CHONDRICHTHYANS FROM THE LISBON–TALLAHATTA FORMATION CONTACT (MIDDLE EOCENE), CHOCTAW COUNTY, SILAS, ALABAMA

In Silas, Alabama, a disconformity with an overlaying lag deposit separates the (middle Eocene, Lutetian) Lisbon and Tallahatta Formations. This lag deposit consists of blankets and lenses of macrofossil residuum that preserves an unreported assemblage of chondrichthyans represented by at least 13 species. This chondrichthyan assemblage is similar to other contemporaneous nearshore faunas found across North America and elsewhere globally and includes: Striatolamia macrota Agassiz, 1843; Brachycarcharias lerichei Casier, 1946; Carcharodon auriculatus Blainville, 1818; Otodus obliquus Agassiz, 1843; Abdounia recticona (Winkler, 1873); Abdounia enniskilleni (White, 1956); Galeocerdo eaglesomei (White,1955); Physogaleus secundus (Winkler, 1874); Scyliorhinus sp.; Rhizoprionodon sp.; Pristis sp.; Myliobatis sp.; and Aetobatis sp. Additional macrofossils with nearshore affinities occurring in the Silas lag include bones and teeth from fish, reptiles, and marine mammals as well as mollusc shells. The accumulation and concentration of macrofossils in the Silas lag are the result of third order eustatic sea level fluctuation prior to late Eocene climatic cooling and global sea level regression associated with the formation of the continental ice sheet on Antarctica. The Silas assemblage supports prior studies that document evolutionary trends throughout the Cenozoic in which carcharhiniforms become the dominant order of chondrichthyans in the modern oceans.