Science Fiction Double Feature: Two new papers on the same day – a strange new fossil porpoise, and vertebrate taphonomy of the Purisima Formation

Yesterday saw the publication of two new papers: the first of which is about a new genus and species of bizarre porpoise from the Pliocene of California, and the second is the published version of my master’s thesis.

The first paper is a collaboration with Rachel Racicot, Brian Beatty, and Tom Deméré and finally describes the extinct odontocete informally known as the “skimmer” or “half-beaked” porpoise. The new fossil phocoenid is named Semirostrum ceruttii, the genus name referring to the dramatically shorter rostrum, and is also an homage to the half-beak fish, Hemiramphus. The species name is in honor of Richard Cerutti, longtime field paleontologist and preparatory for the San Diego Natural History Museum. Mr. Cerutti collected the holotype in the early 90’s from the Pliocene San Diego Formation – I met him during a 2012 visit to the SDNHM.

The holotype skull, earbone, and mandible of Semirostrum ceruttii, and a composite skeletal reconstruction from three specimens. From Racicot et al. (2014); skeletal reconstruction by yours truly.

The new fossil porpoise has a somewhat longer rostrum than modern phocoenids, and is slightly more delphinid-like than modern porpoises as well. Most obviously, Semirostrum has a bizarre lower jaw with an elongate, fused mandibular symphysis that is developed into a laterally flattened and expanded, paddle-shaped process that juts far beynd the edge of the rostrum. The teeth in the mandible do not extend past the rostrum, so the majority of the symphysis is edentulous. The preserved teeth have labial wear facets, which we interpret as being the result of substrate abrasion – the observed patterns of tooth wear differ from extant phocoenids in lacking apical wear facets. When articulated, the wear facets do not match up with occluding upper teeth – indicating that regular tooth wear does not account for the observed pattern. We hypothesize that the elongate mandibular symphysis is a benthic probe, and that Semirostrum pushed its “chin” through the substrate, with sediment streaming along the lateral sides of the toothrow – snatching up any burrowing prey that came into contact with the “chin” or rostrum. In my life restoration, I illustrated Semirostrum as using it’s mandibular symphysis like a benthic plough, ploughing through the uppermost layer of the sediment in its search for burrowing invertebrates. The type specimen consists of a complete skull and mandible with periotic, tympanic bulla, and postcrania – an absolutely gorgeous set of fossils which I first had the opportunity to examine on my first visit to the SDNHM back in 2007.

 

Life restoration of Semirostrum ceruttii, from Racicot et al. (2014) - by yours truly.

My contribution to the paper was describing fossil material of Semirostrum from the Purisima Formation. Although the holotype specimen of Semirostrum ceruttii is from the San Diego Formation, multiple specimens of Semirostrum have been collected from contemporaneous sections of the Purisima Formation. In fact, one of the earliest known specimens of Semirostrum – collected in the mid 1980s by local collector Wayne Thompson – consists of a pair of fused mandibles. The specimen still unpublished and is now at LACM, but I was not able to see it on my last museum visit in November 2013. Material from the Purisima Formation includes a nearly complete skull and isolated mandible, a partial rostrum, and a couple of isolated periotics. None of the material is associated – it’s all scattered material preserved in bonebeds and other inner shelf sediments, presumably scattered across the seafloor by currents or from drifting carcasses. Nevertheless, every referred element exhibits morphological features unique to Semirostrum. Some specimens – including the periotics and the mandible – are morphologically indistinguishable from and identical in age to Semirostrum ceruttii. However, the skull and partial rostrum are somewhat older, from about the Mio-Pliocene boundary; furthermore, the skull exhibits a slightly asymmetrical facial region – which is a bit more primitive than extant phocoenids, and Semirostrum ceruttii. For these reasons, we interpreted this slightly older material as representing an as yet unnamed, slightly older species, and chose to simply identify it as Semirostrum sp.

 

Examples of different preservational features on shark teeth (A), odontocete vertebrae (B), auk (Alcidae) humeri (C), and odontocete periotics (D) - specifically, the top periotic is Parapontoporia wilsoni, and the bottom periotic is a referred periotic of Semirostrum ceruttii figured by Racicot et al. (2014:figure 2).

The second paper is the publication derived from my master’s thesis research at Montana State University. My master’s thesis dealt with the taphonomy of Miocene and Pliocene marine vertebrates preserved in the Purisima Formation of Northern California. I initially got hooked on taphonomy – the science of fossil preservation – thanks to my undergraduate adviser Dave Varricchio, who did his Ph.D. on the formation of “Jack’s Birthday Site”, a multispecies bonebed assemblage from the late Cretaceous Two Medicine Formation. I took his taphonomy course in fall 2005, and read a few articles on the taphonomy of modern whales. At that time, I had just returned from my first summer season of permitted field work in the Purisima Formation, so I was naturally interested in looking into the taphonomy of the unit. Further piquing my interest was marine reptile researcher Pat Druckenmiller’s return to MSU to teach for a year. Pat did his master’s thesis at MSU, where he published the short necked plesiosaur Edgarosaurus from the Thermopolis Formation near Bridger, Montana. Pat had an interest in marine vertebrate taphonomy – and we talked quite a bit about it.

 

Histograms of taphonomic characteristics of bones, teeth, and cartilage from different lithofacies of the Purisima Formation. In general, highest energy conditions are on left, lowest energy on right.

As it turns out, the Purisima Formation had already been the focus of a taphonomic study of fossil invertebrates in the 1980’s. The Purisima Formation is rather unique in that, unlike most rock units which have received taphonomic study, it preserves invertebrate and vertebrate fossils in a number of different depositional environments. This provided Norris (1986) with the unique opportunity of examining across-shelf trends in preservation of marine invertebrate fossil concentrations. Even with such an expansive shelly fossil record, similar studies have been few and far between. No studies investigating across-shelf trends in marine vertebrate taphonomy had ever really been attempted. My own limited field experience at the time indicated that a study of similar scope as Norris’s original paper – but analyzing marine vertebrates from the Purisima Formation instead – would uniquely permit the examination of cross-shelf changes in vertebrate preservation. All previous studies had sampled vertebrates from a single marine unit reflecting a single depositional environment, or a single fossil bed, or a single skeleton. These studies are of course necessary and make up the bread and butter of marine vertebrate taphonomy, but investigating larger scale processes that control or influence the spatial distribution and preservation of vertebrate bones and teeth in the marine environment is (or, was) virgin territory.

I’ll discuss the highlights later on in a dedicated series of posts, but these are takeaway points:

1) vertebrate material is most abundantly concentrated along time-rich hiatal or erosional surfaces – namely, bonebeds and shell beds.

2) taphonomic damage – abrasion, phosphatization, fragmentation, polish – are all positively correlated with both high-energy, shallower water deposits, and time-rich surfaces.

3) this indicates a systematic relationship between sedimentary architecture and marine vertebrate preservation, and that the sheer majority of the marine vertebrate fossil record is controlled by physical sedimentary processes, rather than biotically controlled. From a paleoecological perspective, there is not much hope that using any sort of specimen counting methods (e.g. relative abundance) for faunal analysis will be able to backstrip the rather severe taphonomic overprint.