In the Footsteps 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 kilos of soil to sift through and examine 5 grams at a time. That’s the scale of the project that Pierre-Olivier Antoine and his fellow paleontologists have undertaken on the island of Puerto Rico. It’s 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 by pinpointing the timing,” explains Philippe Münch of the Géosciences Montpellier laboratory. Accompanied by paleontologist Laurent Marivaux ofISEM, the two Professors 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 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 researcher-detectives decided to head to the site. After sifting tirelessly, they were lucky enough to come across 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 is thus confirmed.
Land route
“These are the oldest known rodents in the Caribbean, and they are close extinct cousins of today’s chinchillas, viscachas, and other pacaranas within the chinchilloids, a group of rodents found exclusively in South America,” explains Laurent Marivaux. This is a valuable clue for his fellow geologist. Because if these rodents were able to make the journey, it means there must have been a land bridge… “This means that at that time there was a more or less continuous land bridge between the mainland and the islands—either a single land bridge or 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 taking shape as more fossils are discovered. In fact, 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 when 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 reinforce 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 lookout for new evidence, paleontologists continue to sift through the material. Among these tons of rock, other fossils are sure to reveal their mysteries soon. “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 Research Agency Research 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 its partner for paleontology and biology.
