Large brachythoracid arthrodires (placoderm fishes) from the early Devonian of Wee Jasper, New South Wales, Australia, with a discussion of basal brachythoracid characters

ABSTRACT

Two new brachythoracids are described from the Emsian limestones of the Lake Burrinjuck area, both based on skull remains. They are the largest and most advanced arthrodires so far known from the fauna, and amongst the largest fishes recorded anywhere from the Early Devonian. Dhanguura johnstoni gen. et sp. nov. is based on a single incomplete skull with an estimated length of at least 40 cm, which possesses the derived character of a strongly T-shaped rostral plate, not recorded in any other arthrodire from the fauna. Cathiesichthys weejasperensis gen. et sp. nov. was of similar size, and shows a derived dunkleosteid type of development of the posterior skull margin. In both forms the paranuchal plates have narrow anterior margins, suggesting affinities with the family Homostiidae. Some new material of the genus Taemasosteus is illustrated for comparison. Skull characters used in phylogenetic analyses of brachythoracid arthrodires are reviewed; in Dhanguura johnstoni the similar overlap of the nuchal plate onto centrals, central plates onto preorbitals, and preorbitai plates onto the rostral plate may reflect a single developmental pattern. The preorbitai plates completely separated by the pineal and rostral plates are interpreted as a homoplasy with coccosteomorphs, and instead of a preorbital embayment of the centrals, as seen in that group, this bone has a posterolateral process, as in Arenipiscis. Cathiesichthys is derived in the massive transverse nuchal ridge, one of three defining characters of the Eubrachythoraci, but shows no evidence of an anterior nuchal thickening.