The morphology ofFalcatus falcatus(St. John and Worthen), a Mississippian stethacanthid chondrichthyan from the Bear Gulch Limestone of Montana

Abstract

ABSTRACT The Paleozoic fin spine Physonemus falcatus St. John and Worthen 1883, from the Valmeyeran St. Louis Limestone of St. Louis, Missouri, has been found on sexually mature males of a small, highly sexually dimorphic chondrichthyan from the Chesterian Bear Gulch Limestone of Montana. An amphistylic suspensorium, cladodont teeth and pectoral fin further characterize this species and the group to which it belongs. The first dorsal spine and fin of males are derivable from a form like that of Stethacanthus altonensis. Males also have elongate rostra and dorsal cranial and fin denticles, while females and juveniles lack denticles, first dorsal fin, or spine. The new genus Falcatus (Order Cladodontida, Family Stethacanthidae) is erected for the species and for the related F. hamatus and F. proclivus. Aspects of the sexual dimorphism are discussed; Falcatus falcatus, with many more males than females, fits a behavioral model of male display-courtship and female choice. Possible evidence of lek aggregations...