Dermal skeleton of the stem osteichthyan Ligulalepis from the Lower Devonian of New South Wales (Australia)

When first described based on isolated scales, Ligulalepis was assigned to the Palaeoniscoidea, a basal group of actinopterygians (ray-finned fishes). Recent cladistic analyses, mainly based on skull and neurocranial characters, have mostly recovered the taxon (or, ‘ Ligulalepis ’) as a stem osteichthyan. Here we present information on Ligulalepis dermal elements other than scales and skulls, that include a cheek fragment, a premaxilla, other marginal jaw elements and teeth, an accessory vomer, a partial shoulder girdle, incomplete spine-like elements, and a gular plate. The shoulder girdle and premaxilla compare closely with those of basal actinopterygians, whereas the spine-like element shows some similarity to the distal end of the spines on medial dorsal plates of the Chinese Late Silurian stem osteichthyans Guiyu and Sparalepis , or alternatively to fin rays on the stem osteichthyan Dialipina. One of the jaw elements appears to be a compound jugal plate plus part of the dentate maxilla, an arrangement not previously known in any Devonian stem osteichthyan, or actinopterygian. Histological structure of dermal plates somewhat resembles that of Meemannia , but pore openings in Ligulalepis lead only to the vascular canal network at the base of the ornament layer and not to a pore canal network. Like previous phylogenetic analyses, our analysis incorporating post-cranial dermal skeleton characters also recovered Ligulalepis as a stem osteichthyan.