THE SIGNIFICANCE OF THE DEVELOPMENT OF JACOBSON'S ORGAN IN THE NORTH AMERICAN TAILED FROG ASCAPHUS TRUEI (ANURA: LEIOPELMATIDAE)

Abstract

The North American tailed frog (Ascaphus truei) is presently regarded as one of the primitive members of the Anura owing to the possession of the following primitive characteristics: free bony ribs, the retention of two ‘tail-wagging’ muscles and Jacobson's organ, which in position corresponds to that of more advanced urodeles. Unlike most other anurans, the tailed frog lacks a tympanum and is voiceless. According to Jurgens (1971), the tendency exists for Jacobson's organ in primitive urodeles to migrate from a position ventral to the main nasal sac to a ventrolateral position in phylogenetically advanced urodeles. Furthermore, in primitive anurans Jacobson's organ is situated ventrolaterally to the main nasal sac and the tendency exists for it to shift anteromedially to lie between the septum nasi and main nasal sac in advanced anurans. Because of this character shared by Ascaphus and advanced urodeles, the embryological ventrolateral shifting of the organ in Ascaphus truei could be regarded as a reflection of the hypothetical evolutionary ancestral transition between aquatic and terrestrial vertebrates. Jurgens' opinions on the phylogenetic migration of Jacobson's organ are corroborated by the embryological development of the organ in Ascaphus truei.