Comparative taphonomy and taphofacies analysis of marine vertebrates of the Neogene Purisima Formation, Central California

Abstract

The taphonomy of vertebrate skeletal remains in the marine realm is poorly understood, and the majority of previous studies have focused on single skeletons, lagerstatten, or bonebed genesis. Very few studies have attempted to document environmental gradients in preservation, and as such it has been difficult to establish a concrete taphonomic model for vertebrates in the shallow marine realm. The Neogene Purisima Formation of central California, a richly fossiliferous unit representing depositional settings from nearshore to offshore, offers a unique opportunity to examine preservational trends across these settings. Lithofacies analysis was conducted in order to place the vertebrate fossils within a proper sedimentologic and depositional environmental context. This study examined over 600 vertebrate fossils of elasmobranchs, osteichthyes, aves, pinnipeds, odontocetes, mysticetes, sirenians, and land mammals, and taphonomic data regarding abrasion, fragmentation, phosphatization, articulation, polish, and biogenic bone modification were recorded. These data were then used to compare both preservation of these multiple taxa within a single lithofacies and preservation of one taxon across lithofacies in order to document environmental gradients in preservation. Many of these taphonomic processes resulted in differential preservation between taxa, indicating strong preservational bias within the Purisima Formation. Varying levels of abrasion, fragmentation, phosphatization, and articulation were found to be strongly correlated with physical processes of sediment transport and sedimentation rate. These varying characteristics were used to delineate four taphofacies, which corresponded to inner, middle, and outer shelf settings, and bonebeds which cut across all taphofacies. Application of sequence stratigraphic methods shows a strong similarity between the taphofacies model and the sequence stratigraphic model. Bonebeds mark major discontinuities (sequence boundary, transgressive surface of erosion, marine flooding surfaces), while the packages of rock between these discontinuities consistently exhibit onshore-offshore changes in taphofacies, closely corresponding to onshore-offshore changes in lithofacies and inferred depositional setting. The strong physical control on marine vertebrate preservation and preservational bias within the Purisima Formation has implications for paleoecologic and paleobiologic studies of marine vertebrates.