Friday, June 24, 2011
The Razorback Dolphin is a descendant of the Common Bottlenose Dolphin (Tursiops truncatus) which has converged upon 'River Dolphins'. This isn't as unlikely as might be supposed as T. truncatus is not just pelagic, but also occurs in bays, estuaries, and the lower reaches of rivers (Reeves et al. 2002); individuals even live in the Rio de Ais (Stolen and Barlow 2003). Like the ancestral species, this species has normal-sized eyes, non-paddle-shaped flippers, and a falcate (first) dorsal fin; the beak length, large melon, and body shape appear closer in morphology to Inia or Platanista. I could imagine some rather strange things happening internally in the melon region - it's the cetacean thing to do. Taken together, these features suggest an animal adapted for riverine habits which is still capable of competing in brackish and near-shore marine waters. Ais, after all, does not have an extensive river system which can support an entire species, maybe a sub-population at best.
And there are dorsal finlets. Presumably these formed through a similar genetic pathway as the thunniform dolphin's extra fins despite the radically different body plans and habitats of the species. It's tempting to give the Razorback and Thunniform dolphin a common T. truncatus ancestor, although I suspect these would be too neat and tidy for evolution.
The primary inspiration for this species is once again a cryptid, the Sawtooth Dolphin observed by Jeremy Wade. The individual probably got its appearance from an injury, and I could imagine the coincidental appearance of the Razorback Dolphin causing some confusion for whoever is exploring Ais.
Reeves, R. R., Stewart, B. S., Clapham, P. J., and Powell, J. A. (2002) National Audubon Society Guide to Marine Mammals of the World. Alfred A. Knopf: New York.
Stolen, M. K., and Barlow, J. (2003). A model life table for Bottlenose Dolphins (Tursiops truncatus) from the Indian River Lagoon System, Florida U.S.A. Marine Mammal Science 19(4), 630-649. Available.