FORMATION AND TAPHONOMY OF THE SHARKTOOTH HILL BONEBED, A UNIQUE ACCUMULATION FROM THE MID-MIOCENE OF CALIFORNIA

Since the 1850s, the Middle Miocene Sharktooth Hill (STH) bonebed, near Bakersfield, California, has produced an abundant and species-rich vertebrate assemblage (sharks, fish, turtles, birds, cetaceans, pinnipeds, terrestrial mammals) preserved in a matrix of silt to very fine sand. The STH bonebed is a narrow stratigraphic interval (~30 cm) with an outcrop over a minimum area of 100 km². It may be one of the greatest accumulations of marine vertebrates known in the fossil record, but its formation remains unresolved. Taphonomic data from fossils in the bonebed combined with sedimentologic/stratigraphic data allow us to evaluate different hypotheses about how such marine vertebrate accumulations form.

We collected taphonomic, taxonomic and ontogenetic data in the field and on a large collection of systematically quarried STH vertebrate specimens with minimal collection bias. Taxonomically, the bonebed is composed predominantly of disarticulated indeterminate mammal postcrania and shark teeth (48% and 44%, respectively). The mammals are mostly cetacean individuals of all ages. Many of the bones are abraded and/or broken, though some of the breakage is probably post-burial. Evidence of predation and/or scavenging and epi/endobionts is rare. Invertebrate body fossils are unknown from the bonebed, but the underlying mudstone is highly bioturbated. Total evidence indicates that the STH bonebed formed on a disconformity over a protracted period, likely representing time-averaging over a thousand to hundred-thousand year scale.

The bonebed accumulated shortly after the mid-Miocene climatic maximum near 16 Ma, during a sea-level regression when sediment bypassed a large area. Later, deposition of fine sand and silt preserved occasional articulated marine mammals (often in concretions) above the bonebed. The concentration, mode and extent of the STH bonebed make it a unique depositional situation unlike other known marine vertebrate occurrences, such as the Miocene Calvert, Monterey or Pisco Formations.