Ventral Thoracic Neuromast Lines of Placoderm Fishes

Abstract

IN the past forty years, following Stensö's¹ pioneer work on the arthrodire Macropetalichthys, much information has come to light in support of the view that the Palaeozoic placoderms and the still surviving elasmo-branchs and holocephalians are more closely related to each other than to any other group of fishes². These fishes have thus recently been grouped as the Elasmobranchiomorpha³ (with the possibly erroneous inclusion of the Acanthodii), and a particularly close relationship has been claimed between the ptyctodont placoderms and the holocephalians⁴. Attempts have been made to interpret the cephalic neuromast system of placoderms after an elasmobranch-holocephalian model⁵, but the true nature of the sensory lines on the ventral plates of their thoracic armour has remained obscure. However, some recent observations on the extant selachian Squatina squatina may throw some light on the problem.