The San Sebastián Formation is a geologic formation in Puerto Rico. It preserves fossils dating back to the Oligoceneperiod.[1]
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 chinchilloidrodents and Eleutherodactylusfrogs.[2][3] In addition, taxa that are no longer known from the Caribbean, such as gavialidcrocodilians and geomyoid rodents, have also been recovered from the formation.[4]