Cranial morphology of the Loxommatidae (Amphibia: Labyrinthodontia)

Abstract

The labyrinthodont superfamily Loxommatoidea is now divided into two families. The highly aberrant Spathicephalus is placed in a new family, the Spathicephalidae, to be described elsewhere. The family Loxommatidae is retained for the remaining genera, Loxomma, Megalocephalus and Baphetes. Additional material and further preparation has made possible a redescription of the three loxommatid genera and new skull restorations have been produced for most species. In particular the loxommatid braincase and palatoquadrate are reconstructed for the first time; in many features their structure is more primitive than that hitherto described for any temnospondyl. Since an intertemporal bone is found to be a feature of Baphetes as well as Loxomma, these two genera have been separated on the basis of skull shape and on stratigraphical grounds. A specimen from the Communis zone, Westphalian A, is attributed to Loxomma, as L. rankini sp.nov., while 'Loxomma bohemicum' has been transferred to the genus Baphetes as B. bohemicus (Fritsch). A further specimen, originally associated with Macrerpeton, has also been referred to this genus as B. lintonensis sp.nov. The skull of Megalocephalus pachycephalus can be described in greater detail than that of any other loxommatid species and thus forms the basis for discussion of the functional morphology. Jaw muscles are reconstructed for this species and it is concluded that the antorbital vacuity, which characterizes the Loxommatoidea, evolved as a bulging hole for a large pterygoideus muscle associated with a piscivorous habit and a kinetic inertial system of jaw closure. Consideration of the mechanics of jaw closure sheds light on a further enigma, i.e. the function of the basipterygoid articulation in the primitive temnospondyl skull. The loxommatid skull is considered divisible into two units. The presence of a specialized cranial joint between the quadrate and quadratojugal allows potential for any movement at the basal articulation to be accommodated in the main at this site. It is suggested that the system described for loxommatids represents an alternative design to the mobile cheek region of anthracosaurs and that the articulations represent zones of elasticity, which accommodate the stresses on the skull caused by a powerful jaw musculature.