Abstract
Extant and extinct reptiles exhibit numerous combinations of tooth implantation and attachment. Tooth implantation ranges from those possessing roots and lying within a socket (thecodonty), to teeth lying against the lingual wall of the jawbone (pleurodonty), to teeth without roots or sockets that are attached to the apex of the marginal jawbones (acrodonty). Attachment may be ligamentous (gomphosis) or via fusion (ankylosis). Generally speaking, adaptative reasonings are proposed as an underlying driver for evolutionary changes in some forms of tooth implantation and attachment. However, a substantiated adaptive hypothesis is lacking for the state of acrodont ankylosis that is seen in several lineages of Lepidosauria, a clade that is plesiomorphically pleurodont. The convergent evolution of acrodont ankylosis in several clades of lepidosaurs suggests a selective pressure shaped the evolution of the trait. We hypothesize that acrodont ankylosis as seen in Acrodonta and Sphenodon punctatus, is an adaptation either resulting from or allowing for a stronger bite force. We analyzed bite force data gathered from the literature to show that those taxa possessing acrodont dentition possess a stronger bite force on average than those taxa with pleurodont dentition. Dietary specialists with pleurodont dentition may also possess relatively high bite forces, though body size may also play a role in their ability to bite hard. Furthermore, our results have implications for the evolution of acrodont ankylosis and potential behaviors related to strong bite force that influenced the evolution of acrodonty within Acrodonta and Rhynchocephalia.
Highlights
Acrodont tooth implantation, where the tooth sits at the summit of the tooth-bearing bone, evolved multiple times within Lepidosauria
While direct comparisons between tooth implantation types and other diets are not possible using the available data, we found that pleurodont frugivores exhibit the highest median normalized bite force (NBF) values whereas durophagous pleurodonts exhibit the lowest median NBF values, the latter is based on a small number of measurements (n = 2)
We suggest that the ankylosis and bone deposition seen in Acrodonta and Sphenodon punctatus that accompanies the typical acrodont tooth morphology would aid in resisting tooth failure
Summary
Acrodont tooth implantation, where the tooth sits at the summit of the tooth-bearing bone, evolved multiple times within Lepidosauria. It appears at least twice within squamate reptiles, as seen in Acrodonta (Romer, 1956) and Trogonophidae (Gans, 1960), and once within Rhynchocephalia (Jenkins et al, 2017) (Fig. 1). In Acrodonta and Sphenodon punctatus, the only living representative of Rhynchocephalia, the dentition is strongly ankylosed (i.e., fused) via the adjacent bone In those taxa, teeth and surrounding tissues have been investigated thoroughly via histological studies (Cooper & Poole, 1973; Smirina & Ananjeva, 2007; Kieser et al, 2009; Kieser et al, 2011; Haridy, 2018), CT data
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