Taphonomy of a Middle Pennsylvanian marine vertebrate assemblage and an actualistic model for marine abrasion of teeth

The taphonomy of assemblages of disarticulated remains of marine vertebrates is not well studied. Examination of a Middle Pennsylvanian chondrichthyan assemblage from Kohl's Ranch, Naco Formation, central Arizona, contributes to knowledge of such assemblages and reveals a complex taphonomic history. This vertebrate assemblage is restricted to two thin horizons associated with a concentration of disarticulated and tightly packed invertebrates. The vertebrate specimens are associated with phosphatic internal molds of molluscs and bryozoans. Most vertebrate specimens show abrasion. Several lines of evidence suggest that the specimens were abraded in a nearshore wave environment and subsequently transported offshore by a storm surge, where they were incorporated into an environmentally condensed assemblage. In particular, the presence of abrasion, a concentrated skeletal assemblage, significant amounts of siliciclastic sand, presence of intraclasts, phosphatic molds, and a basal lithologic discontinuity support this hypothesis; similar characteristics are found in many bone beds in the fossil record. To test whether wave-dominated nearshore marine environments are capable of abrading vertebrate teeth, modern elasmobranch teeth (Odontaspis and Myliobatis) were placed in an experimentally simulated abrasive environment of fine siliciclastic sand. Results indicate that progressive degradation of specimens by abrasion and cracking occurs as the duration of abrasion increases. Abrasion occurs along the edges of both the crown and root, and cracking proliferates across the surface of the crown. These data confirm that wave-dominated marine environments can progressively abrade vertebrate teeth, and are consistent with the taphonomic hypothesis presented for the Kohl's Ranch vertebrate assemblage.