First Report of a Pectoral Girdle Muscle in Snakes, with Comments on the Snake Cervico-dorsal Boundary

The morphology, topological relationships, and innervation patterns of m. cervicoquadratus in various snakes, especially basal taxa, were examined. Nearly all aspects of its anatomy suggest that this muscle is homologous with a part of m. cucullaris, most likely m. episternocleidomastoideus, the muscle that connects the skull and pectoral girdle in non-ophidian squamates. This is the first report of any pectoral girdle muscle persisting in snakes. In snakes examined, the most posterior extent of m. cervicoquadratus does not extend more posteriorly than the level of the 11th precloacal vertebra, suggesting that the length of the neck inferred on this characteristic would be quite short compared to the length of the entire precloacal region. However, different anatomical and developmental characteristics are known to indicate various lengths of the neck in snakes. This suggests that such characteristics potentially evolve independently from one another, and thus no single criterion may be sufficient to delimit the neck in snakes.

Figure 1.

Semi-schematic diagrams of the musculature around the cranio-vertebral boundary in (A) Python curtus (FMNH 55503), (B) Typhlops muelleri (FMNH 252062), and (C) Gekko smithii (FMNH 10692).Left lateral views with superficial cutaneous muscles and/or m. constrictor colli removed. In (A), m. cervicomandibularis is also cut and reflected to expose m. cervicoquadratus. Abbreviations: Ame, m. adductor mandibulae externus; Ccs, m. costocutaneous superior; Cl, clavicle; Cm, m. cervicomandibularis; Cq, m. cervicoquadratus; Cq-d and –v, dorsal and ventral heads of m. cervicoquadratus, respectively; Dc, m. deltoideus clavicularis; Dm, m. depressor mandibulae; Ds, m. deltoideus scapularis; Ecm, m. episternocleidomastoideus; Hc, hyoid cornu; Ic, m. iliocostalis group; Lo, m. longissimus group; Lp, m. levator pterygoideus; Mh, m. mylohyoideus; Oh, m. omohyoideus; Ncm, m. neurocostomandibularis; P, m. pterygoideus; Qml, quadratomaxillary ligament; S, skull roof; Sh, m. sternohyoideus; Tr-a and -p, anterior and posterior slips of m. trapezius, respectively; Ts, m. transversospinalis group

Figure 2.

Semi-schematic diagram showing topological relationships among the hyoid cornu, m. cervicoquadratus, m. omohyoideus, and other muscles in Cylindrophis ruffus (FMNH 131765). Medial view of the right side with m. cervicoquadratus cut from the insertion and m. neurocostomandibularis reflected dorsally, showing the origin of m. omohyoideus. Abbreviations: Cci, m. costocutaneous inferior; Ccs, m. costocutaneous superior; Cq, m. cervicoquadratus; Hc, hyoid cornu; Hg, m. hyoglossus; Ncm, m. neurocostomandibularis; Oh-a and -p, anterior and posterior parts of m. omohyoideus, respectively; Ss, mm. squamosquamales