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.

Recent fieldwork in the Purisima Formation, Part 3: mysticete earbones and wildlife

Hey Folks,

Sorry for yet another delay - I've been pretty busy, working on several manuscripts (a thesis-length paper on the Purisima Formation marine mammals from my undergraduate field area in Halfmoon Bay, a new manuscript on shark bitten cetacean bones, and my contribution to a paper of the mollusk and vertebrate assemblage of a late Miocene marine locality in Sonoma County), applying to the University of Otago Doctoral Program to work with R. Ewan Fordyce (in New Zealand), and digging up a new right whale fossil from the Purisima Formation. There are plenty of topics I have thought of to write about on here, but not enough time!


Continuing on with my series of posts about recent Purisima Fm. fieldwork with Dick Hilton, I've written a little about a new mysticete earbone. There are lots of mysticete earbones from the Purisima Fm., from number of different taxa, including cetotheriids, many balaenopterids, and balaenids. Dick had originally spotted this specimen during a field trip earlier in the spring, but was unable to collect it. On our first day of our expedition back in late May, we spotted it easily, and given the easier tides, we were able to quickly excavate it. I was immediately struck with the size of the specimen, and in particular a large knob called the dorsal posterior prominence. This very distinctive earbone morphology is characteristic of the extinct rorqual "Megaptera" miocaena, which Remington Kellogg described in the 1920's from the late Miocene Sisquoc Formation of southern California. Several authors including Deméré et al. (2005) and Dooley et al. (2004) have suggested that it does not belong in Megaptera at all, and that it requires a new genus to be erected. However, it has appeared in some phylogenetic analyses (Bisconti, 2008; Marx, 2010) as a sister taxon to modern Megaptera novaeangliae (the modern Humpback Whale for the uninitiated). It really needs to be reanalyzed and probably redescribed.

Fossil tympanics of "Megaptera" miocaena from the upper Miocene San Mateo Formation (left) and Purisima Formation (right).

Fossils of this taxon are now known from Tortonian and Messinian (6-11 Million Years Old) strata in California, including the Sisquoc Formation, Purisima Formation (two localities), and the Santa Margarita Sandstone (a new specimen of which will soon be donated to UCMP). Other vertebrates from this time period include the dusignathine walruses Pontolis and Gomphotaria, the odontocetes Denebola, Parapontoporia, Albireo, and Piscolithax, as well as other mysticetes such as Nannocetus and Herpetocetus; all of these taxa are now known from multiple strata of this age, suggesting a distinct, and well-represented late Miocene marine mammal fauna from the eastern North Pacific.

Lastly, we spotted some wildlife during the trip, the photos of which are below.

Northern elephant seals spotted from the point.

A peregrine falcon near a nest at the edge of a cliff.

A coyote that ran along the beach while we stopped for a snack.

References:

Bisconti, M. 2008. Morphology and phylogenetic relationships of a new eschrichtiid genus (Cetacea: Mysticeti) from the Early Pliocene of northern Italy. Biol J Linn Soc 153: 161–186.

Deméré, T.A., A. Berta, and M.R. McGowan. 2005. The taxonomic and evolutionary history of fossil and modern balaenopteroid mysticetes. Journal of Mammalian Evolution 12:99–143.

Dooley, A. C., Jr., Fraser, N. C., and Luo, Z.-X. 2004. The earliest known member of the rorqual-gray whale clade (Mammalia, Cetacea). J. Vertebr. Paleontol. 24: 453–463.

Kellogg, R. 1922. Description of the skull of Megaptera miocaena, a fossil humpback whale from the Miocene diatomaceous earth of Lompoc, California. Proc. US Natl. Mus. 61: 1–18.

Marx, F. G. 2010. The more the merrier? A large cladistic analysis of mysticetes,
and comments on the transition from teeth to baleen. J Mammal Evol 18:
77–100.

Recent fieldwork in the Purisima Formation, Part 2: a possible new species of Herpetocetus?

Back in late May, Dick Hilton (Sierra College) and I did a three day field trip collecting fossil vertebrates from a locality in the Purisima Formation I recently got a permit for. On the second day, excavated a large block of sediment with what I assumed at the time were sirenian bones. I had not seen the bones except in cross section, and because they were somewhat dense, I thought they might be from a sea cow. We quickly carved out a large block, and due to the cohesive nature of the sediment, we were able to wrap it in tinfoil and duct tape. It was one of the first finds of the day, and I thought there could definitely be something neat inside. Because we were only a half mile from the cars, I left my pack with Dick and hoofed it back to the car with the thirty pound block, and also to grab some gatorade I had left in my car. When I returned, Dick was taking a siesta, and after some lunch, we headed further down the beach. Only a few hundred feet down I spotted a distinctly potato-shaped thing sticking out next to a piece of bone: it rather looked to me like a tympanic bulla, and I climbed up a bit to check it out. It was in fact a tympanic, and when the rest of the piece came out, I was able to see that it was in fact a nearly complete squamosal, complete with the bulla and posterior process of the petrosal. Dick and I thought the trip had been a success just because of this specimen, especially because it was from a section of cliffs where neither of us expected to find anything.

Dick Hilton digging up a huge baleen whale tympanic.

I couldn't have been more wrong. Unfortunately, I didn't know that I was until after the SATLW (Aquatic Tetrapods) conference. I did have a day or two before the conference to prepare the squamosal, and it did indeed have a plug-shaped posterior process of the petrosal, indicating it belonged to the Herpetocetinae, which includes Herpetocetus, Nannocetus, (probably) Piscobalaena, and Cephalotropis (according to Steeman, 2007). After the conference, I opened up the duct taped jacket and began preparation. After a couple hours the exposed pieces were still not making sense, and then I found a couple of bones that looked like they were adjacent to one another. When I removed them, there was a tiny neck of bone connecting them – and after a little more preparation, I realized it was a Herpetocetus petrosal and posterior process. Damnit, another goddamn Herpetocetus.

The petrosal and posterior process of the new specimen, with the facial nerve canal labeled. Upper left is ventral, lower left is dorsal, and right is medial view.

The skull with (partially incorrectly) articulated petrosal of the new skull in dorsal (top) and ventral (bottom) views.

Once I had enough of the block prepared, I realized I had quite a bit of the ventral portion of a small braincase preserved. It includes both exoccipitals, one occipital condyle, the basioccipital, the right squamosal, and the complete petrosal. After preparation, the petrosal is most similar to petrosals of Herpetocetus. This may be a bit technical, but herpetocetine baleen whales have several peculiar features that define them as a group. The posterior process of the petrosal – which is typically an elongate strap of bone that connects with the skull posteriorly – is very short and plug-shaped in these animals. Additionally, the posterior process (which is rarely found attached in isolated fossil mysticete petrosals) is flat and contributes to the lateral side of the skull, instead of being 'hidden' in a trench between the squamosal and exoccipital bones. Secondly, some herpetocetines have a flattened anterior process that is blade shaped; this structure is typically conical and robust or knoblike in most other mysticetes. Clearly, this specimen exhibits both of these features. Additionally, Herpetocetus spp. exhibit a large triangular flange on the side of the bone, which overhangs the squamosal – also present in this specimen. Additionally, herpetocetines all have extremely small earbones relative to most mysticetes. Unfortunately, the neck of the posterior process appears to have been deformed slightly, and when the main portion is articulated correctly, the posterior process sits in its trough a little wonky, and when the posterior process is articulated correctly, the main portion doesn't articulate well.

The posterior process, squamosal, and tympanic of Herpetocetus bramblei.

The two alternate articulations of the petrosal showing correct articulation of the posterior process (left) and correct articulation of the body of the petrosal (right).

The temporal region of the skull of Herpetocetus bramblei with the petrosal outlined in red.

However – it shows several features that differentiate it from all species of Herpetocetus as well as other herpetocetines like Nannocetus and Piscobalaena. Firstly, the anterior process is medially oriented – it is usually anteriorly facing instead. Second, the posterior process is very transversely narrow and elongate – it is typically more nearly circular in other species. Lastly, the most bizarre feature is that it has a very long anterior fissure of the facial nerve canal which is contorted into an S-shape – something I have not seen in any mysticete, fossil or modern.

Various mysticete petrosals in ventral view, showing two fossil rorquals (Plesiobalaenoptera and Balaenoptera sursiplana), a modern balaenid (Eubalaena japonica), the new specimen, and two other Herpetocetus specimens.

This is pretty exciting, and I am looking forward to preparing the other specimen, which includes part of a squamosal and a tympanic, and most likely a petrosal. It should not be too difficult to get these specimens written up and described.

Further Reading

Geisler, J. H. & Luo, Z.-X. 1996. The petrosal and inner ear of Herpetocetus sp. (Mammalia: Cetacea) and their implications for the phylogeny and hearing of archaic mysticetes. Journal of Vertebrate Paleontology, 70, 1045–1066.

Steeman, M.E. 2007. Cladistic analysis and a revised classification of fossil and recent mysticetes. Zoological Journal of the Linnean Society 150:875–894.

Steeman, M.E. 2010. The extinct baleen whale fauna from the Miocene-Pliocene of Belgium and the diagnostic cetacean ear bones. Journal of Systematic Palaeontology 8:63-80.

Whitmore, F.C., and L.G. Barnes. 2008. The Herpetocetinae, a new subfamily of extinct baleen whales (Mammalia, Cetacea, Cetotheriidae). In C.E. Ray, D.J. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.). Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14:141–180.

Fossil preparation - odontocete tympanic

About two or three summers ago I collected a beautiful little odontocete tympanic from the Purisima. Problem was, I only found out it was beautiful (past tense) after it sat in about twenty or thirty pieces. Because the part that was exposed looked like some other type of bone (and not an odontocete tympanic) I mis-estimated how sturdy the fossil was, and it exploded as I carved matrix away from it. I have since not repeated the mistake. Anyway, the fossil has sat in pieces in a plastic bag for two years, and I finally got the courage to glue it back together. I say courage because 1) I was somewhat embarassed by this damage, and 2) I was nervous to piece together all these tiny fragments. Below is a photo of what I had to work with.

Tympanic fragments prior to preparation.

I began by finding the pieces of the robust involucrum, which is the thick portion of the cetacean tympanic. There are more or less three major portions of the tympanic: the involucrum (frequently the only preserved part), the posterior process, which attaches to the posterior involucrum, and the paper-thin involucrum (which in odontocetes is usually under 1-1.5mm thick, hence the overall fragility of these elements). Then, I started finding matching pieces, and gluing these to the involucrum.

The tympanic after 30 minutes of preparation; some of the outer lip fragments have been glued in place. Lateral view (top photo) and anterior view (bottom photo) - note the matrix-free tympanic cavity.

After a couple more hours, I was able to finish gluing back most of the outer lip of the tympanic, as well as the posterior process.

Tympanic before (left) and after (right) addition of the sigmoid and posterior process.

Tympanic in dorsal (left) and lateral (right) aspects.

After the fossil was glued together, it became very obvious that this was a tympanic from the "river dolphin" Parapontoporia wilsoni, which has a small posterior process, a sharp anterior apex of the bulla, and most characteristically a laterally inflated outer lip, not seen in any other Purisima odontocete (for which tympanics are known, and out of the given tympanic sample from the Purisima Fm.). As far as crania, jaws, periotics, tympanics, and parts thereof go, Parapontoporia is by far the most common Purisima odontocete (i.e. between collections at UCMP, SCMNH, and LACM go, there are roughly a dozen nearly complete crania known, rostrum not included).

Newly prepared tympanic (right) side by side with another very well preserved Parapontoporia wilsoni tympanic.

All in all, I was extremely pleased; in one afternoon I had turned a pile of fragments (which I had virtually no hope for) into a beautiful little specimen. All but three tiny fragments under 5mm in size were glued on; the other ones probably attached to the margin of the outer lip, which may require fragments lost during collection (or, conversely, pieces pulverized). But let's not split hairs here - this by far was the most damage I've ever done to an odontocete tympanic, to the point where I was embarassed to even think about it; and now, it's one of the nicest I have. The positive side to this inadvertently destructive mode of

collection was that all the matrix was absent from the tympanic cavity, unlike the specimen on the left in the above photo (where the matrix inside was actually phosphatized, but phosphatic 'cementation' had fortuitously not formed an overgrowth around the rest of the bulla - best case scenario!). This is one of about a dozen and a half or so odontocete bullae I've recovered from the Purisima.