The neurocranium of the Lower Carboniferous shark Tristychius arcuatus (Agassiz, 1837)

摘要

Tristychius arcuatus, from the late Viséan of Scotland, is known from numerous ironstone nodule-encased specimens. These are reinvestigated using computerised tomography (CT) scanning and 3-D digital reconstruction methods. CT scans of the neurocranium (braincase) corroborate many features of the existing reconstruction, but also reveal new details of the gross proportions, the external morphology and, for the first time, features of the internal morphology, including the otic skeletal labyrinth. The unusual position of the articulation for the palatoquadrate on the ventral surface of the postorbital process is confirmed, but the area for the hyoid articulation is relocated posteriorly and the configuration of nerve foramina within the orbit is changed. A preliminary phylogenetic hypothesis is offered, suggesting a place for Tristychius within a sequence of early branching events in the elasmobranch lineage. In this, Tristychius is not resolved as a hybodontid, but is instead a stem elasmobranch possibly related to genera such as Acronemus, and exemplifying anatomical conditions close to the base of the Euselachii. As such, the neurocranium of Tristychius offers important insight into a defining feature of these early members of the anatomically advanced elasmobranchs: the origin of specialised phonoreception.