FISH COPROLITES FROM THE NANJEMOY FORMATION, LOWER EOCENE, VIRGINIA, USA

The Eocene Nanjemoy Formation crops out in Maryland and on the Virginia Coastal Plain, along the eastern coast of the USA. This formation is composed of glauconitic sands, silts and clays and is divided into the Potapaco and Woodstock members. Fossils of bivalves, sharks, rays, actinopterygian fishes, reptiles, birds and mammals, and of fruits and seeds are common in the Potapaco Member. Here, we present the results of an analysis of more than 2000 vertebrate coprolites from the Potopaco Member at the Fisher/Sullivan Site in Virginia. The chemical composition (phosphatic), inclusions and morphology suggest that only carnivorous scats were preserved. The analyzed specimens were grouped into six morphotypes: (1) the Cylindrical morphotype is a cylinder with rounded ends; (2) the Segmented morphotype is a cylinder segmented with rounded ends, and on some one end is concave; (3) the Oval morphotype is a bean-like coprolite; (4) the Scroll morphotype is cylindrical to conical in lateral view and has coils seen only at the ends; (5) the Folded morphotype is a spiral that is concentrically folded; and (6) the Sinuous morphotype is serpentine with rounded ends. Different surface marks produced by coprophagy occur in different morphotypes, and represent both invertebrate burrows and bite traces made by fishes. The bitten coprolites were at least mouthed by exploring or foraging animals, probably by fishes that are known to be coprophagous. Several invertebrates are known to feed and/or dwell in feces habitually. Larvae of invertebrates are the probable producers of the burrows. All the morphotypes, except the Folded morphotype, contain undigested food material such as fish bones. The mineralogical and chemical analysis shows an early precipitation of phosphate and pyrite minerals, probably induced by the microbial community. All Nanjemoy coprolites were produced by fishes, and the Folded and Scroll morphotypes were made by chondrichthyans, Carcharhiniformes sharks producing the Scroll, and lamniform sharks, probably the genus Carcharias, produced the Folded morphotype. Oval, Cylindrical, Sinuous and Segmented morphotypes were produced by actinopterygian fishes. The surface marks and the lack of flatness on most coprolites indicate different taphonomic histories, suggesting early lithification.