Well, here we are at the end of seriously frickin’ weird cetacean skull week. I hope you’ve all enjoyed it. We’re going to finish with a bang by looking at a few – yes, not one, but a few – of the real way-out-there oddballs among the odontocetes. We start with a famous freak individual…
If you’ve ever read anything about sperm whales Physeter macrocephalus you’ll have read the assertion that broken and deformed lower jaws have often been reported in members of this species. It’s nice to know this, but why are these broken and deformed lower jaws never figured? Here is perhaps the ultimate example: this is the lower jaw of a mature male, about 12 m long, harpooned in Antarctic waters in January 1959. The dentaries curve to the right, then form a spiral, and note that the teeth on the outside of the spiral have been worn down to stumps, or lost (Spaul 1964). This is not the only sperm whale individual in which a coiled lower jaw was reported, and there are also quite a few additional records of sperm whales with strongly bent, hooked or crooked lower jaws. These deformities have been hypothesised to have arisen in diverse ways: as heritable malformations, as the consequences of disease or malnutrition, or as pathologies that resulted from combat. The important thing is that the individuals that possessed these jaws were generally described as healthy and otherwise normal (such was the case for the individual shown here). This strongly indicates that, however sperm whales obtain food, they don’t need nice neat ‘normal’ lower jaws to do it. Exactly how sperm whales do obtain their food has been the topic of great debate, and it’s such an interesting area that I’m not about to begin to cover it now (sorry, I would if I had the time).
Moving on… if you’ve been paying attention you will have seen the bizarre walrus whales Odobenocetops mentioned here recently. It would be wrong not to look at least briefly at these incredible animals. O. peruvianus was described by Muizon (1993) from the early Pliocene part of the Pisco Formation and proved amazing in its combination of bizarre, decidedly un-cetacean combination of characters. Lacking both a rostrum and (almost certainly) a melon, its short skull had a strongly vaulted palate, dorsally facing orbits, anteriorly positioned nares, and posteroventrally projecting maxillary processes that housed tusks. The left tusk was much shorter than the right tusk (c. 25 cm vs over 55 cm). In a second skull discovered later on (Muizon et al. 1999), both tusks were short (c. 25 cm), suggesting that only males possessed the long right tusk. Sharing several derived characters with the monodontids (narwhals and belugas) and regarded as the sister-taxon to this group, Odobenocetops was essentially a ‘walrus whale’ that presumably used its vaulted palate and strong buccal musculature to suction-feed on the soft parts of benthic molluscs.
Even more incredible was the description of a second species, O. leptodon [shown here], that possessed much, much longer tusks (Muizon et al. 1999, 2002). Its left tusk was 25 cm long while the right one was a ridiculous 1.35 m long. O. leptodon is geologically younger than O. peruvianus and appears more specialised in tusk anatomy, in having a longer, broader palate, and in some other characters. However, O. leptodon was more primitive in still possessing a melon and in having orbits that did not allow the same degree of binocular visison as did those of O. peruvianus. It is inferred from these differences that neither species was ancestral to the other, but that both had specialised in distinct ways after diverging. The presence of long, walrus-like tusks in these bottom-feeding suction-feeders strongly implies that tusks act as guides, and are strongly beneficial for this lifestyle. Why the profound asymmetry in (presumed) males? It may be that the suction-feeding was only carried out on the short-tusked left side.
Incidentally, if you’re interested in tusked odontocetes remember to check out the Tet Zoo narwhal article here. I still have to come back to monodontids at some time.
Finally, if you’ve been reading the comments you will already know all about the bizarre, soon-to-be-published ‘half-beaked porpoise’ or ‘chinny-chin-chin porpoise’ or ‘skimmer porpoise’, currently being worked on by Rachel Racicot and colleagues. A Pliocene phocoenid from the San Diego Formation, it possesses an incredibly elongate, fused, toothless dentary symphysis that extends well beyond the tip of the rostrum. It also sports large bony bosses just anterior to the nares, but given that one of the diagnostic characters of Phocoenidae is the presence of convex paired premaxillary eminences just anterior to the nares, these bumps are most likely just hypertrophied versions of these.
But what the hell was the animal doing with that incredible lower jaw? Clearly, it was leaping out of the water, sustaining some sort of flight, and trawling its lower jaw though the water while gliding through the air. No, I am kidding. We have no idea (or I don’t anyway). We await the full description with great interest: an abstract on the taxon (Racicot et al. 2007) is all I’ve seen so far. The photo of the new taxon was taken by Doug Shore and is used with his permission.
So that’s that. For the time being we say goodbye to the Odontoceti. Now that we’re in August I need to start spending time getting geared up for conference season. I’m giving a talk at SVPCA in Dublin early in September, so had better get that ready, and continue the desperate quest for finance.
Refs – -
Muizon, C. de 1993. Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru. Nature 365, 745-748.
- . & Domning, D. P. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134, 423-452.
- ., Domning, D. P. & Parrish, M. 1999. Dimorphic tusks and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes Rendu de l’Academie des Sciences, Paris, Sciences de la Terre et des Planètes 329, 449-455.
Racicot, R., Deméré, T. & Rowe, T. 2007. Morphology of a bizarre new fossil porpoise (Cetacea: Phocoenidae) from the Pliocene San Diego Formation of southern California, USA. Journal of Vertebrate Paleontology 27 (supp. 3), 132.
Spaul, E. A. 1964. Deformity in the lower jaw of the sperm whale (Physeter catodon). Proceedings of the Zoological Society of London 142, 391-395.