by Greg Mayer
In my post on the genitalia of ratfish (which are shark relatives), I noted that although no extant sharks had similar structures, some fossil ones did, so here are what two species of these sharks looked like. Both are members of the family Stethacanthidae, known for its sexual dimorphism.Fig. 1. Female (above) and male Falcatus falcatus (Carboniferous of North America). Note the pelvic claspers on the male, and the roughened denticles atop the head, as well as on the head clasper
How exactly the male ratfish uses his head clasper during mating is obscure (at least to me), and the use of the head clasper (the spine of the first dorsal fin) in the shark Falcatus would also be obscure, except that a pair has been found fossilized in flagrante delicto, the female grasping the head clasper in her mouth, her body parallel to and above the male’s. There would have to be more to their mating than this to bring the male’s pelvic claspers in to position, but it does provide at least a partial picture of mating and courtship in this fossil species. A nice photo of the fossil pair is at the fine website on Fossil Fishes of Bear Gulch maintained by Richard Lund and Eileen Grogan. They also have a photo of the rather similar Damocles serratus (presumably so named because its own sword [clasper] was always hanging over its head).
Equally bizarre is Stethacanthus, with a brush-like set of denticles atop the first dorsal fin, a large first dorsal fin spine, and roughened denticles atop the head. It’s not clear exactly how, or for what, this structure was used, but the fact that it occurs only in males, and that the related Falcatus (and almost certainly Damocles as well) used a similar structure in mating, suggests some sort of sexual behavior function.
Fig. 2. Male (to left) and female Stethacanthus altonensis (Carboniferous of North America).
Lund and Grogan provide further discussion and illustration at their website, and one of Lund’s papers is in the American Museum of Natural History’s digital library of its scientific publications. Matt Celeskey at the Hairy Museum of Natural History has reconstructions of both Falcatus and Stethacanthus.
Filling a demand that I didn’t know existed, in the mid 1990’s two excellent and well-illustrated popular accounts of the history of fishes were published, both emphasizing the fossil record. WEIT readers should enjoy both; Long has more on these odd sharks and ratfish.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore.
Maisey, J.G. 1996. Discovering Fossil Fishes. Henry Holt, New York.