Marine Vertebrate Taphonomy – Purisima Formation, Northern California

taphonomy“Taphonomy” literally means “the laws of burial” and is the study of how dead animals and plants are preserved within the fossil record. The subject is historically underappreciated but integral to interpreting certain long term trends in the fossil record; any paleontologic data involving the relative or absolute number of particular fossils and their distribution (geographic, stratigraphic, or lithologic) have all been affected by the processes of burial – and typically to an unknown (and in some cases, perhaps unknowable) degree.

Studies of taphonomy are typically either historical (analyzing/interpreting fossils) or actualistic experiments of decay, disarticulation, and burial (often having considerable overlap with forensic research). Considerable research effort has focused on terrestrial vertebrates (given how easy it is to monitor a carcass or other experiment that can stay put over a certain period of time) and marine invertebrates (given how abundant their fossils are in the rock record). The study of vertebrate taphonomy in marine settings has only attracted modest interest, unfortunately leaving those of us who study marine vertebrate paleontology with few tools to interpret their preservation. Available studies of terrestrial vertebrate decomposition are not helpful, given that 1) terrestrial and marine settings host completely different assemblages of flesh-eating critters (insects, birds, mammalian carnivores v. crustaceans, sharks, bony fish), 2) soft tissues do not desiccate in subaqueous settings, 3) currents and more rapid scavenging in marine settings can completely disarticulate a vertebrate skeleton in a matter of hours rather than weeks, and 4) differences in the lateral heterogeneity between marine and nonmarine settings. Studies of marine invertebrate preservation are also not helpful owing to differences in 1) skeletal mineralogy, 2) body size and life habits, and 3) anatomy (e.g. multielement endoskeleton rather than exoskeleton, valves, etc.). These problems highlight a need for study of factors that affect vertebrate preservation in marine settings.

I used these issues to frame the large, deep hole of knowledge I would attempt to fill in by studying the preservation of marine vertebrates from the upper Miocene-Pliocene (~7-3 myo) Purisima Formation of Northern California for my master’s thesis at Montana State University (completed 2011). A decent collection (n=1000+) vertebrate specimens from this unit now exist within several museum collections. Crucially, several groups of fossil vertebrates (sharks, bony fish, sea birds, pinnipeds, cetaceans, sirenians) occur within a variety of different depositional settings of the continental shelf, permitting analysis of 1) preservational trends across the continental shelf and 2) comparison of preservation between different groups. I recorded some basic semi-quantitative taphonomic data for every specimen from the Santa Cruz section of the Purisima Formation, scoring specimens for articulation, fragmentation, abrasion, and polish. I also devised a new phosphatization scale which tracks both bone permineralization (based on bone color) and the development of adhering phosphatic nodules.Phosphatization (upper left) and articulation (bottom) scales and preservation examplesPhosphatization (upper left) and articulation (bottom) scales and preservation examples.

Analysis of this data in concert with study of Purisima Formation stratigraphy, sedimentology, and bonebeds yielded a host of related observations and interpretations – I’ll summarize some of the basics here:

-Bonebeds correspond to parasequence boundaries within the Purisima Formation, and therefore were formed during periods of rising sea level (e.g. transgressive lag model in sequence stratigraphy). Bones and teeth within bonebeds were typically the most modified (e.g. damaged).

-A strong correlation between increasing inferred water depth and better preservation exists (e.g. higher articulation, lower bone/tooth damage in deeper settings). This is due to weaker and less frequent current disturbances in successively deeper settings.

-Strong differences in preservaton exist between different taxa and even tissue types. Teeth and whale earbones are less often damaged than regular bones or calcified cartilage. Curiously, phosphatization affects different taxa in different ways; bird bones are rarely phosphatized whereas bony fish are commonly phosphatized with attached nodules, and shark teeth & cartilage are often phosphatized but almost never include attached nodules.

-Bioturbating invertebrates can rework fossils up to three meters down-section (reworking via erosion typically works in the opposite way: older fossils are mixed into younger sediments, rather than younger specimens downward into older sediments).

-Bonebed “architecture” in cross-sectional exposures can be highly informative towards interpreting depositional history but can be completely altered and overprinted (e.g., removing sedimentologic evidence like erosional scours) by bioturbating invertebrates. Other evidence of reworking/submarine erosion (e.g. rip-up clasts, phosphatic nodules) are needed to properly interpret bonebed genesis.

-Biogenic bone modifications (e.g. tooth scrapes, punctures, invertebrate feeding traces) are relatively rare in deposits like the Purisima Formation (<2% of all specimens).

taphofacies modelTaphofacies model of the Purisima shelf.

This study has now been published in the open access journal PLoS One and is freely available here:

Boessenecker, R. W., Perry, F.A., and J.G. Schmitt. 2014. Comparative taphonomy, taphofacies, and bonebeds of the Mio-Pliocene Purisima Formation, central California: strong physical control on marine vertebrate preservation in shallow marine settings. PLOS One 9:3:e91419.