The Relationship Between Cephalic Scales and Bones in Lizards: A Preliminary Microtomographic Survey on Three Lacertid Species

Abstract

In the last two decades, there has been a great deal of interest in the morphology and anatomy of the lizard skull in an ecological and evolutionary perspective. However, the relationship between variations in many key anatomical features remains largely unknown. Using microtomography and geometric morphometrics, we examined the relationship between bones and scales associated with the parietal foramen in the three lizards species most common in the Italian peninsula: Podarcis muralis, P. sicula, and Lacerta bilineata. The imprints of the scales are clearly recognizable on the outer bone surface, and this may suggest a structural interaction between these elements. The temporal osteoderms are visible in the larger males and in the larger females of L. bilineata, but they are absent in the smaller specimens of L. bilineata and in all Podarcis specimens. Two parallel rows of pterygoid teeth are present in all the specimens of L. bilineata and are absent in the smaller male of L. bilineata and in both Podarcis species. Cheek osteoderms occurred only in the largest specimens of our sample (i.e., large L. bilineata), being possibly related to hyperostotic processes and densitometric thresholds more than to phylogeny. Minor differences may be also associated with the form of the parietal foramen. In absolute terms the parietal foramen tends to be largest in L. bilineata but in relation to skull length the foramen tends to be larger in P. muralis. In this latter species the foramen is also more elongated. In all three species the fronto-parietal suture occupies a similar location relatively to the scale spatial organization. A shared allometric pattern shows that the main vault enlargement can be localised at the areas anterior to the fronto-parietal suture, providing further information on the possible morphogenetic dynamics associated with the interaction between scales and bones around this structure.