Cranial Specialization and Locomotor Habit in the Lagomorpha

SYNOPSIS. The skull of modern leporid lagomorphs is structurallyspecialized to permit significant intracranial movement. Thisappears to be the first recorded instance of organized cranialkinesis in mammals. A well-defined intracranial joint encirclesthe braincase between the posterior occipito-otic complex andthe remainder of the cranium. It arises from the retention andelaboration of a zone of patency found in neonates. The jointallows the much heavier anterior region of the cranium ($ mandible)to move relative to the posterior region which is stabilizedby its muscular attachment to the neck. It is hypothesized thatthe kinesis functions as a shock-absorbing mechanism to minimizethe jarring effects (possibly on the visual apparatus) of thelarge impulsive loads associated with running and jumping. Thedissipation of kinetic energy through controlled skull deformationmay be augmented by the hydraulic displacement of intracranialblood through a specialized system of venous channels and sinuseslocated within the fenestrated posterior regions of the cranium.Both biomechanical considerations and behavioral observationsindicate that the relatively massive external ears of cursorialleporids may play a vital role in cranial kinesis by helpingto "reset’ the kinetic mechanism between loading cycles.The evolutionary origin of this specialized cranium, it is suggested,may be associated with the development of a cursorial, saltatorylocomotor habit in lagomorphs. Cranial kinesis is lacking insuch noncursorial forms as the living pikas (=Ochotonidae) andcertain primitive fossil leporids of North America (e.g., Paleolagus,Megalagus).