TSUIHIJI, T.*; KEARNEY, M.; RIEPPEL, O.; Field Museum of Natural History, Chicago; Field Museum of Natural History, Chicago; Field Museum of Natural History, Chicago: Differentiation of axial muscles in the anterior precloacal region of snakes and implications for the cervico-dorsal boundary
Because of the complete lack of the pectoral girdle, it is unclear whether a distinction between the neck and trunk can still be drawn in snakes, or how these regions were modified as snakes evolved from their limbed ancestor. Some colubrid snakes have been described as retaining a myologically distinct �neck� on the basis of the modification of some axial muscles in the anterior precloacal region. We examined various snakes, especially basal taxa, and found that, such myological modifications, present in snakes in general, are similar to those seen in the neck and anterior trunk of other squamates. Therefore, the position of the cervico-dorsal boundary in snakes may be established by comparing the vertebral levels at which such �neck muscles� differentiate in snakes and other squamates. Epaxial muscles inserting on the skull such as m. longissimus cervico-capitis tend to originate more anteriorly in basal typhlopid snakes than in tetrapodal squamates, while those in some higher snakes originate slightly more posteriorly than in the latter. Based on these muscles, therefore, it may be inferred that the neck of typhlopids is shorter than that of tetrapodal squamates while that of some higher snakes may be somewhat longer than in the latter. In contrast, m. rectus capitis anterior, a hypaxial muscle, extends much more posteriorly in the examined snakes (up to the 22nd vertebra) than in tetrapodal squamates, thus potentially contradicting the inference based on epaxial muscles. However, even the longest �neck� suggested by this muscle would still be just a small fraction of the entire precloacal region. Thus, it is likely that the elongated precloacal region in snakes has resulted mainly from elongation of the trunk, not of the neck.
