Stechacanthus altonensis
Stethacanthidae, a large group of predatory elasmobranchs that lived from the Upper Devonian into the Pennsylvanian, and are known as fossils from Montana to Moscow, Russia. S. altonensis ranged to 5 feet in length.
The body lacked scales except for those on the top of the head and the crest of the first dorsal fin of males.The pectoral whip was widened by proteinaceous rays for better control of maneuvering and they had an almost symmetrical high aspect ratio tail, which would make them good crusiers with significant maneuverability.
The stoutly braced jaws bore large, sharp cusped and many pointed cladodont teeth on well-muscled and well-braced jaws. They were major apex predators. We assume from their geographic and temporal range, and apex predator role, that they may have been long-distance migratory cruisers.
Dorsal fin spine
S. altonensis, like the other stethacanthid sharks, displayed conspicuous sexual dimorphism. Males had large spike-like denticles on top of head and the crest of the highly modified first dorsal fin. The fin and spine of S. altonensis are back-to-back upright triangles with the dorsal fin movably hinged to the rear margin of the spine by a tongue-in-groove joint. (The illustration of the spine is a redrafting of the original drawing of the type specimen). The spine itself was not movable, being mounted deep between the epaxial muscles. Females lacked the fin and the spine, as well as any scales at all.
It is likely from the fossil remains we have found in the Bear Gulch Bay that this bay may have been a pupping ground (nursery ground) for large stethacanthids as well as a stopover for feeding during migratory cruising.
Reference:
- Lund, R. 1974. "Stethacanthus altonensis (Elasmobranchii) from the Bear Gulch Limestone of Montana." Ann. Carnegie Museum 45: 161-178.
- Lund, R. 1985. "Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana." Amer. Mus. Nat. Hist. Novitates, 2828: 1-24.