Fusion in the vertebral column of the pachyosteomorph arthrodire Dunkleosteus terrelli (‘Placodermi’)

Abstract

Fusion in the vertebral column has evolved multiple times within jawed vertebrates and for these taxa represents normal physiology, with structures such as the sacrum, notarium and pygostyle providing rigidity and support. The synarcual represents the fusion of the anterior part of the vertebral column and occurs in a number of jawed vertebrates, including a variety of placoderms, chondrichthyans and mammals. Placoderms are an entirely fossil group of armoured fishes (Silurian-Devonian), resolved phylogenetically to the base of the jawed vertebrate clade, with vertebrae comprising neural and haemal arches composed of perichondral bone. The placoderm synarcual preserves substantial developmental information from anterior (oldest) to posterior, where new vertebrae are incorporated. This developmental sequence was described recently in the phyllolepid arthrodire Cowralepis mclachlani and ptyctodonts such as Materpiscis attenboroughi, although finer developmental details were not visible. We describe the synarcual in a subadult specimen of the pachyosteomorph arthrodire Dunkleosteus terrelli, preserving 16 paired vertebral elements showing varying degrees of anteroposterior fusion along the vertebral column. Micro-CT scanning of the synarcual provides details of a transitional zone of vertebral modification (involving four of these paired vertebrae, posteriorly), providing unprecedented information on how each vertebra is altered and incorporated into the synarcual. Some of the synarcual elements still retain substantial vertebral identity, showing less fusion overall, thus being more comparable to other arthrodires such as Compagopiscis. By comparison, synarcuals of other taxa, such as the ptyctodont placoderms, along with batoids, holocephalans (Chondrichthyes) and mammals (syncervical) show more complete fusion of vertebral elements.