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.
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.
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 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.
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).
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.