Monday, November 3, 2008

Reconstruction of a fossil mysticete

At left is a skeletal reconstruction I recently did, which was supposed to go in my talk for SVP this year on a bizarre baleen whale named Herpetocetus bramblei. This skeletal reconstruction is based on a referred skeleton of Herpetocetus sendaicus from the early Pliocene of Japan.

Herpetocetus is a very weird type of baleen whale, and I will most likely be posting about some of its oddities in the future.

For example, there is a very odd 'style' of the suturing between the rostral elements (e.g. maxillae, premaxillae, nasals) and the posterior cranium (particularly the frontals). The mandible of this taxon is extremely strange, as is the jaw joint itself (e.g. glenoid fossa), and the auditory/temporal region of the cranium. The postcranial skeleton is, however, fairly normal for a cetacean.

Lastly, most species of Herpetocetus would have been in the ballpark of 4-5 meters in length, while H. bramblei, the largest known species, would have been in the 5-6 meter range. Still, in American, thats about 15-18 feet, for a large Herpetocetus. Most modern mysticetes occur in the 10 meter-plus range (again, 30 feet or more in American). There are a couple of smallish freaks, such as the Dwarf Minke Whale (an unnamed subspecies of Balaenoptera acutorostrata) and the Pygmy Right Whale (Caperea marginata), in the 6-8 meter and 4-6 meter range, respectively.

Another odd feature about fossil mysticetes is the tendency towards small size with respect to extant mysticetes. For one, this could be climatically controlled (large mysticetes are pretty much relegated to cold, nutrient rich waters much of the year). This also, however, could be sampling bias; several authors have suggested that because the majority of fossil mysticete crania are small (e.g. Herpetocetus), then fossil mysticetes probably were all that small (strongly paraphrased). However, the problem with this statement is that the skull of a 90 foot blue whale is over 12 feet long and 6+ feet wide; thats not exactly easy to excavate as a fossil, and not exactly easy to find complete, either.

Size in fossil cetaceans is worth a post all by itself; that'll come later.


Morgan said...

I kind of expect that "giant" mysticetes were probably fairly rare until recently, and it's an artifact of preservation. THat said, I have heard of some fragmentary stuff representing larger taxa, but I think most of it is scrap

Robert Boessenecker said...

So far, I'm not convinced one way or the other; an estimate of size evolution based on something like bizygomatic width of mysticete skulls is probably going to be faulty, based on the relative rarity of mysticete skulls to begin with.

Mainly since they appear to have no other use... and since they are far, far, FAR more common, it could be more useful to look at samples of mysticete vertebrae from the latest miocene and Pliocene. I'm sure that vertebral size gives a less accurate guage of body size, but the potential sample size is larger by several orders of magnitude.

Admin said...

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