Comparative Taphonomy, Taphofacies, and Bonebeds of the Mio-Pliocene Purisima Formation, Central California: Strong Physical Control on Marine Vertebrate Preservation in Shallow Marine Settings

Abstract

BackgroundTaphonomic study of marine vertebrate remains has traditionally focused on single skeletons, lagerstätten, or bonebed genesis with few attempts to document environmental gradients in preservation. As such, establishment of a concrete taphonomic model for shallow marine vertebrate assemblages is lacking. The Neogene Purisima Formation of Northern California, a richly fossiliferous unit recording nearshore to offshore depositional settings, offers a unique opportunity to examine preservational trends across these settings.Methodology/Principal FindingsLithofacies analysis was conducted to place vertebrate fossils within a hydrodynamic and depositional environmental context. Taphonomic data including abrasion, fragmentation, phosphatization, articulation, polish, and biogenic bone modification were recorded for over 1000 vertebrate fossils of sharks, bony fish, birds, pinnipeds, odontocetes, mysticetes, sirenians, and land mammals. These data were used to compare both preservation of multiple taxa within a single lithofacies and preservation of individual taxa across lithofacies to document environmental gradients in preservation. Differential preservation between taxa indicates strong preservational bias within the Purisima Formation. Varying levels of abrasion, fragmentation, phosphatization, and articulation are strongly correlative with physical processes of sediment transport and sedimentation rate. Preservational characteristics were used to delineate four taphofacies corresponding to inner, middle, and outer shelf settings, and bonebeds. Application of sequence stratigraphic methods shows that bonebeds mark major stratigraphic discontinuities, while packages of rock between discontinuities consistently exhibit onshore-offshore changes in taphofacies.Conclusions/SignificanceChanges in vertebrate preservation and bonebed character between lithofacies closely correspond to onshore-offshore changes in depositional setting, indicating that the dominant control of preservation is exerted by physical processes. 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. Evidence of preservational bias among marine vertebrates suggests that careful consideration of taphonomic overprint must be undertaken before meaningful paleoecologic interpretations of shallow marine vertebrates is attempted.

A reappraisal of Cerebavis cenomanica (Aves, Ornithurae), from Melovatka, Russia

“…This pattern of attrition and abrasion is typical of reworked bone material in other phosphorite bone accumulations (Walsh & Martill, ; Boessenecker et al. ), suggesting the specimen was reworked prior to redeposition in a phosphatic lag deposit.…”

“…However, more severe attritional damage and rounding through abrasion is apparent in the caudal region of the skull (see below). This pattern of attrition and abrasion is typical of reworked bone material in other phosphorite bone accumulations (Walsh & Martill, 2006;Boessenecker et al 2014), suggesting the specimen was reworked prior to redeposition in a phosphatic lag deposit.…”

Multispecies Shark Feeding in the Trans-Saharan Seaway: Evidence From Late Cretaceous Dyrosaurid (Crocodyliformes) Fossils From Northeastern Mali

“…This helps to explain the abundance of decimeter-scale coprolites and other vertebrate fossils, while smaller fossils such as shark teeth were likely removed by wave currents. Boessenecker et al (2014) also reported a preservation bias against shark teeth in certain facies of the Purisima Formation of California, which includes phosphatic bonebeds of similar lithology and paleoenvironment to those at Mali-8.…”

Turtles From an Arkadelphia Formation—Midway Group Lag Deposit (Maastrichtian—Paleocene), Hot Spring County, Arkansas, USA

“…Prior models of lag deposit formation during the Late Cretaceous and Cenozoic of North America indicate that many concentrations of vertebrate fossils occur at the boundary between third order sea level cycles (e.g., [34][35][36][37][38][39][40][41][42][43]). These sea level cycles can be recognized in outcrop by disconformities and lag deposits that occur along geologic contacts between clay-dominated and sand or limestone dominated lithologies (e.g., [44][45][46]). Vertebrate fossils concentrated in these lags can differ on the order of several million years and represent organisms with different habitat preferences (e.g., [37,46,47]).…”

“…These sea level cycles can be recognized in outcrop by disconformities and lag deposits that occur along geologic contacts between clay-dominated and sand or limestone dominated lithologies (e.g., [44][45][46]). Vertebrate fossils concentrated in these lags can differ on the order of several million years and represent organisms with different habitat preferences (e.g., [37,46,47]).…”