Abstract

The anatomy of the external naris and anterior palate is described in detail for Lycaenodon longiceps, a morphologically conservative member of the primitive therapsid clade Biarmosuchia. Therapsids are distinguished from pelycosaur-grade synapsids by numerous features of the naris and palate. As in most early therapsids, the septomaxilla in Lycaenodon consists of a short intranarial portion and a long, posterodorsal, facial process. The septomaxillary foramen is reduced to a small, longitudinal slit between the septomaxilla and maxilla. On the palate, the unpaired vomer is clasped anteriorly by long palatal processes of the premaxillae. The choanae are elongate structures whose anterior portion housed the lower canine when the jaws were closed. The palatine and maxilla bear a small shelf (crista choanalis) on their medial surface that has been suggested to represent an early stage in the development of a secondary palate. The parallel acquisition of a crista choanalis and a bony secondary palate in several therapsid groups supports the hypothesis that respiration-related changes were an important aspect of early therapsid evolution.

The genus Lycaenodon may be diagnosed by at least three autapomorphies: the premaxilla bears a scroll-like choanal process that roofs the anterior portion of the internal nares, the vomer is widest at the level of the upper canines with its downturned edges beginning more posteriorly, and the dorsal surface of the postfrontal is convex. Another specimen referred to Lycaenodon preserves additional features that might be autapomorphic, but are not preserved in the holotype.

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