Mastication in the Tuatara, Sphenodon punctatus (Reptilia: Rhynchocephalia): Structure and Activity of the Motor System

The masticatory pattern of Sphenodon punctatus, the sole remaining rhynchocephalian, now restricted to islands off the coast of New Zealand, has been analyzed by detailed anatomy, cinematography, cinefluoroscopy, and electromyography. Food reduction consists of a closing, crushing bite followed by a propalineal sliding of the dentary row between the maxillary and palatine ones. The large, fleshy tongue can be protruded to pick up small prey, and also plays a major role in prey manipulation. The rotational closing movement of the j aw, supporting the basic crushing movement, is induced by the main adductor musculature. I t is followed by a propalineal anterior displacement relying heavily on the action of the M. pterygoideus. The fiber lengths of the several muscles reflect the extent of shortening. The most obvious modification appears in the M. pterygoideus, which contains a central slip of pinnately arranged short fibers that act at a period different from that of the rest of the muscle; their action increases the power during the terminal portion of the propalineal phase. This also allows the animal to use its short teeth in an effective shearing bite that cuts fragments off large prey. The action of single cusped dentary teeth acting between the maxillary and palatine tooth rows provides a translational crushing-cutting action that may be an analog of the mammalian molar pattern. However, this strictly fore-aft slide does not incorporate capacity for later development of lateral movement. As the sole Recent representative of the order Rhynchocephalia, the tuatara Sphenodon punctatus has been the subject of a disproportionately large number of anatomic studies (see Dawbin, '62; Robb, '73). The skull of Sphenodon differs from that of other lepidosaurians in being fully diapsid. A quadratojugal bone, a squamosal-jugal brace, and a fixed quadrate are retained, although there is some question about the degree of cranial kinesis (Versluys, '12; Kuhn-Schnyder, '54; Ostrom, '62; Iordansky, '66; Rieppel, '78). Both the dentitional pattern (Robinson, '76) and the cephalic musculature (Haas, '73; PoglayenNeuwall, '53) show further differences. Earlier investigations, both descriptive and functional, relied on preserved material. More recently, field investigations (Crook, '75; Gans, '82) and laboratory studies of living specimens (e.g., Hill and Dawbin, '69; McDonald and Heath, '71; Gans and Wever, '76, Ireland and Gans, '77) have begun to increase our understanding of the physiology and behavior of the tuatara (Robb, '77). The present report of the relation of cranial architecture, musculature, and dentitional patterns to feeding behavior and muscle action in the tuatara provides a base line for comparisons among lepidosaurians. Lepidosaurians feed in various ways and their varied feeding behaviors are reflected in the structures they use. Snakes swallow their prey whole by means of alternating, unilateral movements of their highly kinetic skulls (Cundall and Gans, '79; Gans, '61). Amphisbaenians bite into their prey, ripping pieces loose by rotation of the body (Gans, '74). Herbivorous lizards crop vegetation, then swallow it with minimal reduction (Throckmorton, '76). Carnivorous lizards chew small food items (Frazzetta, '62) or utilize inertial feeding to bolt down large prey (Gans, '61). Neither the occurrence nor the absence of cranial kinesis correlates clearly with particular feeding strategies. The skull of Sphenodon differs from those of Recent lizards because it has two fixed and parallel rows of teeth on each side of the upper Work carried out while H.I. Rosenberg was Visiting Associate Professor at The University of Michigan on leave from the Department of Biology, University of Calgary, Calgary, Alberta, Canada. This paper is dedicated to the memory of George Haas in appreciation of his profound contributions to the cephalic anatomy of Recent and fossil reptiles. 0362-252518211713-0321$09.0