Monday, September 5, 2011

A new specimen of Parapontoporia

Last fall I made a couple posts detailing an excavation (here and here) of a new odontocete skull from a relatively young (middle-late Pliocene) horizon in the Purisima Formation. This specimen was collected over a six hour period, and the excavation was pulled off and completed just before sunset. The specimen is still not completely prepared, but it does include a complete braincase, the posterior half of the rostrum, both petrotympanics (articulated petrosal and tympanics), and part of one of the lower jaws.

The new skull of Parapontoporia sternbergi in oblique dorsolateral view; bottom photo is labeled.

Three species of Parapontoporia have been described: Parapontoporia pacifica, from the late Miocene Almejas Formation in Baja California, Parapontoporia sternbergi, from the Pliocene San Diego Formation, the Mio-Pliocene San Mateo Formation, and possibly from the Pliocene Wilson Grove Formation, and of course, Parapontoporia wilsoni from the Mio-Pliocene Purisima Formation. Most of the Purisima material is referable to P. wilsoni, which is characterized by a very deep "basin" at the base of the rostrum (shown poorly in the photo below) and by a facial region that is longer than it is wide. P. sternbergi has a shallower basin, and has a facial region wider than it is long (and also appears to me to be smaller in general; P. wilsoni crania can be up to 20% larger than those of P. sternbergi). P. pacifica is not known from a complete braincase, and it has a flat base of the rostrum (i.e. no rostral basin). The new specimen exhibits a shallow rostral basin, and has a braincase that is wider than long, and is damn tiny - all suggesting that it is assignable to Parapontoporia sternbergi rather than P. wilsoni. "But the Purisima Fm. species is Parapontoporia wilsoni!", you might say. Just like modern cetaceans, fossil cetaceans likely had a cosmopolitan distribution - and Parapontoporia sternbergi is primarily known from late Pliocene rocks (specimens identified as P. sternbergi from Miocene strata should probably instead be called Parapontoporia sp.). The type locality of P. wilsoni is about 5.3 Ma, slightly older than the San Diego Fm. Likewise, the new specimen from the Purisima is from a horizon about 10 meters below an ash bed dated at 3.35 Ma, and therefore probably late Pliocene also.

Parapontoporia sternbergi, the most completely known species within Parapontoporia, exhibits an extremely elongate rostrum, filled with about 80 teeth per quadrant; that's a total of 320 teeth. I don't know the specifics for other toothy odontocetes (such as Eurhinodelphinids), but this strikes me as being a terrifyingly large number of teeth for a mammal, and I would not be surprised if Parapontoporia was the toothiest of all mammals. Additionally, the skull of parapontoporia is asymmetrical, unlike Pontoporia and more like the now extinct chinese river dolphin Lipotes. Parapontoporia was obviously named because it is closely related to the La Plata River Dolphin, Pontoporia, right? Right? Believe it or not, this new specimen weighs in on the phylogenetic relationships of Parapontoporia.

A reconstruction of a nearly complete skull from the San Diego Formation referred to Parapontoporia sternbergi by Barnes (1985).

The holotype of Parapontoporia wilsoni from the Purisima Formation.

When Larry Barnes published is major study of Parapontoporia in 1985 (the year I was born...), there were no skulls of Parapontoporia with associated earbones. Because of the similarity of Parapontoporia to modern river dolphins like Pontoporia and Lipotes, he looked through museum collections and tried to identify possible petrosals that could be referred to this new taxon. Oddly enough, one set of six petrosals from the San Diego Formation appeared very similar to modern Lipotes, although Barnes felt that the skull was more similar to Pontoporia.

Isolated petrosals from the San Diego Formation referred to Parapontoporia sternbergi by Barnes (1985).

Because of this discrepancy, there has been some disagreement (not in the published literature - in discussion only) about what the real petrosal of Parapontoporia looks like. At a UCMP visit once, Nick Pyenson showed me another petrosal which he felt might be a better match than the ones Barnes referred. This is a fairly serious matter, because these isolated petrosals have been used in various cladistic analyses to fill in the data matrix - and given the key phylogenetic position of Parapontoporia as a sister taxon to Delphinoidea - means that if the petrosals are not in fact from Parapontporia, there could be some serious errors in previously phylogenetic studies.

The petrotympanic complexes of the new Purisima Formation specimen of Parapontoporia sternbergi in dorsal/cerebral aspect.

The petrotympanic complexes in (near) ventral aspect.

This specimen is the first skull of Parapontoporia with well-preserved petrotympanic complexes associated with it. These petrosals are identical to those referred to this taxon by Barnes (1985), and confirm his referral. Interestingly, this also confirms the Lipotes-like morphology of the earbones. Previous cladistic analyses (e.g. Muizon 1988) have resulted in Parapontoporia being the sister taxon to Lipotes rather than Pontoporia, and several authors have suggested such a relationship, despite the hand-drawn cladogram and assertions of Barnes (1985). This new find indicates the Lipotes-like ear morphology does belong to Parapontoporia.

Petrosals of A) Inia, B) Pontoporia, and C) Lipotes.

Another Purisima Fm. skull of Parapontoporia wilsoni with a petrotympanic complex, in a private collection.

I'd be lying if I said that this were the first specimen ever found with petrosals and tympanics; two other specimens are known. One is a specimen of Parapontoporia pacifica from the Capistrano Formation at the San Diego Natural History Museum, which has a crushed tympanic and petrosal. The other specimen is also from the Purisima Formation, shown above, but has been unavailable for study. The petrosal is barely exposed and stuck within the concretion. This specimen remains in a private collection. This no longer bothers me, as the new specimen has both petrotympanics fully freed from the skull, and one of which has already been CT-scanned at UT Austin for a study by Manuel Martinez and Jonathan Geisler (among other authors, which I am dead last for this minor contribution). Look out for it at SVP!

References and further reading:

BARNES, L. G. 1984. Fossil odontocetes (Mammalia: Cetacea) from the Almejas Formation,
Isla Cedros, Mexico. Paleobios 42:1–46.

BARNES, L. G. 1985. Fossil pontoporiid dolphins (Mammalia: Cetacea) from the Pacific coast
of North America. Contributions in Science, Natural History Museum of Los Angeles
County 363:1–34.

FORDYCE, E., AND C. DE MUIZON. 2001. Evolutionary history of cetaceans: a review. Pages
169–233 in J. -M. Mazin and V. de Buffrenil, eds. Secondary adaptation of tetrapods to
life in water. Verlag Dr. Friedrich Pfeil, Munich, Germany.

GEISLER, J. H., AND A. E. SANDERS. 2003. Morphological evidence for the phylogeny of
Cetacea. Journal of Mammalian Evolution 10:23–129.

MUIZON, C. de. 1988. Les relations phylog`en´etiques des Delphinida (Cetacea, Mammalia).
Annales de Paleontologie 74:159–227.

PYENSON, N. D. 2009. Requiem for Lipotes. Marine Mammal Science 25:714-724.


Chuck Powell said...

Afternoon Bobby, just to bring you up to date on dating of some of the formation you reference. The San Diego Formation is considered Pliocene to middle Pleistocene (about 4.2 to 1.5 Ma; Vendrasco and others, in press for a synopsis) and the Wilson Grove Formation is late Miocene to early Pliocene ( Just FYI. Good post - keep it up.



Robert Boessenecker said...

Hey Chuck,

Thanks - with respect to the San Diego Formation, because that work is not yet published, I was referring to earlier age estimates used by Demere (1983, 1994, etc.) For the Wilson Grove Formation - the Parapontoporia specimen is from the Trenton locality, which in Powell et al. 2004 is summarized as late Pliocene.