If the occasional Caperea really does have a supernumerary dorsal fin, it would only be a minor anomaly compared to the skeletal madness within:
Caperea has vertebrae counts and proportions that are strikingly different from any other whale. Cetaceans have four types of vertebrae: cervical (neck), thoracic (with ribs), lumbar, and caudal (tail, with chevrons sticking out below); unlike most mammals, there are no sacral vertebrae, which articulate with the hips. Perhaps the most striking difference between Caperea and the Fin Whale (Balaenoptera physalus) is the relative size of the ribcage. Caperea has 17 to 18 thoracic vertebrae, more than any other cetacean, but not much more than Fin Whales, which have 14 to 15 (Buchholtz 2010, True 1904). The extra length of the ribcage is thus mostly due to the elongation of the thoracic vertebrae themselves (Buchholtz 2010) and as a result, the relationship between vertebrae count and length is unlike that of any other cetacean (Buchholtz 2007). Another striking trait of Caperea is the very low number of lumbar vertebrae, with most individuals having one and one individual having none (Buchholtz 2010). In other words, Caperea has a tail coming (almost) straight out of its ribcage. Comparatively, Fin Whales have 14 to 16 lumbars (True 1904) and no other baleen whale has fewer than 10 (Tinker 1988). The River Dolphin Inia reportedly has as few as three lumbars, but it also has 13 thoracic vertebrae (Best & da Silva 1993), which is totally normal. It is likely there are some functional similarities shared between Inia and Caperea, but the proportions of Caperea reminded me more strongly of another aquatic mammal, and it’s not a cetacean.
Yes, a manatee, Trichechus manatus. Bear with me here. There are 17 to 18 thoracic vertebrae and 1 to 2 lumbars (Buchholtz et al. 2007), which overlaps with Caperea. The thoracic vertebrae are also elongate (Buchholtz et al. 2007) and judging from the comparison above, it’s roughly to the same degree as Caperea. However, the patterning is not quite the same, since the longest vertebrae in Caperea are near the thoracic/lumbar/caudal region and those of the manatee are about mid-thoracic (Buchholtz et al. 2007; Buchholtz 2010). The ribs of both species are also quite wide, particularly the posterior ones. Unlike Dugongs, Manatees lack sacral vertebrae (Buchholtz et al. 2007). These are some curious parallels, and a purposefully ignorant reconstruction of Caperea as a whale-a-tee was all but inevitable:
Contrary to what hypothetical future (or alternate universe?) palaeontologists may think, Caperea doesn’t look like a manatee at all. It pretty much looks like a Minke with an arched jaw.
Not only does Caperea look nothing like a manatee in life, it also doesn’t obviously function like one, being oceanic and reportedly a fast swimmer (Kemper 2009). Caperea is reportedly highly flexible (Kemper 2009), as is Inia (Fish 2002), so this makes me wonder if lumbar reduction results in a more flexible body, and that perhaps Caperea and manatees achieved this through a similar mutation. As documented in the three-part series from Tet Zoo (Part 1, Part 2, Part 3) Caperea has other bizarre morphology not shared with other cetaceans or manatees including huge and overlapping transverse processes as well as ribs that appear curiously loosely-connected. As for why it has any of this morphology or would need to be flexible, I have no idea.
Te Papa’s Blog has lots of entries documenting the dissection of a juvenile Caperea, and it is really invaluable for seeing how the soft tissue and skeleton fit together. It’s certainly interesting that soft tissue doesn’t necessarily mean that animals were weirder than their skeletons would indicate, some externally look far more “normal” than they have any reason to.
Best, R. & da Silva, V. (1993) Inia geoffrensis. Mammalian Species 426, 1—8. Available
Bisconti, M. (2012) Comparative osteology and phylogenetic relationships of Miocaperea pulchra, the first fossil pygmy right whale genus and species (Cetacea, Mysticeti, Neobalaenidae). Zoological Journal of the Linnean Society 166(4) 876—911. Supplement available
Buchholtz, E. (2010) Vertebral and rib anatomy in Caperea marginata: Implications for evolutionary patterning of the mammalian vertebral column. Marine Mammal Science. Available
Buchholtz, E. et al. (2007) Vertebral anatomy in the Florida manatee, Trichechus manatus latirostris: a developmental and evolutionary analysis. Anatomical Record 290(6) 624—637.
Buchholtz, E. (2007) Modular evolution of the Cetacean vertebral column. Evolution & Development 9(3) 278—289. Available
Fish, F. (2002) Balancing Requirements for Stability and Maneuverability in Cetaceans. Integrative and Comparative Biology 42(1) 85—93. Available.
Kemper, C. (2009) Pygmy Right Whale IN: Perrin, W. et al. (eds.) Encyclopedia of Marine Mammals.
Tinker, S. (1988) Whales of the World. Partially Available
True, F. (1904) The whalebone whales of the western North Atlantic. Smithsonian Contributions to Knowledge 33 1—332. Available