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Exceptionally complete material of a new stethacanthid chondrichthyan, Akmonistion zangerli, gen. et sp. nov., formerly attributed to the ill-defined genera Cladodus and Stethacanthus, is described from the Manse Burn Formation (Serpukhovian, Lower Carboniferous) of Bearsden, Scotland. Distinctive features of A. zangerli include a neurocranium with broad supraorbital shelves; a short otico-occipital division with persistent fissure and Y-shaped basicranial canal; scalloped jaw margins for 6–7 tooth files along each ramus; a pectoral-level, osteodentinous dorsal spine with an outer layer of acellular bone extending onto a brush-complex of up to 160% of neurocranial length; a heterosquamous condition ranging from minute, button-shaped, flank scales to the extraordinarily long-crowned scales of the brush apex; and a sharply up-turned caudal axis associated with a broad hypochordal lobe. The functional implications of this anatomy are discussed briefly. The rudimentary mineralization of the axial ske...

The microscopic anatomy of the peculiar spine-“brush” complex of the Paleozoic shark Stethacanthus altonensis (Symmoriida: Stethacanthidae) reveals that the spine consists of trabecular dentine throughout—no shaft and mantle components are discernible. Parts of the trabecular dentine contain abundant fibers that suggest attachment surfaces for tendons and other connective tissue membranes to the epaxial musculature. The “brush” consists of parallel, membranous tubes and connecting membranes that form an intricate, internal cavity system. In section, this structure resembles sections of erectile tissues in man. The “brush” may thus have been inflatable. The “brush” is covered with a patch of large, massive, dermal denticles pointing forward. Associated with this spine-“brush” is a second patch of enlarged dermal denticles on top of the head whose crowns face backward. The entire complex consists of a variety of gross anatomical, histological, and microscopic anatomical elements. Sexual display or ...

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...