Tetrapod Zoology

Blunt-nosed paedomorphic cutie

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.

i-2934f1d4e67a8857157095fdb2391ccb-Orcaella C. McHenry left lateral view.jpg

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…

i-6e14ec63b274693a096745087edc650b-Orcaella C. McHenry dorsal view.jpg

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.

i-b3a909216c6c69c9099621afa5f2008c-Orcaella good at squirting water.jpg

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.

Comments

  1. #1 chris y
    August 2, 2008

    Did you see the movie yesterday? They’re great little critters, aren’t they?

    http://news.bbc.co.uk/1/hi/world/asia-pacific/7536830.stm

  2. #2 Graham King
    August 2, 2008

    Neat! Dolphin/cetacean skulls I think must be among those skulls with the greatest capacity to surprise/mystify onlookers familiar only with the outward, live appearance of the creatures.
    It just goes to show how important soft tissue can be in shaping the ‘look’ of an animal.

    Globicephaline means ball-head, basically, yes?

    In the Scots vernacular, ‘baw-heid’ is an insult, disparaging one’s wits (or rather, lack thereof).
    Odd, that, since the more bulbous the cranium, the higher the degree of encephalisation (braininess) one might infer.

    But in cetaceans, of course, their bulbous forehead profile is due to the ‘melon’ – not brain tissue.

    Yet cetaceans are brainy among mammals.

    It’s all very confusing.

    (But it’s comforting to know that there are still more cetacean forms than I knew.)

  3. #3 Shadow
    August 2, 2008

    And I’ve decided to go for the definition of ‘week’ that incorporates seven, rather than five, days

    Hurray!

    Of course, this all meant missing out on the Montauk monster, but you can’t have everything.

    I was wondering if you’d be weighing in on that. *Snaps fingers* I had my mother in here to have a look at it this morning. Her first reaction was a pleasing, “What the hell is that?” – and then we had a good time tossing around the various hypotheses and pointing out all the things that Weren’t Quite Right.

  4. #4 Dallas Krentzel
    August 2, 2008

    Is that slender protrusion of bone from the squamosal to (what looks like) the maxilla the jugal bone? I think I see the same structure in the Kogia skull but it’s hard to tell.

  5. #5 Darren Naish
    August 2, 2008

    Is that slender protrusion of bone from the squamosal to (what looks like) the maxilla the jugal bone?

    Yes, that’s the jugal. It’s delicate and weakly attached (to the lacrimal anteriorly and zygomatic process of the squamosal posteriorly) and is frequently lost in skulls (I mean lost during the taphonomic process, not lost in the phylogenetic sense).

  6. #6 Dallas Krentzel
    August 2, 2008

    So then, in the river dolphins is the jugal still supposed to be fused to the squamosal and in those particular specimens was only lost taphonomically?

  7. #7 Darren Naish
    August 3, 2008

    The jugal isn’t fused to the squamosal in any crown-group cetaceans (so far as I know), but it is fused to the lacrimal in lots of odontocetes (though this doesn’t stop it getting lost during decomposition). Among river dolphins, Lipotes, Pontoporia and Inia possess a splint-like jugal like that of delphinids. Platanista differs in having a more robust jugal: this is almost certainly the primitive condition among odontocetes as it’s present in basal whales, mysticetes, sperm whales, and basal ziphiids (among others). Again, however, the jugal is often lost in specimens and is missing from the specimen you’ve been looking at here at Tet Zoo: an attached jugal can however be seen in this specimen. This should help show how tiny the orbits are in this taxon.

  8. #8 Hai~Ren
    June 23, 2010

    Requesting help from Darren and other Tet Zoo folks to identify a carcass that washed up on a beach in Singapore.

    http://singaporeseen.stomp.com.sg/stomp/3552/4124/399582

    The person who took the photos once again demonstrates some of the major mistakes where it comes to documenting these carcasses – no close-ups of any unique identifying features such as fins, flippers or tail flukes, no shots from multiple angles, and worst, no measurements, no estimate of size, and nothing to show a sense of scale at all!

    I suspect it’s either Irrawaddy dolphin (Orcaella brevirostris) or finless porpoise (Neophocaena phocaenoides), but I hope someone more familiar with the anatomy of these species can give a more informed guess. These are the only 2 inshore small cetacean species here in Singapore that lack a beak; the Indopacific humpbacked dolphin (Sousa chinensis is more commonly seen but has a very distinct beak). Based on the presence of tiny tooth sockets at the front of the lower jaw, I doubt that it’s a dugong.

    It seems that no effort was made to contact the authorities or at least get the Raffles Museum of Biodiversity Research to come collect the carcass, so the fate of the carcass is currently unknown.

  9. #9 Zoee Kookers
    March 17, 2011

    These dolphins are endangered but, so CUTE!

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