Frog Postcranial Morphology: Identification of a Functional Complex

Abstract

Three features that have traditionally been treated as independent characters in frog systematic studies belong to a single functional complex. Ilial length, posterior transverse process length and condition of the dorsal crest on the ilium are significantly correlated with degree of sacral expansion. Those frog species with expanded diapophyses have long ilia and short posterior processes. Species which lack sacral expansion have relatively shorter ilia, longer transverse processes and a dorsal crest on the ilium. The correlated characters are part of a functional complex involved with directional movement at the ilio-sacral joint. Expanded diapophyses, long ilia and short transverse processes are associated with pelvic movement in the horizontal plane at the joint in both antero-posterior and lateral directions. Cylindrical diapophyses, short ilia and longer processes are found in frog species that have, predominantly, vertical rotation of the pelvis at the sacrum.