Let’s face it, all the frickin’ weird cetacean skulls we’ve looked at so far have belonged to frickin’ weird cetaceans: sperm whales and river dolphins. Time for something less frickin’ weird, though still frickin’ weird, if you get my meaning. It’s a boring old dolphin. But is it just a boring old dolphin? No, of course not.
Here is the skull of Orcaella brevirostris [the skull shown here is of USNM 199743, image © Smithsonian United States National Museum, courtesy C. McHenry]. Variously termed Irrawaddy or snubfin dolphins, the Orcaella species are entirely tropical and restricted to the coastal waters of south-east Asia and northern Australasia (they might also occur around the Philippines. While known from the southern and western coasts of New Guinea, they may also occur around the northern and eastern coasts). They also inhabit the major rivers in their range, and members of some populations spend their entire lives in freshwater. A population of large animals with unerupted teeth inhabit two freshwater lakes on Borneo and have been intimated at times to represent a distinct taxon…
The rostrum of Orcaella is very short for a delphinid while the braincase is particularly large. The orbital margins are robust and the mesethmoid (which form a vertical plate-like structure between the premaxillae) is particularly large and obvious and sports a dorsal crest.
The big, wide braincase and short rostrum of Orcaella make it look decidedly paedomorphic, and this is supported by the fact that adults exhibit several skull roof characters not normally present in adult delphinids. The frontals still exhibit an extensive dorsal exposure, as do the parietals, for example (Marsh et al. 1989, Stacey & Arnold 1999). O. heinsohni differs from O. brevirostris in usually possessing three ossicles in the nasal region as opposed to two (I say ‘usually’ as the number of nasal ossicles varies in O. heinsohni from 0 to 6), and in having an accessory ossification between the nasals and the posterior border of the mesethmoid (Beasley et al. 2005)
What makes Orcaella particularly interesting (in my opinion) is that there has been much debate as to exactly what it is. Conventionally regarded as a delphinid, Nishiwaki (1963) thought that it was weird enough to get its own ‘family’, Orcellidae [sic], and there were a few suggestions in the 1970s and 80s that it might be a monodontid, and specifically a close relative of the beluga Delphinapterus. Kasuya (1973) even split the monodontids up, classifying Orcaellinae (for Orcaella) and Delphinapterinae (for Delphinapterus) within the new ‘family’ Delphinapteridae.
Today, morphological and genetic data have confirmed that Orcaella is a delphinid, but what sort of delphinid? Its short, blunt rostrum and broad braincase have let to suggestions that it must be allied with the pilot whales, false killer whales and relatives (the globicephalines). May-Collado & Angnarsson (2006) recovered Orcaella as the sister-taxon to Orcinus, the killer whales, in which case it’s a member of the delphinid clade Orcininae. If this is correct, then the closest living relative of the killer whale is a blunt-nosed, paedomorphic little cutie (which is, as you can see from the adjacent photo, good at squirting water).
And I’ve decided to go for the definition of ‘week’ that incorporates seven, rather than five, days, so just one more to go. Of course, this all meant missing out on the Montauk monster, but you can’t have everything.
Refs – –
Beasley, I., Robertson, K. M. & Arnold, P. 2005. Description of a new dolphin, the Australian snubfin dolphin Orcaella heinsohni sp. n. (Cetacea, Delphinidae). Marine Mammal Science 21, 365-400.
Kasuya, T. 1973. Systematic consideration of Recent toothed whales based on the morphology of the tympanoperiotic bone. Scientific Report of the Whales Research Institution 25, 1-103.
Marsh, H., Lloze, R., Heinsohn, G. E. & Kasuya, T. 1989. Irrawady dolphin Orcaella brevirostris (Gray, 1866). In Ridgway, S. H. & Harrison, R. (eds) Handbook of Marine Mammals Volume 4. Academic Press (London), pp. 101-118
May-Collado, L. & Angnarsson, I. 2006. Cytochrome b and Bayesian inferences of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344-354.
Nishiwaki, M. 1963. Taxonomical considerations on genera of Delphinidae. Scientific Report of the Whales Research Institution 17, 93-103
Stacey, P. J. & Arnold, P. W. 1999. Orcaella brevirostris. Mammalian Species 616, 1-8.