A new stethacanthid chondrichthyan from the Lower Carboniferous of Bearsden, Scotland

Exceptionally complete material of a new stethacanthid chondrichthyan, Akmo nistion zangerli, gen. et sp. nov., formerly attributed to the ill-defined genera Cladodus and Stethacanthus, is described from the Manse Burn Formati on (Serpukhovian, Lower Carboniferous) of Bearsden, Scotland. Distinctive f eatures of A. zangerli include a neurocranium with broad supraorbital shelv es: a short otico-occipital division with persistent fissure and Y-shaped b asicranial canal. scalloped jaw margins for 6-7 tooth files along each ramu s; a pectoral-level, osteodentinous dorsal spine with an outer layer of ace llular bone extending onto a brush-complex of up to 160% of neurocranial le ngth; 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 rudimen tary mineralization of the axial skeleton and small size of the paired fins (relative to most neoselachian proportions) are contrasted with the massiv e, keel-like, spine and brush complex: Akmonistion zangerli was unsuited fo r sudden acceleration and sustained high-speed pursuit of prey. Cladistic a nalysis places Akmonistion and other stethacanthid genera in close relation to the symmoriids. These taxa are located within the basal radiation of th e chondrichthyan crowngroup, but more detailed affinities are uncertain. Th ey may represent a plesion series on the holocephalan stem lineage, or a di screte clade branching from the base of the elasmobranch lineage.