ELEMENTAL AND MINERALOGICAL CHARACTERIZATION OF CENOZOIC SHARK SKELETAL MATERIAL: IMPLICATION FOR QUANTIFYING DIAGENESIS

Fossilized vertebrate skeletal tissues have recently received considerable attention as geochemical archives of paleoecological and paleoenvironmental information. In these kinds of studies, fossil tooth enamel has been the preferred material for analysis. Tooth enamel has several advantages over bone. which make it less susceptible to diagenesis: (1) it is relatively non-porous; (2) has larger crystallites; (3) and has low organic content (consisting of >95% carbonate hydroxyapatite). Bone on the other hand, is porous, contains small crystallites, and has high organic content (about 25-30%), all of which make it more susceptible to diagenesis. However there are certain situations in which fossil bone is: (1) either the only skeletal material available for study (e.g., in those vertebrates that lack teeth, such as most birds), or (2) is preferred (over teeth) because certain skeletal elements archive incremental growth. The purpose of this study is to quantify diagenesis of shark vertebral centra through analysis of a suite of physical and chemical characters including crystallinty index, carbonate content, and major, minor, trace and rare earth (REE) elemental concentrations. Although shark skeletons are initially cartilaginous (a soft supporting tissue that does not fossilize), the vertebral centra replace the cartilage with carbonate hydroxyapatite during the growth of the individual. The sharks are all from the group known as the Lamnoidea, which includes the modern great white (Carcharodon carcharias), and five closely related extinct species from Eocene to Pleistocene age. The modern shark species is included in this study to provide an unaltered “end-member” in which initial physical parameters and elemental concentrations can be determined. Understanding diagenesis will determine if the geochemical signal preserved in fossils is the original biological signal, a diagenetic signal, or some combination of both.