Saturday, May 28, 2011

Dorudon Was Not A Monster

The external shape of cetaceans is very much defined by blubber and other soft tissues†. In a previous article, I argued that if a cetacean were to be naïvely reconstructed by what the skeleton (or rotten carcass) 'suggests', it could end up looking more like a reptilian horror than, say, a fat, charismatic monodontid we all know and love. It's below the monstrous footnote.

† But don't just take my word for it - the Woods Hole Oceanographic Institution has excellent CT scans showing the interplay between skeleton and external shape. Aside from the caudal peduncle and (occasionally) the tip of the snout, toothed whales are cocooned in blubber. The heads of the False Killer Whale and Narwhal provide sufficiently extreme examples. Contrarily, the Minke Whale has a skull which roughly correlates with the external shape... in a dorsal view; shrink-wrapping the skull at a different angle shows that soft tissue still plays a considerable role in determining overall shape. Dorudon probably looked a great deal more like toothed than baleen whales, however more basal 'baleen' whales (stem-Mysticeti) lacking the hyper-derived skull are potentially very informative. Thanks to Markus Bühler for the link.

This intentionally incompetent Beluga bears an unexpected similarity to some reconstructions of Dorudon... notwithstanding the blowhole and fur, of course. This is partially due to the offending illustrations depicting Dorudon atrox with almost no blubber, which makes as much sense as reconstructing a fossil bird without feathers. The other factor is that skeletally, Dorudon is broadly similar to modern toothed whales, despite being basal to the toothed/baleen whale split:

Delphinapterus leucas skeleton from Wikipedia Commons.
Dorudon atrox2, taken and modified from Wikipedia Commons. Note that the arm is held at an angle and was not, in fact, really really short.
White-sided dolphin, taken and modified from Wikipedia Commons.

Above Dorudon is a Beluga, which is similar in size and also has non-fused neck vertebrae (Uhen 2004). What I find particularly striking is the similar depth of the ribcages and the comparatively short spinous processes of Dorudon. Beaked whales also have non-fused neck vertebrae and Ziphius in particular has been compared in size to Dorudon (Uhen 2004) - judging by this photo of Ziphius, the species also has a deep ribcage and relatively enormous spinous processes†. Below Dorudon is a Lagenorhynchus dolphin (either L. acutus or L. obliquidens) which has numerous highly derived characteristics (Buchholtz and Schur 2004), and thus makes for strong contrast. The ribcage seems relatively streamlined and shallower and the spinous processes of the vertebrae are extremely developed. There's still a broad similarity between Dorudon and that highly derived taxon, which makes portrayals of Dorudon as some anguilliform quasi-reptilian horror appear increasingly bizarre.

† Aside from which, the lumbar/anterior caudal region gives off a strong Basilosaurus vibe due to the elongated vertebral bodies and lack of interlocking processes. Hmm.

So why have I been talking so much about Dorudon atrox as opposed to D. serratus, Chrysocetus, Ancalecetus, or some other 'dorudontine'? Dorudon atrox is the best-known 'archaeocete', and at present "[r]elationships among the Dorudontinae are not well-defined, either by morphology or stratigraphy... [i]n addition, the relationships among the Dorudontinae cannot be determined until the taxa within the Dorudontinae are clearly delimited" (Uhen 2004). Additionally, it's become apparent that I've been citing Uhen (2004) quite a bit so far, and that source just so happens to be a massive, book-length treatise on D. atrox which is freely available. The publication is outstanding... aside from the frontispiece, which was credited as being made in cooperation with the author, but seems to contradict several points made within the publication and looks more like a zombie than a fairly close relative of extant cetaceans.

I think I can do Dorudon a bit more justice... next post.

Well, I've actually already done it for the banner - but the explanation will be in the following post! Which won't be in a month, I swear.


Buchholtz, E. A., and Schur, S. A. (2004). Vertebral osteology in Delphinidae (Cetacea). Zoological Journal of the Linnean Society 140, 383–401. Available.

Uhen, M. D. (2004). Form, Function, and Anatomy of Dorudon atrox (Mammalia, Cetacea): An Archaeocete from the Middle to Late Eocene of Egypt. University of Michigan Papers on Paleontology 34, 1-222. Available.


Allen Hazen said...

Those see-through scanned whale heads you link to are BEAUTIFUL!!!! Thank you for pointing them out to me!
A lot of the head above the bony rostrum in many modern odontocetes is the melon. Do you (does anybody) know at what stage this evolved? Assuming Dorudon was basal to the Odontocete/Mysticete split, it probably didn't have one. Which is consistent with your top drawing.

Cameron McCormick said...

Some of the other taxa are certainly worth checking out. Even some amphibians have their skeletons encased deeply within soft tissue.


The 'splashguard' of mysticetes and melon of odontocetes are apparently homologous; since Janjucetus apparently couldn't echolocate, it is most probable that melons occur solely in odontocetes. I gave the banner illustration some sort of proto-splashguard considering how close Dorudon is to the cetacean crown-group... it is admittedly hard to picture what is going on in that general region.

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