On the trail of Caribbean rodents

Rodent teeth and humans. An exclusive investigation carried out in Puerto Rico by a team of paleontologists, geologists and biologists who are looking at 30 million-year-old clues to solve one of the great enigmas of science...

700 kilos of soil to be sifted and examined 5 grams by 5 grams. This was the scale of the project undertaken by Pierre-Olivier Antoine and his paleontological colleagues on the island of Puerto Rico. A painstaking task with an ambitious goal: to understand howthe first land mammals arrived in the Caribbean.

" Today, this question is one of the thorniest mysteries in the natural sciences "says the paleontologist fromMontpellier's Institut des sciences de l'évolution. One mystery hides another: how were these islands formed, geologically speaking? " We've already defined the main patterns, but the contribution of paleontology is enabling us to refine the scenarios, and in particular to specify the timing", explains Philippe Münch of the Montpellier Institute of Evolutionary Sciences.explains Philippe Münch, from the Géosciences Montpellier laboratory. Accompanied by paleontologist Laurent Marivaux fromIsemin February 2019, the two teacher-researchers brought together an international team in Puerto Rico to unearth the clues they needed to move the investigation forward...

- 30 million years

But why Puerto Rico?" A Puerto Rican paleontologist had published a stunning discovery there in 2014: a fossil rodent incisor ". While there was nothing exceptional about this discovery in itself, its analysis yielded many surprises. " It is thought 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 would have passed from the South American continent to the Caribbean as early as this very early period. That's when the team of detective researchers decided to visit the site. After relentless sifting, they were lucky enough to come across three more teeth, molars " easier to identify with certainty than incisors "says Pierre-Olivier Antoine. Age? - 30 million years. The scenario of a very ancient presence of these rodents in the Caribbean is thus confirmed.

Overland route

" These are the oldest known rodents in the Caribbean, and are close extinct cousins of today's chinchillas, viscaches and other pacaranas within the chinchilloids, a strictly South American group of rodents. "explains Laurent Marivaux. A valuable clue for his geologist colleague. For if these rodents were able to make the journey, it's because there was a passageway... ". This means that at that time there was a more or less continuous land route between the mainland and the islands, a land route or a myriad of closer islands that would have enabled them to reach Puerto Rico and the Greater Antilles". The team of geologists is therefore actively searching the islands and the bottom of the Caribbean Sea for clues to the existence of these ancient islands, which have now disappeared.

This scenario is becoming clearer as more and more fossils are discovered.Isembiologists,including Pierre-Henri Fabre, have shown by studying the genetic variations of these rodents that there may have been several waves of arrival in the Caribbean. " There may therefore have been several periods when continental surfaces allowed this passage," explains Philippe Münch. These ancient islands must therefore have disappeared underwater and re-emerged several times. A highly complex geological history.

Well-kept secrets

Another clue should soon reinforce these hypotheses: " we expect the results of ancient DNA analysis by the end of 2020. ". Not on these famous teeth, which are far too old to contain any usable DNA, but on more recent fossils of the descendants of these first settlers, discovered during the last campaign in February 2020.

And the investigation doesn't stop there. Always on the lookout for new evidence, paleontologists are constantly sifting. Among these tons of rock, other fossils should soon reveal their mysteries. " Puerto Rican rodents have not yet revealed all their secrets "says Pierre-Olivier Antoine. Enough to set paleontologists' teeth on edge...

This research was supported by the Agence Nationale de la Research (ANR), programme GAARAnti (ANR-17-CE31-0009), led by Philippe Münch (Geosciences MontpellierUniversity of Montpellier) [INSU], whose partner for palaeontology and biology is theMontpellier Institute of Evolutionary Sciences [INEE].