From lizard body form to serpentiform morphology: The atlas–axis complex in African cordyliformes and their relatives

The comparative vertebral morphology of the atlas–axis complex in cordyliforms, xantusiid and several skinks is studied here. These lizards are particularly interesting because of their different ecological adaptations and anti‐predation strategies, where conformation ranges from the lizard‐like body to a snake‐like body. This transition to serpentiform morphology shows several evolutionary patterns in the atlas–axis complex: 1) the zygapophyseal articulations are lost in the early stage of the transition. In contrast to mammals, the atlas is more or less locked to the axis in lepidosaurs, but the absence of zygapophyseal articulation releases this locking for rotation. However despite its serpentiform morphology, Chamaesaura is different, in possessing this articulation; 2) the first intercentrum of Chamaesaura and Tetradactylus africanus (serpentiform grass‐swimmers) is fully curved anteriorly, underlying the occipital condyle. While this limits ventral skull rotation beyond a certain angle, it locks the skull, which is a crucial adaptation for a sit‐and‐wait position in grassland habitats that needs to keep the head stabilized; and 3) in Acontias, most of the atlas articular surface with the occipital condyle is formed by the lateral aspect of the articulation area relative to the area located in the dorsal region of the slightly reduced intercentrum. A similar state occurs in amphisbaenians, most likely reflecting a fossorial lifestyle of the limbless lizards. Although Chamaesaura and Tetradactylus live sympatrically in grasslands, Chamaesaura differs in several ways in atlas–axis complex: for example, aforementioned presence of the atlas–axis zygapophyseal articulation, and long posterodorsal processes. Its occipital condyle protrudes further posteriorly, placing the atlas–axis complex further from the endocranium than in Tetradactylus. Hence, adaptation in the same niche, even among sister clades, can lead to different atlas–axis morphology due to different lifestyle strategies, for example, different foraging mode, while similar atlas–axis morphology can evolve in two lineages occupying different niches, as in Ablepharus and Scelotes. J. Morphol. 277:512–536, 2016. © 2016 Wiley Periodicals, Inc.