Meeting Abstract
Thecodonty has traditionally combined two components of anatomy: the geometry of tooth emplacement in the jaw with deep sockets and the presence of a periodontal ligament anchoring the tooth to the jaw. Extant mammals employ three attachment tissues (cementum, a periodontal ligament, and alveolar bone) in forming a gomphosis between the teeth and jaw. Among fossil synapsids, thecodonty and gomphosis are considered derived states restricted to mammaliaforms and some derived, non-mammalian cynodonts. By contrast, most other fossil synapsids are thought to employ reptile-like ankylosis, where the tooth is fused to the jaw. Here we report on the presence of thecodonty and gomphosis in Permian and Triassic therapsids. Using fossil histological techniques, we examined the geometry and tissues involved in tooth attachment in dinocephalians, gorgonopsians, and cynodonts. Gomphosis appears to be ubiquitous within our sample. However, the extent to which a tooth is implanted into the jaw (i.e. root length) varies markedly. The remnants of the tissues involved in gomphosis were detected in at least tapinocephalid dinocephalians (early herbivorous therapsids) along with some evidence of cementum in gorgonopsians and traversodontid cynodonts. Our findings bring the first appearance of mammal-like tooth implantation back to at least the middle Permian (>260 MA). Additionally, variation in root length and periodontal space across Therapsida and within its subclades suggests that the evolution of tooth implantation in synapsids may be more strongly influenced by factors like diet than phylogeny.
