Cranial anatomy of Libognathus sheddi Small, 1997 (Parareptilia, Procolophonidae) from the Upper Triassic Dockum Group of West Texas, USA.

Libognathus sheddi, a leptopleuronine procolophonid from the Upper Triassic Cooper Canyon Formation, Dockum Group, West Texas, was based on an isolated left dentary and partial coronoid. New material referable to Libognathus sheddi, from the Cooper Canyon Formation, provides new information on the cranial anatomy. This new cranial material includes the antorbital portion of a skull, a left maxilla and premaxilla, quadratojugals, and dentaries, including intact tooth rows in the upper and lower jaws. Libognathus shows autapomorphies including; dentary deep with ventral margin oblique to tooth row immediately from the symphysis at ≥23°; anterior projecting coronoid contacting the lingual surface of the dentary underlying the last two dentary teeth; reduced contact between the lacrimal and the nasal; suborbital foramen formed by the maxilla and ectopterygoid, excluding the palatine; a posterior supralabial foramen shared by the maxilla and jugal; a Y-shaped antorbital pillar formed by the palatine, and massive orbitonasale and facial foramina (shared with unnamed southwest USA leptopleuronines). Phylogenetic analysis indicates that Libognathus is a highly derived leptopleuronine procolophonid, closely related to Hypsognathus fenneri and other southwest USA Revueltian leptopleuronines, which fall out as sister taxa to Hypsognathus, a relationship supported by a maxillary dentition restricted anterior to the orbital margin, a possibly synapomorphic orbitonasale septum in the form of an "antorbital pillar" created by the palatine, an anteroventral process of the jugal, and the presence of a small diastema between the first dentary tooth and the more posterior dentition. Libognathus exhibits a possible ankylosed protothecodont tooth implantation with frequent replacement, differing from some other proposed procolophonid implantation and replacement models. Chinle Formation and Dockum Group leptopleuronines are restricted to the Revueltian teilzone/holochronozone, making them possible Revueltian index taxa.