trail of Caribbean Rodents
Teeth from Rodents and Humans. An exclusive investigation conducted in Puerto Rico by a team of paleontologists, geologists, and biologists who are examining 30-million-year-old clues to solve one of science’s greatest mysteries…
700 kilograms of soil to sift through and examine 5 grams at a time. That is the scale of the project undertaken by Pierre-Olivier Antoine and his fellow paleontologists on the island of Puerto Rico. It is a painstaking task with an ambitious goal: to understand how the first land mammals arrived in the Caribbean.
“This question is now one of the most challenging mysteries in the natural sciences,” notes the paleontologist fromthe Institute of Evolutionary Sciences in Montpellier. A mystery that hides another: how were these islands formed from a geological perspective? “We’ve already outlined the broad patterns, but paleontology allows us to refine the scenarios—particularly to pinpoint the timing,” explains Philippe Münch of the Géosciences Montpellier laboratory. Accompanied by paleontologist Laurent Marivaux ofISEM, the two Professors s assembled an international team in Puerto Rico in February 2019 to work together to uncover clues that would help them advance their investigation…
– 30 million years
But why Puerto Rico? “A Puerto Rican paleontologist had published a stunning discovery there in 2014: a fossilized rodent incisor.” While this discovery in and of itself is nothing out of the ordinary, its analysis held many surprises. “It appears to belong to a lineage of rodents originating in South America, and has been dated to nearly 30 million years ago!”
This would mean that the rodents in question had migrated from the South American continent to the Caribbean as early as that very distant period. That’s when the team of detective-like researchers decided to head to the site. After sifting tirelessly, they were fortunate enough to find three more teeth—molars, which, as Pierre-Olivier Antoine explains, “are easier to identify with certainty than an incisor.” Their age? Thirty million years. The theory of a very ancient presence of these rodents in the Caribbean has thus been confirmed.
Land route
“These are the oldest known rodents in the Caribbean, and they are extinct close cousins of today’s chinchillas, viscachas, and pacaranas—all members of the chinchilloids, a group of rodents found exclusively in South America,” explains Laurent Marivaux. This is a valuable clue for his fellow geologist. For if these rodents were able to make the journey, it’s clear that there was a land bridge… “This means that at that time there was a more or less continuous land bridge between the mainland and the islands—a land bridge or even a myriad of closer islands—that would have allowed them to reach Puerto Rico and the Greater Antilles.” The team of geologists is therefore actively searching the islands and the depths of the Caribbean Sea for evidence of the existence of these ancient islands that have since disappeared.
A scenario that is becoming clearer as more fossils are discovered. In fact, biologists atISEM, including Pierre-Henri Fabre, have shown—by studying the genetic variations in these rodents—that there were likely several waves of arrival in the Caribbean. “There must therefore have been several periods during which land bridges allowed this migration,” explains Philippe Münch. These ancient islands must therefore have submerged and reemerged several times. It is indeed a very complex geological history.
Well-Kept Secrets
Another piece of evidence should soon support these hypotheses: “We are expecting the results of ancient DNA analyses by the end of 2020.” Not from those famous teeth—which are far too old to yield usable DNA—but from more recent fossils of descendants of those early settlers, discovered during the last excavation campaign in February 2020.
And the investigation doesn’t stop there. Always on the hunt for new evidence, paleontologists continue to sift through the rock. Among these tons of rock, other fossils should soon reveal their mysteries. “Puerto Rican rodents haven’t given up all their secrets,” says Pierre-Olivier Antoine. That’s enough to keep paleontologists on their toes…
This research was supported by the French National Agency for Research on Research s (ANR), under theGAARAnti program(ANR-17-CE31-0009), led by Philippe Münch (Géosciences Montpellier, University of Montpellier) [INSU], withthe Montpellier Institute of Evolutionary Sciences [INEE] serving as the partner for paleontology and biology.
