Prosalirus bitis and the anuran caudopelvic mechanism

Abstract

ABSTRACT Cranial features of Prosalirus bitis, an Early Jurassic anuran from the Kayenta Formation, include a dentigerous premaxilla and maxilla; a parasphenoid with a lanceolate cultriform process and posterolateral alae; a well ossified, unpaired sphenethmoid; and a stapes of modern form. The atlas bears a notochordal fossa between the cotyles. The number of vertebrae is unknown but anterior, middle, and posterior presacrai vertebrae are represented. The sacrum, which lacks postzygapophyses, possesses elongate, narrow alae and a cartilaginous intercentral articulation with a short urostyle. The appendicular skeleton exhibits a relatively primitive glenoid, prolongation of the iliac shaft, elongation of the hind limbs, radio-ulnar and tibiofibular fusion, and unfused, elongate proximal tarsals. Mobile sacro-urostylic and ilio-sacral joints are important kinetic links in the anuran caudopelvic mechanism for saltation. The angular appearance of the back common among living anurans reflects a pronounced flexure at the sacro-urostylic and ilio-sacral joints. During the launch phase of a jump, the sacrum + presacral vertebral column extends at these joints by the action of the longissimus dorsi muscles, aided by forelimb thrust. The urostyle remains fixed between the ilia by the coccygeo-iliacus muscles that transmit propulsive thrust from the ilia to the urostyle, and thence to the sacrum and presacrai column. The juxtaposition of a shortened tail between elongate, anteriorly directed ilia in the Early Triassic salientian, Triadobatrachus mussinoti, represents a primitive stage in the evolution of the anuran caudopelvic muscular linkage.