Shark and ray fauna of the Tirabuzón Formation (Pliocene) in El Álamo Canyon, Sierra de Reforma - Aguajito, Baja California Sur, Mexico

Abstract

In this paper we present the results of a stratigraphic and paleontological study of new fossil localities found in the La Reforma and El Aguajito Calderas, Baja California Sur, Mexico, approximately 30 km north of the town of Santa Rosalía. The studied sedimentary succession is part of the substrate of the Pleistocene Calderas and is related to the Tirabuzón Formation (Pliocene) of the Santa Rosalía basin. During the Pliocene-Pleistocene the boundary of the Santa Rosalía basin extended beyond these volcanic structures and therefore the El Álamo canyon site recorded stratigraphic and paleontological evidences of the Tirabuzon Formation with abundant shark teeth and ray fossils and interbedded volcanic deposits. Shark fauna had previously been reported for the Tirabuzón Formation, to the south of the study area, but not at the El Álamo canyon. The same lithology and fossil assemblage was found in other locations: Corkscrew Hill, Santa María, Cueva Amarilla, El Morro Prieto and El Gringo. The studied material consists of 500 individual specimens of elasmobranch fossil teeth (macroteeth). We report 17 species and two genera of sharks, one species, and six genera of rays and, for the first time, a Batoid fauna for the Tirabuzón Formation. Discordantly on top of the Cimarrón siltstone appears the 2.4 Ma Cueva Amarilla ignimbrite of the Inferno Formation. The faunal assembly corresponds to a shallow and warm marine environment, similar to the current fauna of the Eastern Tropical Pacific.