PhD: Enamel structure, development, and evolution in acrodont lizards

Most reptiles can continuously replace their teeth throughout their lives, but a diverse group of lizards, known as the acrodonts, have lost this ability. Acrodont lizards cannot replace their adult teeth, similar to mammals, meaning that their teeth must be adapted to resist tooth wear for long periods of time. How their dental enamel, the most wear-resistant parts of their teeth, is adapted to suit the disparate diets of this diverse group of extant lizards is not known. This thesis project will therefore involve extensive histological sampling of enamel from representative species across seven subfamilies of acrodont lizards: Brookesiinae (leaf chameleons), Chamaeleoninae (chamaeleons), Leiolepidinae (butterfly lizards), Hydrosaurinae (sailfin lizards), Amphibolurinae (bearded dragons and their relatives), Agaminae (agamas), and Draconinae (gliding lizards and their relatives). This survey of extant acrodont species with disparate diets and ecologies will reveal for the first time how modern reptile enamel structure and chemistry can evolve to cope with the functional demands imparted on the dentition when the ability to replace teeth has been lost.

Histological analyses will be accomplished through CT scanning, hard-tissue histology, Scanning Electron Microscopy (SEM), and SEM-Energy Dispersive x-ray Spectroscopy (SEM-EDS). The student will also collect soft tissue data through paraffin sectioning and Transmission Electron Microscopy (TEM) of developing teeth in a select number of acrodont species. These will include close extant relatives of uromastycines, a group of acrodont lizards that evolved mammal-like enamel to determine how this stereotypically mammalian feature may have evolved from an ancestrally “simple” form of reptilian enamel.

The PhD student will work closely with a second PhD who specializes on the prismatic enamel of uromastycines. As such, the major findings from both projects will contribute to a comprehensive phylogenetic analysis, focusing on evolutionary changes in enamel structure, chemistry, and mechanical properties across acrodont lizards. This comparative phylogenetic methods approach will address the questions of (1) when; (2) how many times; and (3) under what ecological or functional scenarios different enamel structures are likely to appear within an evolutionary lineage. Moreover, comparisons of the mechanical properties of prismatic enamel with its prismless counterparts within other lizards will be used to test how the evolution of prismatic enamel may have contributed to the evolutionary success of acrodont lizards, or if it is associated with specific dietary and ecological niches.