24.4 Wednesday, Jan. 4 Longitudinal Variation in the Axial Muscles of Snakes NICODEMO, Philip*; JAYNE, Bruce C.; Univ. of Cincinnati; Univ. of Cincinnati email@example.com
In snakes, as in other vertebrates, the axial muscles are segmented, but snakes are notable for having individual segments that span several vertebrae. Consequently, muscles that extend anteriorly have a constraint on their length as their origins are located closer to the skull. However, this and other aspects of longitudinal variation in axial muscle morphology are poorly documented for snakes. We compared patterns of segmentation and morphology of the anterior trunk spinalis muscle (SP) in 26 species of phylogenetically and morphologically diverse snakes with midbody lengths of SP segments ranging from 9 to 46 vertebrae. In all species the contractile tissue of a single SP segment usually originates posteriorly from multiple slips attached to several adjacent vertebrae, whereas the anterior site of insertion is a long tendon attached to a single vertebra. Generally, the anterior segment of an adjacent pair of SP muscles both originates, and inserts one vertebra anterior to the adjacent posterior segment, thus identical SP muscle origins and insertions are simply translated along the axial skeleton. Three modifications of this pattern occurred in the anterior trunk. First, the posterior-most site of origin was translated by more than one rather than a single vertebra. Second, adjacent SP segments fused to a single tendon of insertion as indicated by an increased number of muscular slips. Third, in a few highly arboreal colubrids, the anterior tendons of multiple SP segments fused, but they ultimately attached to a single vertebra. The reduction in the length of the SP segments resulted primarily from shortening tendon rather than contractile tissue. Consequently, the ratio of contractile tissue to tendon length of serial homologues varies both longitudinally within individual snakes and between different species.