Another "new" publication: pilot whale-like delphinids from the Purisima Formation

The second paper that I published this year (freely available online here at App.pan.pl), and did not get around to discussing on this blog, was a paper I published with Frank Perry (Santa Cruz Museum of Natural History) and Jonathan Geisler (NYIT College of Osteopathic Medicine) in Acta Palaeontologica Polonica. This study reported some new fossils of Pliocene globicephaline whales from the Purisima Formation near Santa Cruz. Generally, most papers I have published are concerned with fossils I’ve dug up myself, with a few exceptions – this being one of them. In summer 2009, I received a forwarded email from Frank Perry including some photographs of a fragmentary odontocete skull in a partial concretion. Frank (at first) was not excited, but I was puzzled at the relative size of the scale bar – it was one of those clear plastic rulers made for kids, and after looking at the whole image at large size, I noticed that it was not a 6” ruler, but a 12” ruler – indicating the preserved fragment was about a foot and a half long (~40 cm). This was no porpoise or small dolphin – this was something big, pilot whale or beluga sized – a substantial beast indeed. 

The prepared fossil globicephaline skull from the Purisima Formation, collected by Robin Eisenman in 2009 from the shore of Monterey Bay. From Boessenecker et al. (2013).

I emailed Frank about this, who gave me the contact information for the collector – Robin Eisenman, then of Aptos – who after only a few minutes on the phone happily agreed to contribute the fossil to science. So, I picked it up, and took it back to Montana State University to prepare it. Preparation did not take long, owing to great separation of the rock from the bone. On a subsequent visit back home, I drove down to Watsonville to check out the collection of Stan Jarocki, a dedicated amateur paleontologist with one of the best eyes for finding fossils I’ve ever come across, professional or otherwise. I’ve been in the field several times with Stan, and every time it’s amazing what he’s able to pull out from just a couple square millimeters of exposed bone. Stan is perhaps best known for establishing a rather large collection of bird fossils from the Purisima Formation, including numerous beaks and braincases, not to mention prized complete humeri and other diagnostic postcranial bones. I was quite amazed to see the first drawer in his collection, which was an entire drawer filled with nothing but dolphin ear bones -  baleen whale ear bones were in a separate drawer. Among these new finds were several Herpetocetus periotics which he permitted me to take for donation to UCMP and study, in addition to a neonatal or perinatal partial mandible of Herpetocetus; these specimens will be described in detail whenever I get around to writing up my Purisima Herpetocetus research. In the dolphin ear bone drawer I saw dozens of periotics which were obviously the “river” dolphin Parapontoporia, various phocoenid periotics, and several delphinine periotics (often identified in the literature as “Delphinus or Stenella”). Among the phocoenid periotics, two looked superficially similar to a phocoenid mentioned by Larry Barnes in his Ph.D. dissertation as “aff. Tursiops” which is now known to be a Haborophocoena-like phocoenid with an asymmetrical skull – but were much, much larger. I looked at them, and concluded they must be some sort of large delphinid.

The skull in lateral and posterior view, with a line drawing of endocast features. From Boessenecker et al. (2013).

Now, at first I didn’t know what the skull represented – I thought it was some sort of massive monodontid, something a bit weirder and larger than the pilot whale convergent monodontid Denebola brachycephala. It wasn’t until I was flipping through the plates of Van Beneden and Gervais (1880) – one of the great cetological monographs – that I realized that the skull was a good match for a globicephaline whale. Not too long after this, I contacted Jonathan Geisler, and proposed we present it as a poster at the 2009 SVP meeting – which I presented at the Bristol meeting of SVP, and if you look at the abstract, it says we identified it as Globicephala sp. We eventually decided it wasn’t referable to the genus, but certainly the subfamily. The fossil exhibits some features distinctive to the subfamily, such as large size, anteriorly widening premaxillae, and premaxillae that slope medially on the base of the rostrum; however, the development of a longitudinal ridge on the premaxilla precluded referral to any extant globicephaline. Furthermore, the ventral side of the skull actually had some features that were more like Pseudorca than Globicephala, something that Jonathan Geisler and I found out on a visit to the AMNH in January 2011.

The two globicephaline periotics collected by Stan Jarocki from the Purisima Formation in Santa Cruz County. From Boessenecker et al. (2013).

On a short tangential side note, Jonathan Geisler had invited me to contribute to a webpage on the NYCOM (Now NYIT College of Osteopathic Medicine) website about cetacean evolution – but due to a quirk in New York laws, I’d have to fly out to Long Island and sign everything in person. So, Jonathan graciously paid for me to fly out, sign the paperwork, and hang out in New York for a week – including a day trip to the AMNH. Even though it’s only a one hour drive from Manhattan, it took us two hours by train and subway to get there from NYIT. Upon arriving from my flight and getting a cab ride to Old Westbury (the driver had no idea where it was, and it took him until 30 minutes into the drive to figure out where it was) – two pieces of bad news presented themselves. The first was that I had been rejected from the SDSU-UCR joint doctoral program; the second was that I had taken the globicephaline fossil with me, and it had broken in transit. I noted that it would be an easy fix, but still… talk about circumstance kicking someone when they’re down.

Bivariate plot of promontorium length and maximum width of bony nares for Delphinida (based on adult crania), with horizontal line for the skull, and vertical lines for the two periotics. This indicates that the periotics are probably too small to belong to the same taxon as the skull, and that two globicephalines are recorded in the Purisima Formation. From Boessenecker et al. (2013).

So, enough with the backstory, time for some more science. After examining the two periotics that Stan Jarocki collected, Jonathan noted that they appeared a bit on the small side to go along with the skull that Robin Eisenman collected – which got him on to thinking that there may have been two separate globicephalines preserved in the Purisima Formation. In order to evaluate this, Jonathan used a bunch of measurements that he had amassed as part of an NSF funded project on delphinidan phylogeny. He used two measurements in particular: external bony nares width, and the length of the promontorium. When these measurements for fossil and modern delphinidans are plotted on a graph, a distinct trend can be seen – although there is quite a bit of scatter around the line of best fit. Regardless, the results do indeed suggest that the periotics are far too small to represent the same globicephaline as the skull: using the line of best fit, the skull size (narial width = 110mm) would be associated with periotics that had a 21mm long promontorium on the periotic, and the periotics instead have ~15mm long promontoria, which would be associated with a skull approximately half the size (narial width = 60mm). This line of evidence suggests that there were indeed two globicephalines present in the Purisima Formation. Interestingly, this is one of the only localities in the Pacific realm to include more than one globicephaline.

Geographic distribution of late Miocene, Pliocene, and Pleistocene fossil globicephalines. Black denotes described or figured specimens. From Boessenecker et al. (2013).

Lastly, we reviewed the globicephaline fossil record, and found that the subfamily was already worldwide in distribution by the early Pliocene. Molecular divergence date estimations for the clade include estimates as old as 8 Ma (Vilstrup et al., 2011) and as young as 4 Ma (McGowen et al., 2009). Few globicephalines are any older than 6 Ma, and the late Miocene is one of the most intensely sampled parts of the cetacean fossil record; we suggested that divergence dates as old as 8 Ma are probably too old, and likewise by 4 Ma we already have globicephaline fossils – indicating that 4 Ma is certainly too young. Divergence dates of about 5.5 Ma (Cunha et al., 2011) are probably more accurate. Discovery of additional (and better preserved) globicephaline fossils will help refine this picture.

References

Boessenecker, R.W., Perry, F.A., Geisler, J.H. 2013. Globicephaline whales from the Mio-Pliocene Purisima Formation of central California, USA. Acta Palaeontologica Polonica.

Cunha, H.A., Moraes, L.C., Medeiros, B.V., Lailson-Brito, J. Jr., da Silva, V.M.F. 2011. Phylogenetic status and timescale for the diversification of Steno and Sotalia dolphins. PLoS ONE 6 (12): e28297.

McGowen, M.R., Spaulding, M., and Gatesy, J. 2009. Divergence date estimation and a comprehensive molecular tree of extant cetaceans. Molecular Phylogenetics and Evolution 53: 891-906.

Vilstrup, J.T., Ho, S.Y.W., Foote, A.D., Morin, P.A., Kreb, D., Krützen, M., Parra, G.J., Robertson, K.M., Stephanis, R. de, Verborgh, P., Willerslev, E., Orlando, L., Gilbert, M.T.P. 2011. Mitogenomic phylogenetic analyses of the Delphinidae with an emphasis on the Globicephalinae. BMC Evolutionary Biology 11:65: 1-10.