RECENT ADVANCES IN CARIBBEAN PALEONTOLOGY: A CASE STUDY OF OLIGOCENE FAUNA FROM THE SAN SEBASTIÁN FORMATION, PUERTO RICO

The arrival of terrestrial vertebrates in the Caribbean region has been a topic of great discussion, yet one that remains poorly understood. Recent paleontological discoveries in Puerto Rico have shed light on the vertebrate faunal composition of the West Indies. Here we review the latest progress pertaining to the fossil evidence obtained from the Oligocene deposits (late Chattian-early Rupelian; ~30-26.5 Ma) of the San Sebastian Formation along Rio Guatemala, San Sebastián, PR. Previously known vertebrate remains from the San Sebastian Fm. include cranial and postcranial material of the gavialoid Aktiogavialis puertoricensis, postcranial material of pelomedusoid turtles, the dugongid Priscosiren atlantica, osteichthyans (e.g. Balistidae) and chondrichthyans (e.g. myliobatoids and ginglymostomatids), as well as invertebrates, plant material (palynomorphs, leaves, seeds and wood), and trace fossils. More recent discoveries include a distal humerus belonging to the Antilles’ oldest anuran (Eleutherodactylus sp.) along with dental elements of new fossil rodent taxa. The morphology of the dental remains represents two distinct species of chinchilloid caviomorphs, the first one described as Borikenomys praecursor and a larger species not named yet. Phylogenetic analysis suggests these new chinchilloid caviomorph species have a close relation to the South American dinomyid chinchilloids, bringing, in turn, significant biogeographic implications. The possible subaerial exposure of the Aves Ridge around the Eocene-Oligocene transition (~34 Ma) might have resulted in a land bridge or group of closely-spaced islands connecting the Greater Antilles to northwestern South America (GAARlandia hypothesis), allowing an early dispersal of land mammals and other vertebrates in this region. Nonetheless, the unexpected discovery of Caribeomys merzeraudi, a geomyin geomorph rodent with North American affinities in Puerto Rico, reestablished the history of the vertebrate colonization in the West Indies. Because there were no subaerial land connections between North America and the Greater Antilles during the Paleogene, the arrival of geomorph rodents via an overwater dispersal event appears to be responsible for their colonization of the West Indies. These discoveries provide a rare glimpse into the evolutionary and biogeographical history of the terrestrial vertebrate faunas of the West Indies and paint a more complex history than previously hypothesized.