New arthrodires (Family Williamsaspididae) from Wee Jasper, New South Wales (Early Devonian), with comments on placoderm morphology and palaeoecology

Two new arthrodire species represented by articulated trunk armours from the Early Devonian (Emsian) limestones of the Burrinjuck area are placed in a new genus Elvaspis (E. tuberculata, E. whitei), assigned to the Family Williamsaspididae. On new evidence of the dermal neck-joint and shape of the median dorsal plate this family is reassigned from the Phlyctaeniina to the Brachythoraci. The strongly ornamented post-branchial lamina of the trunk armour relates to a recent hypothesis that special post-branchial denticles in placoderms are homologous to pharyngeal denticles of crown-gnathostomes rather than modified external tubercles. Variable development of the post-branchial lamina and its ornament in different placoderm subgroups, with specific resemblance to the external ornament characteristic of that subgroup, indicates that modification of normal external dermal ornament is the most parsimonious interpretation. A comparison of fish diversity in modern and ancient tropical reef environments is consistent with an equilibrium model for species diversification through time.