The Atlas‐Axis Complex in Chamaeleonids (Squamata: Chamaeleonidae), with Description of a New Anatomical Structure of the Skull

Abstract

The comparative vertebral morphology of different chamaeleonid genera has been generally neglected and some aspects such as the comparative anatomy of the neck region remain poorly known. The atlas and axis of all chamaeleonid genera (Brookesia, Rieppeleon, Archaius, Rhampholeon, Nadzikambia, Bradypodion, Chamaeleo, Calumma, Furcifer, Kinyongia, and Trioceros) are studied here. Considerable morphological differences are revealed. Additionally, some taxa exhibit sexual dimorphism in the atlas and axis. An extremely long, divided posterodorsal process is present in males of the Trioceros johnstoni 1 Trioceros jacksonii clade. The solid and well-developed morphology of the posterodorsal process in males of this taxon could reflect its competitive behavior-males fight with their horns and attempt to dislodge one another from branches during encounters. An additional area of insertion for the cervical musculature may indicate an incremental cervical musculature mass and cross sectional area that can add extra support and stability to the head and assist during combat involving lateral pushing. This character is not present in females. Heterochronic processes have played a role in the evolution of chamaeleonids, as evidenced in many characters of the atlas-axis complex. A new hypothesis of an anterior shifting of synapophyses of the axis is erected and a new derived anatomical structure of the parietal of Chamaeleo calyptratus is described (the processus parietalis inferior).

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

“…This present comparative study and the study of Čerňanský et al () highlight that the atlas–axis complex is a useful source of taxonomic characters, because both investigations confirm significant differences between taxa. Together with skull structures, these can represent another rich source for phylogenetic analyses.…”

“…They are not distinctly dorsally sloped in posterior view, however, but lie more or less on the lateral level. A small bulge is also present on their dorsal aspect; reminiscent of the postzygapophyseal dorsal process reported in Trioceros (see Čerňanský et al, ). The neural canal is huge, pentagonal in shape and has high lateral branches.…”

“…This is most likely a plesiomorphic state among all lizards. Thin laminae in several chamaeleonid taxa can be therefore regarded as derived (see Čerňanský et al, ). Except of Smaug , Gerrhosaurus , and Broadleysaurus , the laminae of the neural arch remain unfused (absence of the dorsal mutual contact of the right and left one).…”

“…Standard anatomical orientation is used throughout this article, and vertebrae osteological terminology follows Hoffstetter and Gasc () and Čerňanský et al (). I used Mesquite (Version 2.75; Maddison and Maddison, ) to optimize characters using parsimony.…”

“…This highlights the importance of recognizing other morphological data sources. New work of Čerňanský et al () about detailed anatomy of the atlas–axis complex in chamaeleonids or Vaugh et al () about the atlas–axis complex in geckos suggest that the squamate atlas–axis complex is one such alternative source. The atlas–axis interface with the basicranium (occipital condyle) allows cranio‐cervical movement (see e.g., Romer, ; Hoffstetter and Gasc, ).…”