Evolution of Cranial Kinesis in Lower Tetrapods

Abstract

The earliest tetrapods had pleurokinetic skulls (with movable palatoquadrates and associated dermal bones of palate and cheeks). Pleurokinesis developed from the splanchnokinesis of the crossopterygian skull; the crossopterygian neurokinesis was lost in the primitive tetrapods. Recent amphibians also have pleurokinetic skulls. The stegocrotaphic Gymnophiona retain the primitive pattern of pleurokinesis; in other extant amphibians this is modified and combined with the rhynchokinesis (Anura) or prokinesis (Urodela). Primitive reptiles attained a pleurometakinetic condition of the skull. Cranial kinesis was most advanced in the diapsid reptiles. The improvement of kinesis in the amphikinetic skull of lizards included either development of hyperstreptostyly from simple streptostyly, or a flexible palate from a firm palate. Snakes show both of these trends. Cranial kinesis involves multiple adaptive functions of the entire skull, with the improvement and changes of its adaptive roles in tetrapod phylogeny discussed here. The most important evolutionary mechanisms for increased cranial kinesis include morpho-functional and ontogenetic protoadaptations. Evolution of these occurred by means of ontogenetic heterochrony (especially fetalization), and were controlled by stabilizing selection, which is genetic reflection of modifications, arising due to the influence of altered mechanical tasks.