Tetrapod Zoology

Welcome to day 2 of seriously frickin’ weird cetacean skull week, and here we look at one of my favourites: Platanista, the Asian river dolphins or susus. Susu is a Hindi onomatopoetic name based on the exhalation noise these dolphins make, and other local names include susuk, sishuk, shushuk and sishumarch. There are two species: the Indus river dolphin P. minor Owen, 1853 of the Indus and Chenab in Pakistan, and the Ganges river dolphin P. gangetica Roxburgh, 1801 of the Ganges, Meghna and Brahmaputra of India, Nepal, Bhutan and Bangladesh, the Karnaphuli in Bangladesh, and (possibly) the Sangu in Bangladesh and the Chinese part of the Brahmaputra (Reeves & Brownell 1989, Carwardine 1995). Both species are critically endangered thanks to pollution, hunting, and damming and resulting population fragmentation.

i-4210cb5d86df53a00516f2e5ebb6a62f-Platanista gangetica BMNH unnumbered right lat.jpg

And, of course, their skulls are very, very weird…

It’s well known that the eyes of Platanista are very small, have degenerate optic nerves and lack a lens, and it’s thought that Platanista is essentially blind, though still able to distinguish light from dark. This is reflected in the skull, as the orbits are tiny while the zygomatic processes of the squamosals are comparatively huge and bulky [the skull shown here is of a BMNH P. gangetica specimen, image © Natural History Museum (London), courtesy C. McHenry]. The elongate, slender rostrum is laterally compressed and widest at its tip. It’s apparently proportionally longer in females than in males (making up to 20% of the length of the animal): another example of sexual dimorphism similar to what we recently looked at in birds. Because, in the first detailed account of Platanista in life, John Anderson wrote in 1878 of finding plant and insect fragments in the stomach, it became assumed in the following decades that Platanista made a living by grubbing around in the mud. Some even proposed that it pushed its long rostrum into sediment like a wading bird. It doesn’t do this (so far as we know), but grabs fishes, shrimps and crabs either from the water column or from the bottom.

A captive P. gangetica killed a goose and a wild one was seen to attack a goose (Ellis 1982, Sylvestre 1990), though I’d like to know exactly what ‘goose’ means here (the label ‘goose’ is applied to small, duck-sized species as well as to mid-sized and large species). Because of its bird-eating proclivities, live herons are apparently used as bait by fishermen when hunting P. minor (Sylvestre 1990). The jaws can be opened very wide, apparently at an angle approaching 90°; (Watson 1981), and can be snapped shut very rapidly. The teeth at the anterior ends of the jaws are long and slender and extend beyond the jaw edges when the mouth is closed, while those toward the back of the jaws are short and stout. The lower jaw teeth are longer than the uppers. These teeth aren’t just used to grab fishes and crustaceans (they’re reported to eat fishes of up to 35 cm in length), but might also be used in predation on turtles and, as we’ve seen, birds. The palate of Platanista is odd in that the palatines don’t meet along the mid-line, as is normal, but are separated [look at the bottom left image in the composite shown here: it shows the skull and jaws in palatal view; you can see the separated palatines along the midline, in between the rami of the lower jaws]. I don’t know why.

i-3fa5a58a32ded9b8539f1218979e3637-Platanista skull composite resized.jpg

The oddest and most distinctive features of the Platanista skull are the two massive, ragged-edged maxillary crests that project anteriorly over the facial region and virtually encircle the melon [in composite above, anterior view of skull shown at bottom right]. They’re bigger in P. gangetica than in P. minor. What the hell are these structures? Asymmetrical and skewed to the left, their ventral surfaces are covered by thin, flat air sacs that seem to have grown dorsally from the pterygoid air sinus system (Fraser & Purves 1960, Berta & Sumich 1999). Purves & Pilleri (1973) proposed that the crests might function in focusing sound out from the melon, performing in a manner analogous to that of a megaphone or a pair of cupped hands around the mouth. As near-blind dolphins that rely on echolocation more than the majority of other odontocetes, it is conceivable that Platanista might have evolved hypertrophied sound-transmitting and/or sound recepting structures, but so far as I know this purported function remains speculative. I’m unaware of other any work that includes speculations on the function of the crests, but that doesn’t mean that it doesn’t exist.

Oh, and Platanista is also weird in often swimming on its side, in having a slit-shaped blowhole, and in having females that are markedly larger than the males.

The squalo-susu hypothesis

i-3485acd46a771176317f2866cf951709-Zarhachis from Kellogg resized.jpg

I can’t leave Platanista without commenting briefly on its inferred relationships. While the modern river dolphins of southern Asia, China and South America were traditionally regarded as close allies, morphology and genetics indicate that they represent several independent freshwater invasions (Cassens et al. 2000, Hamilton et al. 2001, May-Collado & Angnarsson 2006). Platanista shares characters with the squalodontids, squalodelphids, dalpiazinids and kin of the Oligocene and Miocene, and has been hypothesised by some to be the last survivor of this once diverse group, conventionally termed Squalodontoidea but more properly termed Platanistoidea (Muizon 1987, 1994, Fordyce 1994). Geisler & Sanders (2003) termed this the ‘squalo-susu’ hypothesis (and failed to support it, incidentally). Close relatives of Platanista – the pomatodelphinines – were swimming the Miocene seas of eastern North America and Europe and include some of the most spectacular of fossil odontocetes, like swordfish-snouted Zarhachis [shown here] from Maryland. Pliny the Elder was referring in the 1st century to a large ‘fish’ in the Ganges that he called the platanista, but not until 1801 did these amazing animals become known to science. It will soon be the end of the line for what was once a widespread and important group.

i-fafad505edf579b8c5eeac853f374bb6-Odontoceti cladogram resized to be so small as to be useless.jpg

Here is a very simplified ‘consensus’ cladogram of Odontoceti, incorporating data from studies that support the ‘squalo-susu’ hypothesis. Some of the relationships shown here have since been contested by Geisler & Sanders (2003).

More tomorrow…

Refs – -

Berta, A. & Sumich, J. L. 1999. Marine Mammals: Evolutionary Biology. Academic Press, San Diego.

Carwardine, M. 1995. Whales, Dolphins and Porpoises. Dorling Kindersley, London.

Cassens, I., Vicario, S., Waddell, V. G., Balchowky, H., Van Bell, D., Ding, W., Fan, C., Lal Mohan, R. S., Simões-Lopes, P. C., Bastida, R., Meyer, A., Stanhope, M. J. & Milinkovitch, M. C. 2000. Independent adaptation to riverine habitats allowed survival of ancient cetacean lineages. Proceedings of the National Academy of Sciences 97, 11343-11347.

Ellis, R. 1983. Dolphins and Porpoises. Robert Hale, London.

Fordyce, R. E. 1994. Waipatia marewhenua, new genus and new species (Waipatiidae, new family), an archaic Late Oligocene dolphin (Cetacea: Odontoceti: Platanistoidea) from New Zealand. Proceedings of the San Diego Society of Natural History 29, 147-176.

Fraser, F. C. & Purves, P. E. 1960. Anatomy and function of the cetacean ear. Proceedings of the Royal Society B 152, 62-77.

Geisler, J. H. & Sanders, A. E. 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution 10, 23-129.

Hamilton, H., Caballero, S., Collins, A. G. & Brownell, R. L. 2001. Evolution of river dolphins. Proceedings of the Royal Society, London B 268, 549-556.

May-Collado, L. & Angnarsson, I. 2006. Cytochrome b and Bayesian inferences of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344-354.

Muizon, C. de 1987. The affinities of Notocetus vanbenedeni, an Early Miocene platanistoid (Cetacea, Mammalia) from Patagonia, southern Argentina. American Museum Novitates 2904, 1-27.

- . 1994. Are the squalodonts related to the platanistoids? Proceedings of the San Diego Society of Natural History 29, 135-146.

Purves, P. E. & Pilleri, G. 1973. Observations on the ear, nose, throat and eye of Platanista indi. Investigations on Cetacea 5, 13-58.

Reeves, R. R. & Brownell, R. L. 1989. Susu Platanista gangetica (Roxburgh, 1801) and Platanista minor Owen, 1853. In Ridgway, S. H. & Harrison, R. (eds) Handbook of Marine Mammals, Volume 4. Academic Press, pp. 69-99.

Sylvestre, J.-P. 1993. Dolphins & Porposes – A Worldwide Guide. Sterling Publishing, New York.

Watson, L. 1981. Whales of the World. Hutchinson, London.

Comments

  1. #1 Hai~Ren
    July 29, 2008

    Platanista as a close relative of the squalodontids? I had no idea!

    Different sources seem to differ on whether there are two separate species, or a single species distributed over two major South Asian river systems.

    Like you, I would be very interested to see what sort of ‘geese’ were attacked by this dolphin, whether these were actual Anser or some other much smaller anatid, like say, Nettapus, which are known as ‘pygmy geese’. And likewise, I’m very curious as to what sort of herons would have been used as bait – actual Ardea or Egretta? Or the much smaller Butorides or Ardeola. In any case, I would think herons make very dangerous prey, even if a dolphin were able to sneak up on one.

    Given the predatory habits, this looks like a cetacean that’s really trying quite hard to be a crocodilian, even if it’s all but blind and relies on sonar to navigate.

    And man, that skull is just frickin’ weird… I still find it so unbelievable that all this bizarre cranial anatomy evolved in a lineage that apparently has their roots among the artiodactyls.

  2. #2 Sordes
    July 29, 2008

    Very interesting post. When I saw for the first time a skull of a Ganges River dolphin in the Museum Schloss Rosenstein (Stuttgart) I was really amazed by its absolute weirdness. The bird-hunting behavior is also really cool, I have never heard about it before, but I already speculated that at least Amazon river dolphin include besides fish and small turtles a lot of other animals in their diet.
    I have also a photo of the skull from Stuttgart, the photo is a bit brighter and shows possible some of the anatomical features better:
    http://www.petermaas.nl/extinct/gallery/displayimage.php?album=19&pos=6
    Here is also a photo of a model of a River Dolphin (I am not sure if it was an Indus or Ganges dolphin): http://www.petermaas.nl/extinct/gallery/displayimage.php?album=19&pos=5

  3. #3 JW Tan
    July 29, 2008

    Anyone know if there’s any hope for staving off extinction?

  4. #4 Colin McHenry
    July 29, 2008

    Great blog, as usual. And isn’t it just amazing that the longest snouted living cetacean lives in the same river system as the longest snouted living crocodilian?

    Speaking of river dolphins, is *Lipotes* gone for good?

  5. #5 Leonardo
    July 29, 2008

    …I know it’s not about dolphin…sorry, but I need yuor help ;-)

    On my blog
    [geomythology.blogspot.com] there’s an opinion poll about dinosaur/infancy for a sociological survey (quantitative informations)…
    I’m writing some notes about the relation between inconscious/children/dinosaurs and mythical archetypes…well, I’m looking for some help!

    After its dead-line, you’ll find a discussion of the results…
    [if possible spread this info; thanx a lot for your efforts; I'm also looking for souvenirs/recollections, things remembered from the past infancy 'en liason' with first dinosaur ever seen (e.g. skeleton, book, newspaper, etc)...Any kind of info is kindly appreciated; you can post it on the comments' side of the last post. Thanx once more]

    Leonardo

  6. #6 Andreas Johansson
    July 29, 2008

    Oh, and Platanista is also weird in … having females that are markedly larger than the males.

    I was, possibly for some reason, of the impression this was the norm in cetaceans. How common/uncommon is it more precisely? Is it a mysticete-odontocete thing? – looking in Wikipedia females seem to be larger in all mysticetes whose articles address sexual dimorphism.

  7. #7 Sordes
    July 29, 2008

    In mysticeti the females or usually larger than the males, but in odntoceti it is normally the oppostite. You can find this in a extreme in sperm whales, the males can grow to more than two times of the length of the females. In beaked whales the females are sometimes larger than the males, females of Baird´s beaked whale are for example the second largest of all toothed whales and are even longer than female sperm whales.

  8. #8 Andreas Johansson
    July 29, 2008

    Thanks!

  9. #9 Cameron
    July 29, 2008

    Live herons, wow. I’m wondering how Platanista recognizes a wader as a food source.

    Hyperoodon ampullatus males also have maxillary crests that nearly or entirely encircle the functional melon, but since Platanista males lack an enormously enlarged forehead and are smaller than females they seem to be unlikely headbutters. Maybe they have another reason to protect their functional melon. Do any other cetaceans have encircling maxillary crests?

    Sordes, the median length for Baird’s beaked whale is around 10 m, which I think is about the same as a female sperm whale. The 12.8 m record for B. bairdii seems exceptional and MacLeod’s paper on beaked whale size (well, the one attached to his thesis) lists the known maximum at 11 m. I haven’t seen any mass estimates that I consider reliable for that species.

  10. #10 Nathan Myers
    July 29, 2008

    A large, rigid, well-innervated structure in a water creature practically spells out “phased array sonar”. I’m betting the rostrum is involved too.

    The narwhal tusk must have a similar sensory use.

  11. #11 Boesse
    July 30, 2008

    Great post – I definitely rank Platanista as one of the weirdest cetaceans of all time. It also has another closely related fossil relative, also from the Calvert Formation, just like Zarchachis: Araeodelphis natator. Stephen Godfrey recently gave a pretty neat talk on new material of Araeodelphis, at the Neoceti Symposium at SVP ’06. Araeodelphis also had those strange maxillary crests.
    I wish I had a better image of Zarchachis – there are some in one of Kellogg’s later papers (1957 perhaps??), but the skull looks heavily reconstructed (not surprisingly, knowing Kellogg).

  12. #12 Darren Naish
    July 30, 2008

    I’d also like to see better pics of Zarhachis. While it’s been thought for a while that Zarhachis, Araeodelphis and Pomatodelphis are platanistids, Barnes (2002) proposed that Platanista is the sister-taxon to a platanistid clade he called Pomatodelphininae. Prepomatodelphis from Austria is also a pomatodelphinine, as is unnamed material from the Miocene of Belgium (Lambert 2006).

    Maxillary crests have been used as a synapomorphy of Platanistidae (Muizon 1987), though they aren’t as prominent in the pomatodelphinines as they are in Platanista.

    Refs – -

    Barnes, L. G. 2002. An Early Miocene long-snouted marine platanistid dolphin (Mammalia, Cetacea, Odontoceti) from the Korneuburg Basin (Austria). Beiträge zur Paläontologie 27, 407-418.

    Lambert, O. 2006. First record of a platanistid (Cetacea, Odontoceti) in the North Sea Basin: a review of Cyrtodelphis Abel, 1899 from the Miocene of Belgium. Oryctos 6, 69-79.

    Muizon, C. de 1987. The affinities of Notocetus vanbenedeni, an Early Miocene platanistoid (Cetacea, Mammalia) from Patagonia, southern Argentina. American Museum Novitates 2904, 1-27.

  13. #13 qbsmd
    July 30, 2008

    Swimming on their sides is not as surprising given the asymmetric bones around the melon. They could swim level, and sense fish more below them then they could otherwise. It would be interesting to see if the asymmetry has reached a stable position, or if it is in the process of evolving further. It would also be interesting to see if they point it out of the water to catch birds.

  14. #14 Sordes
    July 30, 2008

    Cameron, thanks for the additional information. I already knew that the 12.8m are a record, but in Hadoram Shirihai´s great book “whales dolphins and Seals” I found a medium length for female sperm whales of 9m, what would be even shorter than an average Baird´s beaked whale. Actually female sperm whales seem to attain sometimes weights of 20 tons what is much heavier than any beaked whale. But it is in any way really astonishing how large Baird´s beaked whale actually grows.

  15. #15 Graham King
    August 2, 2008

    St Andrews University Zoology Department is home to the Bell Pettigrew collection, a museum that’s well worth a visit and a passionate favourite of mine as a student.
    It held then (and I assume still does) a skull of Platanista gangetica, a name I’ve treasured now for a quarter-century, relishing its lovely sound.
    It also holds other cetacean skulls including that of a rare twin-tusked narwhal – having left AND right tusks, BOTH with a leftward twist (ie the tusks are similar, not mirror-image, helices). Like these

  16. #16 DDeden
    August 2, 2008

    I wonder if the new ‘snubby’ dolphin has a similar crest but lacks the long rostum.

    http://www.theaustralian.news.com.au/story/0,,24110275-30417,00.html?from=public_rss
    http://news.yahoo.com/video/2470;_ylt=AmWzVN2u1pFSVL4pvS_2v9WzvtEF
    (rare dophin… video)

  17. #17 Tom Ford
    August 13, 2008

    it is interesting to note the lack of teeth in the posterior of the lower jaw, while the upper jaw retains posterior teeth.

    That indicates an adaptation for the easier passage of whole prey items to the esophagus along the lower jaw while the upper teeth prevent escape of they prey. Neat.

    T.v Ford

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