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

Resumen

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.