Flemingostrea elegans, n. sp.: guide fossil to marine, lower Coniacian (Upper Cretaceous) strata of central New Mexico

Abstract

The marine oyster Flemingostrea elegans, n. sp., appears suddenly in lower Coniacian (Upper Cretaceous) strata of central New Mexico. It has no immediate ancestor in the Western Interior of the United States and has not been found anywhere outside central New Mexico. Flemingostrea elegans occurs in nearshore sandstones in the Mulatto Tongue of the Mancos Shale in Socorro County and the Gallup Sandstone of Lincoln County. This medium-sized oyster, with its distinctive terebratuloid fold, is an excellent guide fossil to the lower Coniacian, and is a great aid in distinguishing the Mulatto Tongue from other tongues of the Mancos Shale in Socorro County and in differentiating Coniacian from Turonian sandstones in Lincoln County. It occurs in great numbers, often as articulated shells, and is easily distinguished from all other Turonian through Coniacian oyster species by the fold in its lower valve. Its presence above coal beds in the lower part of the Crevasse Canyon Formation provides definitive evidence for a third cycle of transgression/regression of the western shoreline of the Late Cretaceous Seaway as far south in New Mexico as central Socorro County. The Santonian dwarf species, Flemingostrea nanus (Johnson 1903), known only from Santa Fe County, New Mexico, is redescribed and illustrated. Flemingostrea nanus, F. elegans, n. sp., and the upper Cenomanian F. prudentia (White 1877) are the only species of Flemingostrea known from the Western Interior. Ostrea elegantula Newberry 1876, which has been confused in the literature with F. elegans, n. sp., should be considered formally as a nomen oblitum (a forgotten name) and not used again. Ostrea elegantula was named but not described or illustrated by J. S. Newberry in his geological report of Captain J. N. Macomb’s 1859 San Juan exploring expedition. F. B. Meek, who wrote the paleontological report on the Cretaceous fossils collected on that expedition, did not describe, illustrate, or mention it. Newberry’s type specimens were illustrated in 1883 by C. A. White, again without description. Attempts to recover Newberry’s type locality along the Canadian River, Colfax County, New Mexico, were unsuccessful.