TSUIHIJI, T.: Primary homologies of cervical muscles in avian and non-avian diapsids

Homologies of the cervical muscles in birds and other diapsids have yet to be clearly established due mainly to the highly derived cervical morphology of birds and to different nomenclatures developed for avian and non-avian diapsids. The axial muscles attaching on the lateral and dorsolateral surfaces of the cervical vertebrae are especially problematic. The avian mm. intertransversarii complex forming the principal lateral musculature of the neck consists of multipennate muscles and is apparently not homologous with muscles of the same name in other diapsids. The homology of m. ascendens cervicalis lying dorsal to mm. intertransversarii has also been debated. To examine the homologies of these and other cervical axial muscles, I dissected various squamates, crocodylians, and birds. Topological relationships among muscles, bones, and spinal nerves were used as criteria for muscular homology assessment. My observations suggest that the avian mm. intertransversarii represent “composite” muscles that include the m. longissimus and m. iliocostalis groups of the epaxial musculature and a part of m. intercostalis externus of the hypaxial musculature in other diapsids. m. ascendens cervicalis is considered here to be the most lateral part of the m. transversospinalis group. Among diapsids, the morphology of mm. intertransversarii and m. ascendens cervicalis in modern Aves is apparently a derived condition. However, the cervical vertebrae of Hesperornis, a basal ornithurine, bear a process that is morphologically similar to the knob-like origin of an aponeurosis shared by those muscles in extant birds. This suggests that a part of the derived features of those muscles might have been acquired before the origin of the crown clade Aves.