Monday, November 2, 2009

New lower jaw of the extinct lipotid Parapontoporia

Before I dive back into fossil odobenids, I'd like to show off something I just finished preparing. I collected this specimen from underwater in July in Santa Cruz County; I arrived at the exposure several days after I initially discovered it and coated it with vinac. Unfortunately, upon my return when I intended to collect it, the tide wasn't low enough; the specimen was exposed on the apron of a cliff, and was about 6" underwater. Unfortunately, I was there at low tide, and in the intervening days, the sand on the beach had locally eroded, allowing waves to go *just* a bit higher on this 5m stretch of beach.

Oblique view of the lower jaw of Parapontoporia wilsoni.

Anyway, after forty minutes of carving out a pedestal, cursing like mad because I thought I was going to destroy the fossil, and being investigated (and probably secretly laughed at) by a sea otter and a sea lion, I decided to undercut the pedestal. The pedestal was about 14" long and 5" wide, and I was worried that it might crack in half during undercutting - any bone exposed in that crack might fall out (and be swept away by the surf), and then I wouldn't be able to connect the bone from the two pieces of the pedestal back together. Needless to say I was shocked (and endlessly pleased) when the pedestal popped off perfectly.

Dorsal aspect of the fused dentaries of Parapontoporia wilsoni.

Lateral aspect of the fused dentaries of Parapontoporia wilsoni.

Parapontoporia is a very conspicuous member of late Neogene marine vertebrate assemblages in California and Baja California, and has also been reported from Japan. In California, it is known from late Miocene (Tortonian - 9 Ma) through late Pliocene (Piacenzian - 2 Ma) strata, including the San Mateo, San Diego, Capistrano, Pismo , Purisima , and Wilson Grove Formations. Three species are known - Parapontoporia pacifica from the late Miocene Almejas Formation of Baja (Barnes, 1984), Parapontoporia wilsoni from the Mio-Pliocene Purisima Formation (Barnes, 1985), and Parapontoporia sternbergi from the San Diego Formation (Barnes, 1985).

The cranium and jaws of Parapontoporia sternbergi, on display at the San Diego Natural History Museum.
Since description, P. pacifica is still only known from one partial cranium, while P. sternbergi is now represented by about a dozen well preserved crania, and several nearly complete lower jaws. While only the partial holotypic cranium of Parapontoporia wilsoni has been described, however, there are now probably around two dozen crania known, in addition to around 50-75 periotics. Only two lower jaws are known, though - one crappy fragment at UCMP, and a neat (but highly abraded) fragment of an articulated rostrum with teeth at CAS. Two more well preserved jaws are known, both from the early Pliocene of the Purisima - one I collected with my girlfriend in 2006, and the specimen I collected this summer. The 2006 specimen has one tooth, but is better preserved than this specimen.
This specimen may not represent P. wilsoni; the P. wilsoni holotype is about a million years older, and it is certainly possible that crania from this stratum represent P. sternbergi due to their younger age; description of material from the San Mateo Formation is needed to investigate this further. In fact, a huge body of fossils of Parapontoporia need to be described.
Closeup of the teeth of the new jaw of Parapontoporia wilsoni.

Parapontoporia was originally named for its similarity to the extant La Plata River dolphin, or Franciscana (Pontoporia blainvillei; Barnes, 1984, 1985). However, subsequent studies have placed it within the Lipotidae, as the sister taxon of the now extinct Yangtze River Dolphin (Lipotes vexillifer; Geisler and Sanders, 2003; Muizon, 1988), which was only described in 1918. Parapontoporia has an extremely long rostrum and mandibular symphysis, and *may* have the most teeth of any mammal (which, if it isn't Parapontoporia, I'm sure it's some kind of eurhinodelphid or other longirostrine odontocete from the Chesapeake Group of the east coast).

Wherever Parapontoporia occurs, it dominates the odontocete assemblage - in the Purisima, up to 38% of isolated periotics are referable to Parapontoporia. The most abundantly known odontocete crania from the Purisima belong to this taxon. Interestingly, despite decades of construction in San Diego, there are now more crania of this taxon known from the Purisima than from the San Diego Formation. Many of these Purisima crania are still in concretions, but nonetheless, they exist, and an excellent opportunity for a study of ontogenetic and stratigraphic variation is possible given this sample (a project Nick Pyenson was bugging me to do, but I simply didn't have the time as an undergrad). In fact, I picked up two partial crania this summer (both in nodules, though; one weighed about 55 pounds).

Nick Pyenson (2009) recently published a pretty neat (albeit depressing) paper in marine mammal science about the consequences of the extinction of Lipotes, given its 'colorful' evolutionary history. But this post is long enough as is, and I could do several more posts just on Parapontoporia; I'll save discussion of that paper for later.

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.
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: an evolutionary perspective on marine mammal extinction. Marine Mammal Science 25:714-724.


Brian Lee Beatty said...

Nice dentary! Wow, it is amazing how much that looks like Lipotes, I see why it has been found to be related.
I bet Pomatodelphis inaequalis had more teeth. Do you want to make it a formal bet? I know P. inaequalis and Eurhinodelphis better than the west coast thugs, maybe we could really give this a try. It would be a fun way to motivate some lingering projects for me, at least.

Alton Dooley said...

Very nice specimen, and it sounds like quite an adventure collecting it.

That is one crazy-long mandibular symphysis!

Robert Boessenecker said...

Yes, it is a veeery long symphysis! This one as far as I know is the most complete P. wilsoni jaw, and there is still less than half of it preserved.

Well, go ahead and count 'em up, and we'll cowrite a post about it; I gotta read up on adaptations of long rostra and all that stuff.

Doug said...

Crazy long indeed! Kinda like that skimmer porpoise at the San Diego Museum.

Markus said...

Really a great find, congratulations! Every time I see fossils or depictions of Eurhinodelphids or other extremely longsnouthed odontoceti I ask me why we don´t have a single comparable species today. Okay, river dolphins have long and very narrow jaws, at least some of the species, but they can´t rival the fossil forms. And the presence of a highly elongated snout is not such an evolutionary side issue like the super-bizarre heads of Makaracetus or Odobenocetops, but something what occured for many times among completely different animals, from fish to temnospondyls, various reptiles and even birds (to some degree at least), and for several times independetly among whales. So the question remains why such whales are no more around.

Robert Boessenecker said...

I wouldn't go so far as to say that current 'river dolphins' aren't functionally analogous to other longirostrine taxa; additionally, some analyses show quite a few of these longirostrine taxa ('platanistoids', eurhinodelphids, lipotids, iniids, pontoporiids, some kentriodontids) as stem taxa just outside or within delphinoidea, and the only 'brevirostrine' taxa in crown group delphinoidea (exceptions being Brachydelphis, and wherever the hell Delphinodon fits). This is of course discussing odontoceti diverging later than Physeterida.

Other phylogenetic studies indicate that extant river dolphins survived due to independent adaptation to fluvial environments (can't remember the ref, but Fordyce is a coauthor), preserving the archaic nature of these early diverging members of Delphinida (and stem taxa just outside delphinida, if I remember correctly).

Markus said...

Yes, river dolphins are not really comparable to the extinct longirostrine forms, especially because they are so highly adapted to their environment, and show a lot of unique traits. I just mentioned them because at least some of them have very elongated jaws compared to other extant species, but are still by far exceeded in this trait by many extinct forms.
BTW, your drawing of the Dorudon skull looks really great!

Robert Boessenecker said...


Thanks for the compliments. As far as river dolphins as analogues go... I wholeheartedly disagree, since many fossil platanistoids of presumably marine ecology also have long rostra; additionally, platanistoids in many phylogenies in combination with lipotids and inioids bracket different groups of extinct longirostrine odontocetes (e.g. eurhinodelphids, and kentriodontids, depending where both "groups" or parts thereof plot out). The point is that everything that these extant 'river dolphins' were related to during the Miocene also had long rostra, and that fact means that the relevancy of these extant species can't be ruled out so easily. Additionally, recall the bizarre cranial morphology of Platanista. It looks way too weird to be in the rock record. Except for Araeodelphis, and Zarchachis - both of which have bizarre maxillary crests (Araedelphis is currently under study by Stephen J. Godfrey). Both of these taxa are from marine rocks (and granted, they could be 'bloat and float'-derived fossils, but actual fluvial records (a la Potamodelphis in Florida) await discovery. Additionally, Pontoporia is distributed in the shallow marine realm. I suspect the majority of fluvial/riverine adaptations are sensory (i.e. loss of eyesight, changes in echolocation, if any; I can't recall) and physiological (although I can't recall much there, either), rather than feeding adaptations.