San Sebastián Limestone
The San Sebastián Formation is a geologic formation in Puerto Rico. It preserves fossils dating back to the Oligocene period.
It was primarily deposited as limestone in a marine environment, but some localities with a significant amount of terrestrial fauna appear to have been deposited in a deltaic environment. It contains some of the earliest fossils of terrestrial Caribbean vertebrates, including chinchilloid rodents and Eleutherodactylus frogs. In addition, taxa that are no longer known from the Caribbean, such as gavialid crocodilians and geomyoid rodents, have also been recovered from the formation.