CUNDALL, David; FORD, Neil; Lehigh University; University of Texas, Tyler: Xenopeltis unicolor: Evolution of Unilateral Jaw Motion and Macrostomy in Snakes.

Xenopeltis, a snake with unusual skull and cephalic muscle forms, is currently hypothesized to be a basal macrostomatan. Clade membership of Xenopeltis is reassessed on the basis of prey capture and transport mechanics, manipulations of anesthetized animals, and reanalysis of cranial anatomy from dissection and histological sections. These data show that Xenopeltis uses a simple lunge and a driving scissors strike to catch prey. The palatomaxillary apparatus is relatively immobile. Xenopeltis transports prey using occipital and snout shifting and palatomaxillary (PM) packing, a previously undescribed variant of a plesiomorphic mechanism used by Cylindrophis. PM packing depends on ratchet functions provided by hinged teeth on all dentigerous bones and on active depression of the snout, palatine and anterior end of the maxilla. These movements are activated by a large slip of the adductor externus superficialis connecting the prefrontal and dentary. Transport is aided by compressing and extending the anterior trunk. Xenopeltis lacks folded interscale regions between infralabial scales and loose connections between mandibles and skin, limiting separation of mandibular tips and, hence, maximum gape size. All these features indicate that Xenopeltis is not a basal macrostomatan but basal to macrostomatans. Further, rapid transport of relatively large diameter prey by Xenopeltis using snout and occipital shifting suggests these movements are basal to the pterygoid walk. A unilateral pterygoid walk occurs only in clades with the palatines liberated from the snout. Macrostomy evolved secondarily in clades with a unilateral pterygoid walk because other types of transport are ineffective when the mandibular tips are separated widely.