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

Abstract

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.

Vertebral Comparative Anatomy and Morphological Differences in Anguine Lizards With a Special Reference to Pseudopus apodus

“…The elongated, serpentiform body and leg reduction have evolved convergently in numerous reptilian and amphibian lineages for adaption to different environments, and thus such a transformation represents an important biological phenomenon. As was previously observed by Čerňanský (), the transition from lizard body form to serpentiform morphology shows some evolutionary patterns in the atlas–axis complex as well. Although zygapophyseal articulation between atlas and axis is well developed in limbed gerrhonotine anguids, for example, Abronia (Fig.…”

“…All extant members of Anguinae are legless having serpentiform morphology. The limb reduction is not restricted to Anguinae, but is frequently observed in other lizard clades such as Scincoidea, Gekkota, or Amphisbaenia (see e.g., Lande, ; Gans and Montero, ; Lee et al, ; Čerňanský, ). In addition to Anguinae, three other clades belong to the Anguidae: Gerrhonotinae, Diploglossinae, and Glyptosaurinae (Conrad, ).…”

“…This will serve as a reference for other comparative anatomical studies of any other anguimorph, extant, or extinct (such a study is crucial for the determination of the abundant record of the fossil postcranial elements). To test the hypothesis regarding the loss of the zygapophyseal articulation between the atlas and axis during the transition from limbed to serpentiform body form. Čerňanský () observed that in members of the clade Scincoidea such a loss is present during this transition. Our aim is to find out whether similar morphological changes occurred also within the members of the clade Anguidae.…”

Descriptive osteology and patterns of limb loss of the European limbless skink Ophiomorus punctatissimus (Squamata, Scincidae)

“…, ; Wiens et al. ; Čerňanský, ). The evolution of limblessness in non‐snake squamate lineages tends to be associated with one of two snakelike ectomorphs: the burrowing ecomorph and the grass‐swimming ecomorph.…”

The axial anatomy of monitor lizards (Varanidae)

“…Recent, detailed anatomical analysis of the axial skeleton has been undertaken in this group by Čerňanský et al. () and in Cordyliformes generally (Čerňanský, ). A comparative study of the axial skeleton in the neck region was carried out in lizards by Al‐Hassawi ().…”