The braincase of a primitive shark

The Devono-Carboniferous chondrichthyan Stethacanthus is best known for its striking 'spine-brush' complex. Here we provide the first detailed descrip tion of the neurocranium, using several well-preserved specimens from Bears den, Scotland (Serpukhovian; Pendleian El). Significant features include an otico-occipital region of about the same length as the ethmosphenoid divis ion; short but distinct lateral otic processes; a short; broad endolymphati c fossa; a persistent otico-occipital fissure; Y-shaped basicranial canals indicating origin of the lateral aortae anterior to the occipital level; an d minimal suborbital shelves limited to small, ethmoidally located outgrowt hs. Using placoderm, osteichthyan and acanthodian neurocrania for outgroup comparison, PAUP analysis of a matrix of 23 neurocranial characters produce d the following branching sequence: (Helodus (Stethacanthus, Cobelodus (Tam iobatis (Xenacanthus (Cladodus wildungensis (Tristychius, Hybodus)))))). Th e location of Cladoselache in this analysis is uncertain and probably resul ts from missing data and a morphologically restricted character set. Plesio morphic conditions for chondrichthyan braincases are found to include the p resence of a persistent otico-occipital fissure, projection of the median d orsal aorta anterior to the occipital level, and absence of suborbital shel ves. These results may have important implications with regard to primitive gnathostome interrelationships, and especially the phylogenetic position o f placoderms.