Abstract

Squamates present a unique challenge to the homology and evolution of tooth attachment
tissues. Their stereotypically pleurodont teeth are fused in place by a single
“bone of attachment”, with seemingly dubious homology to the three-part tooth attachment
system of mammals and crocodilians. Despite extensive debate over the
interpretations of squamate pleurodonty, its phylogenetic significance, and the growing
evidence from fossil amniotes for the homology of tooth attachment tissues, few
studies have defined pleurodonty on histological grounds. Using a sample of extant
squamate teeth that we organize into three broad categories of implantation, we
investigate the histological and developmental properties of their dental tissues in
multiple planes of section. We use these data to demonstrate the specific soft- and
hard-tissue features of squamate teeth that produce their disparate tooth implantation
modes. In addition, we describe cementum, periodontal ligaments, and alveolar
bone in pleurodont squamates, dental tissues that were historically thought to be
restricted to extant mammals and crocodilians. Moreover, we show how the differences
between pleurodonty and thecodonty do not relate to the identity of the tooth
attachment tissues, but rather the arrangements of homologous tissues around the
teeth.