Taxonomic status of the early Permian Helodectes paridens Cope (Diadectidae) with discussion of occlusion of diadectid marginal dentitions

Abstract

Helodectes paridens Cope (1880) is a problematic Early Permian taxon from Texas that has been considered variously as a diadectid or captorhinid, or simply indeterminate. It was based on a poorly preserved jaw and adjoining elements of the left side of a small skull that was believed to possess a double row of marginal teeth. Thorough preparation of the holotype reveals that one row represents a normal complement of rooted premaxillary and maxillary teeth and the other a row of crowns derived from the opposing teeth of the absent dentary. There are no detectable differences from the commonly encountered Diadectes of the same age, and Helodectes should be regarded as a junior synonym of that genus. As the holotype of H. paridens exhibits no features on which to base specific identity, it is referred to as Diadectes sp. The same interpretation undoubtledly applies to the double row of premaxillary-maxillary marginal teeth in the lost and only known speciment of the equally small holotyes of H. isaaci Cope (1880) from the same locality as H. paridens. A second example of Helodectes-like dental preservation is described in the opposing upper and lowr jaw elements in a very small Diadectes specimen from the Early Permian of Texas. The double-toothed row preservation of the marginal teeth in these specimens promots discussion of aspects of the occlusion of the cheek teeth in Diadectes and those in the closely related Late Pennsylvanian Desmatodon. It is speculated that, in addition to occulsion between the upper and lower cheeck teeth, mastication also occurred through contact between the maxillary cheek teeth and the inner surface of the parapet of the dentary, and between the cheek teeth of the dentary and the ventral surface of the secondary palatal shelf of the palatine. Changes in the pattern of attrition indicate that with increased molarization the occlusion between the upper and lower cheek teeth shifted from a strictly side-to-side contact in extremely small juveniles to a strictly dorsoventral, vertical-alignment contact between the upper surfaces of the crowns in adults. In the adult pattern of occulsion the upper and lower cheek-tooth series were only partially aligned dorsoventrally, and it is assumed that the labial margin of the upper series and the lingual margin of the lower series occluded with the inner surface of the dentary parapet and the ventral surface of the secondary palatal shelf of the palatine, respectively. These changes in the molarization and occlusion from extremely small juveniles to adults may have been accompanied by a shift in diet.