The function of the intramaxillary joint in the Round Island boa, Casarea dussumieri



The bolyeriid snakes Casarea dussumieri and Bolyeria multocarinata are unique among vertebrates in possessing an intramaxillary joint that separates the maxilla into anterior and posterior segments. In contrast to previous studies, which suggest that this joint permits enhanced elevation of the anterior maxillary teeth, our films of live Casarea show that the snout and anterior maxillary teeth are actively depressed 15° 20° below rest position through bilateral retraction of the palatomaxillary arches. Patterns of bone movement in living Casarea support the hypothesis that a caudally directed force is transmitted to the snout via the medial bones of the palatomaxillary arch, suggesting functional affinities between Casarea and higher henophidians.

The intramaxillary joint, in conjunction with the curvature of the mandibles, allows the jaws of Casarea to encircle hard, cylindrical prey held transversely in the mouth. Because the Mauritian terrestrial vertebrate fauna lacks mammals and is dominated by skinks and geckos, which Casarea is known to consume, we suggest that the intramaxillary joint functions in a manner analogous to that achieved by quite different maxillary modifications in colubrid scincivores. Although the origin of the bolyeriid intramaxillary joint remains unclear, its structural refinement and evolutionary stability may be due to selection pressures arising from limited prey diversity.