A reevaluation of the unusual abdominal musculature of squamate reptiles (Reptilia: Squamata).

The abdominal muscles of lizards and snakes (Squamata) have been the subject of periodic attention from anatomists, embryologists, and systematists. Until now, the presence of a superficial portion of the m. rectus abdominis, named the m. rectus abdominis lateralis, has been considered a key synapomorphy of the clade Autarchoglossa, which includes all extant squamates save Gekkota and Iguania. However, the precise anatomical relations of the m. rectus abdominis lateralis have never been fully investigated. Here, I show that the m. rectus abdominis lateralis is present in Iguania. Its absence in Gekkota represents rare gross anatomical support for recent molecular-structure-based hypotheses of squamate relationships placing geckoes as sister to the remaining squamates. Where present, it is the most superficial trunk muscle, exterior to the m. obliquus externus. The separation of the m. rectus abdominis lateralis from the m. rectus abdominis occurs as the m. obliquus externus aponeurosis and part of the m. obliquus internus aponeurosis emerge superficially to form the outer portion of the rectus sheath. In Autarchoglossa, the contralateral mm. recti abdomines laterales meet at the midline and are attached to the imbricae of the transverse scale rows characteristic of the clade, suggesting developmental, functional, and evolutionary association. Because the m. rectus abdominis lateralis is sometimes continuous with the pectoralis, its exclusive association with the m. rectus abdominis is questionable. It may be a neomorphic layer that is part of the abaxial developmental system, comprising those muscles whose connective tissue is largely derived from lateral plate as opposed to somatic mesoderm.