Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution.

Abstract

The skull of '<i>Ligulalepis</i>' from the Early Devonian of Australia (AM-F101607) has significantly expanded our knowledge of early osteichthyan anatomy, but its phylogenetic position has remained uncertain. We herein describe a second skull of '<i>Ligulalepis</i>' and present micro-CT data on both specimens to reveal novel anatomical features, including cranial endocasts. Several features previously considered to link '<i>Ligulalepis'</i> with actinopterygians are now considered generalized osteichthyan characters or of uncertain polarity. The presence of a lateral cranial canal is shown to be variable in its development between specimens. Other notable new features include the presence of a pineal foramen, the some detail of skull roof sutures, the shape of the nasal capsules, a placoderm-like hypophysial vein, and a chondrichthyan-like labyrinth system. New phylogenetic analyses place '<i>Ligulalepis</i>' as a stem osteichthyan, specifically as the sister taxon to 'psarolepids' plus crown osteichthyans. The precise position of 'psarolepids' differs between parsimony and Bayesian analyses.