Thursday, August 9, 2012

Parabalaenoptera baulinensis: the fossil baleen whale from Bolinas, Marin County, California


Until relatively recently, fossil balaenopterids have been avoided by modern paleocetologists like the plague. Modern balaenopterids include the humpback whale (Megaptera novaeangliae) and species of Balaenoptera, including the Minke, Blue, Fin, and Sei whales. Although balaenopterids have very distinctive and easy to identify crania, they are really only common in latest Miocene and Pliocene marine rocks, and early work by Kellogg and others yielded fossil baleen whales with much more primitive skulls, formerly called "cetotheres" sensu lato, also jokingly referred to by some paleocetologists as "Kelloggitheres".

Fossil balaenopterids have been plagued by a particularly nasty taxonomic situation since the late nineteenth century, when P.J. Van Beneden began describing fossil mysticetes collected during the construction of a series of forts around Antwerp. Unfortunately, many of these fossils which names like Plesiocetus and Herpetocetus are based upon were isolated finds, which were subsequently arranged into type 'series' with other skeletal parts based on a preconceived notion of what each taxon should have looked like. The end result was a series of chimaeras, some of which represented by potentially informative but often fragmentary material, lacking type specimens, the associated names of which have been dragged through the systematic mud by subsequent authors, and not allowed to simply die gracefully.

The skull and mandibles of the holotype specimen of Parabalaenoptera baulinensis from the late Miocene Santa Cruz Mudstone of Marin County, California.

Many of these fossil balaenopterids are difficult or impossible to diagnose: which specimen of the series would you designate as a lectotype? Is that specimen diagnostic? What do you do with the other specimens? Is an isolated mandible diagnostic or not? What about a piece of a braincase with unassociated bullae? According to Bosselaers and Post (2010), many of Van Beneden's taxa are based on "syntype" collections that are not possible to unambiguously diagnose: diagnostic syntype elements are either too incomplete, or include multiple diagnostic elements but from multiple individuals (and occasionally taxa; i.e. a balaenid bulla was grouped with the lectotype mandible of Herpetocetus scaldiensis by Van Beneden). Accordingly, Bosselaers and Post (2010) declared all of Van Beneden's Pliocene balaenopterid taxa to be nomina dubia: for the time being, I think this is probably the "safe", appropriate, and right thing to do.

The discovery and description of Parabalaenoptera baulinensis was one of the first important advances in balaenopterid paleontology: it was one of the first balaenopterids described from a nearly complete skull with associated mandibles and postcrania. Some other previously published fossil balaenopterids were described on somewhat complete remains: Megaptera miocaena (late Miocene of California), Megaptera hubachi (late Miocene of Chile), Protororqualus cortesii (Pliocene of Italy), "Balaenoptera" cortesi var. portisi (Pliocene of Italy), and Cetotheriophanes capellinii (...also Pliocene of Italy). Unfortunately, the holotype skeleton of Protororqualus was destroyed during bombing in World War II, and M. miocaena only includes earbones and a braincase; furthermore, the other Italian balaenopterids have been plagued with nomenclatural issues for over a century (see Demere et al. 2005).

The exhibit at the Drakes Beach visitor's center showing the holotype skeleton of 
Parabalaenoptera baulinensis.

In 1973, a large mysticete skeleton was discovered by Carl Zeigler of the College of Marin, weathering out of cliffs near Bolinas in Marin County, California. Bolinas is a quaint artist community on the Marin County coast and has changed little since the 1960's and 70's; it is predominantly settled by ex-hippies, who generally don't like visitors from out of county, and have continually removed the exit sign for "Bolinas: 2 miles" off of highway 1, to the point where the California Dept. of Transportation (CalTrans) has given up putting up new signs. Tales abound of visitors with out of county or out of state license plates having car tires popped or vandalized, and nails and other tire-popping objects being intentionally laid out onto dirt roads in town. My car had a San Rafael Honda license plate holder, so I never had this problem.

The assembled holotype skull of Parabalaenoptera at California Academy of Sciences, photographed by fellow Otago Ph.D. student Felix Marx.

Exposed along the southern tip of Point Reyes is a unit formerly identified as the early late Miocene Monterey Formation; this was subsequently reevaluated after Domning (1978) suggested that sea cow fossils from the Bolinas locality were too derived to be from such an old unit. Afterwards, microfossils suggested a much younger age, closer to the Mio-Pliocene boundary (6-6.8 Million years old), and the formation was reidentified as the Santa Cruz Mudstone, which has only been mapped in Santa Cruz County.

Anterior view of Parabalaenoptera.
The fossil occurred in indurated, blocky mudstone, and was collected over a ten year period as the blocks incrementally eroded from the cliff. The lead authors - Gordon ("Gordie") Chan and Carl Zeigler of the College of Marin in Kentfield, and their field assistants - would have to travel over the hill and out to Bolinas (nearly an hour's drive through some of the windiest vomit-inducing roads in Northern California) on a monthly basis during the summer, and much more often during the winter during periods of intense erosion, and sometimes daily, anticipating falling blocks. After collection, the blocks were prepared, and some were glued together - but left as a series of blocks that could be lined up and assembled. The holotype was prepared at College of Marin, and eventually molded, casted, and donated to the California Academy of Sciences. Mounted casts of Parabalaenoptera baulinensis are currently on display at College of Marin in Kentfield and at the Drake's Beach visitor center at Point Reyes National Seashore. Sadly, before the paper could be published on the fossil - Carl Zeigler and Gordon Chan passed away. Chan passed away in 1996 of Lou Gehrig's disease; I could not find information on Zeigler, though I seem to recall hearing that he was killed by a drunk driver. Dr. Lawrence Barnes of the Los Angeles County Museum of Natural History finished the manuscript and brought it to publication in the California Academy of Sciences in 1997.

The braincase and vertex of Parabalaenoptera.
Parabalaenoptera baulinensis is a medium-sized balaenopterid with a 2.2 meter skull, slightly larger than minke whales (Balaenoptera acutorostrata; a 10 meter long whale with a 2m skull), but has a number of features that are too divergent to warrant inclusion within humpbacks (Megaptera) or Balaenoptera. These include the very elongate and somewhat swollen zygomatic processes, narrow intertemporal region (the skull is less 'telescoped than in modern balaenopterids), and extremely long and narrow nasal bones. The mandibles are strongly outwardly bowed like in Megaptera, and have an elongate coronoid process - somewhat like blue and fin whales (Balaenoptera musculus and physalus). Many of these features suggest that Parabalaenoptera baulinensis was capable of lunge feeding just like modern rorquals. It is additionally convergent with Balaenoptera musculus in having a supraorbital process of the frontal that is somewhat triangular and narrows laterally, whereas in Balaenoptera and Megaptera, the posterior and anterior margins are either parallel, or the posterior margin is perpendicular to the midline. Unfortunately, the holotype specimen is not preserved very well, and it appears that a significant amount of bone was accidentally removed or ground away during preparation, and details of the basicranium are almost totally indiscernible. Parabalaenoptera has been found in many phylogenetic analyses to be a stem-balaenopterid - in other words, a primitive member of the clade (family Balaenopteridae) that does not belong to the clade formed by humpback whales and modern species of Balaenoptera - the Megaptera + Balaenoptera clade, if you will. These two modern genera have been traditionally grouped into the "Megapterinae" and "Balaenopterinae" - Zeigler et al. (1997) even went so far as to name a new subfamily, the Parabalaenopterinae. However, given that none of these subfamilies have really shown to be stable or even consistent in cladistic analyses, it's unclear what the utility of such taxon names even is.

Reconstruction of the holotype skull and mandibles of Parabalaenoptera. Unfortunately, certain features (e.g. squamosal morphology) of the actual skull don't really look like how they're portrayed in this figure. From Zeigler et al. (1997).

Nevertheless, the description of Parabalaenoptera was a hallmark in balaenopterid paleontology; however, given the stagnated taxonomic situation of fossil balaenopterids, little else was published on fossil balaenopterids until Michelangelo Bisconti started revisiting Italian fossils starting in 2007. In the last five years, a number of informative balaenopterid finds have been described - but it is only the tip of the iceberg. Perusing late Miocene and Pliocene marine mammal assemblages in Museums, it is apparent that balaenopterids comprise nearly two-thirds of the more recent baleen whale fossil record (it is at least the case for California marine mammal assemblages). In California, at least four to five unpublished balaenopterids await description from the Pliocene alone. Additional fossils that may represent Parabalaenoptera - potentially a new and slightly younger species - have been collected from the Purisima Formation near Santa Cruz (and are in much better condition than the holotype). The future of balaenopterid paleontology is bright!

References

Bosselaers, M., and Post, K. 2010. — A new fossil rorqual (Mammalia, Cetacea, Balaenopteridae) from the Early Pliocene of the North Sea, with a review of the rorqual species described by Owen and Van Beneden. Geodiversitas 32:331-363. 

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


Domning, D. P. 1978. — Sirenian evolution in the North Pacific Ocean. University of California Publications in Geological Sciences 18:1-176.


Zeigler, C. V., Chan, G. L., and Barnes, L. G. 1997. — A new late Miocene balaenopterid whale (Cetacea: Mysticeti), Parabalaenoptera baulinensis, (new genus and species) from the Santa Cruz Mudstone, Point Reyes Peninsula, California. Proceedings of the California Academy of Sciences 50(4):115-138.


1 comment:

XZombiesAteMyToastX said...

My Grandpa was Carl Zeigler and its nice to read something on his studies considering he died when my sister was a newborn.