In the Footsteps of Caribbean Rodents
Rodent and human teeth. An exclusive investigation conducted in Puerto Rico by a team of paleontologists, geologists, and biologists who are examining clues dating back 30 million years 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 that Pierre-Olivier Antoine and his fellow paleontologists have undertaken 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 have already outlined the broad patterns, but the contribution of paleontology allows us to refine the scenarios, particularly to pinpoint the timing,” explains Philippe Münch of the Montpellier Geosciences Laboratory. Accompanied by paleontologist Laurent Marivaux ofISEM, the two Professors an international team in Puerto Rico in February 2019 to search together for clues that would help them advance the 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 itself is nothing out of the ordinary, its analysis held many surprises. “It appears to belong to a line 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 South America to the Caribbean as early as that very distant period. That is when the team of detective-like researchers decided to head to the site. After sifting tirelessly, they were lucky enough to come across three more teeth—molars, which “are easier to identify with certainty than an incisor,” notes Pierre-Olivier Antoine. Their age?—30 million years. The theory of a very ancient presence of these rodents in the Caribbean is thus confirmed.
Land route
“These are the oldest known rodents from the Caribbean, and they are extinct close cousins of today’s chinchillas, viscachas, and pacaranas within the chinchilloids, a group of rodents found exclusively in South America,” explains Laurent Marivaux. A valuable clue for his fellow geologist. Because if these rodents were able to make the journey, it means there was a passage… “This implies that at that time there was a more or less continuous land bridge between the mainland and the islands—a land bridge or perhaps 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 clues to the existence of these ancient islands that have since disappeared.
A scenario that is becoming clearer as more fossils are discovered. Indeed, biologists atISEM, including Pierre-Henri Fabre, have shown—by studying the genetic variations of 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 passage,” explains Philippe Münch. These ancient islands must therefore have disappeared beneath the water and re-emerged on several occasions. A decidedly very complex geological history.
Well-kept secrets
Further evidence is expected to support these hypotheses soon: “We are awaiting 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 latest excavation campaign in February 2020.
And the investigation doesn’t stop there. Always on the lookout for new evidence, paleontologists continue to sift through the material. Among these tons of rock, other fossils are expected to reveal their mysteries soon. “Puerto Rican rodents haven’t given up all their secrets,” says Pierre-Olivier Antoine. Enough to keep paleontologists on their toes…
This research was supported by the French National Research Agency Research ANR) under theGAARAnti program(ANR-17-CE31-0009), led by Philippe Münch (Géosciences Montpellier, University of Montpellier) [INSU], withthe Institute of Evolutionary Sciences in Montpellier [INEE].
