On the trail of West Indian rodents

700 kilos of soil to be sifted and examined 5 grams at a time. That's 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 how the first land mammals arrived on the Caribbean islands.

"Today, this question is one of the thorniest mysteries in the natural sciences," stresses the paleontologist fromMontpellier's Institut des sciences de l'évolution (Isem*). One mystery hides another: how did these islands form geologically? " We've already defined the main patterns, but the contribution of paleontology enables us to refine the scenarios," explains Philippe Münch, from the Géosciences Montpellier laboratory**.

Accompanied byIsem paleontologist Laurent Marivaux (CNRS), the two UM professors and researchers assembled an international team in Puerto Rico in February 2019, where a fossil rodent incisor, dated to around 30 million years ago, had already been unearthed in the early 2010s. Although not very informative, this small tooth revealed the indisputable presence of these animals in the Greater Antilles at this very remote time... But what type of rodent?

The answer appeared in the sieves: armed with patience, the scientists were lucky enough to discover three other teeth, molars, also dated to 30 million years ago. Rodents were indeed around in those distant times, but where did they come from? Most likely from the South American continent. " These are the oldest rodents known from the Caribbean islands, and are close cousins of a strictly South American group, the chinchilla," explains Laurent Marivaux.

A precious clue for geologists. For if these little beasts were able to make the journey, it's because there was a passageway... ". This means that at the time, there was a more or less continuous land route between the mainland and the islands, or a myriad of closer islands that would have enabled them to reach Puerto Rico and the rest of the Greater Antilles," explains Philippe Münch. 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. And the paleontologists are still digging deeper. " Puerto Rican rodents haven't given up all their secrets," confides Pierre-Olivier Antoine.

photos © Pierre-Olivier Antoine, Laurent Marivaux, Philippe Münch, Jorge Vélez-Juarbe

*Isem (UM - CNRS - IRD - EPHE)
**Géosciences Montpellier
(UM - CNRS - Université des Antilles) Marivaux, L., Vélez-Juarbe, J., Merzeraud, G., Pujos, F., Viñola López, L. W., Boivin, M., Santos-Mercado, H., Cruz, E. J., Grajales, A., Padilla, J., Vélez-Rosado, K. I., Philippon, M., Léticée, J.-L., Münch, P. & Antoine, P.-O. (2020). Early Oligocene chinchilloid caviomorphs from Puerto Rico and the initial rodent colonization of the West Indies. Proceedings of the Royal Society B.