Comparative musculoskeletal anatomy of chameleon limbs, with implications for the evolution of arboreal locomotion in lizards and for teratology

• Published in: Journal of morphology (1931) 2017-09, Vol.278 (9), p.1241-1261
• Main Authors: Molnar, Julia L., Diaz, Raul E., Skorka, Tautis, Dagliyan, Grant, Diogo, Rui
• Format: Article
• Language: English
• Subjects: Anatomy, Comparative; Animals; arboreal locomotion; Biological Evolution; contrast staining; Extremities - anatomy & histology; Forelimb - anatomy & histology; Forelimb - physiology; lizard; Lizards - anatomy & histology; Lizards - physiology; Locomotion - physiology; MicroCT; Muscle, Skeletal - anatomy & histology; Musculoskeletal System - anatomy & histology; myology; PTA; syndactyly; Teratology; Trees; X-Ray Microtomography; zygodactyly
• ISSN: 0362-2525; 1097-4687
• DOI: 10.1002/jmor.20708

Chameleon species have recently been adopted as models for evo‐devo and macroevolutionary processes. However, most anatomical and developmental studies of chameleons focus on the skeleton, and information about their soft tissues is scarce. Here, we provide a detailed morphological description based on contrast enhanced micro‐CT scans and dissections of the adult phenotype of all the forelimb and hindlimb muscles of the Veiled Chameleon (Chamaeleo calyptratus) and compare these muscles with those of other chameleons and lizards. We found the appendicular muscle anatomy of chameleons to be surprisingly conservative considering the remarkable structural and functional modifications of the limb skeleton, particularly the distal limb regions. For instance, the zygodactyl autopodia of chameleons are unique among tetrapods, and the carpals and tarsals are highly modified in shape and number. However, most of the muscles usually present in the manus and pes of other lizards are present in the same configuration in chameleons. The most obvious muscular features related to the peculiar opposable autopodia of chameleons are: (1) presence of broad, V‐shaped plantar and palmar aponeuroses, and absence of intermetacarpales and intermetatarsales, between the digits separated by the cleft in each autopod; (2) oblique orientation of the superficial short flexors originating from these aponeuroses, which may allow these muscles to act as powerful adductors of the “super‐digits”; and (3) well‐developed abductor digiti minimi muscles and abductor pollicis/hallucis brevis muscles, which may act as powerful abductors of the “super‐digits.” Muscles of left forelimb of chameleon (Chamaeleo calyptratus) in medial view, and inset showing detailed muscle anatomy of distinctive syndactylous manus. Considering the remarkable structural and functional modifications of the distal limb in chameleons, the number and attachments of muscles are surprisingly conservative.