Rethinking thecodonty: The influence of two centuries of comparative dental anatomy on our understanding of tooth evolution

"Thecodont" refers to teeth implanted in sockets within the jaw,
a condition traditionally associated with living mammals and crocodylians,
which also coincidentally have teeth attached by ligaments to the socket
walls (gomphosis). For over a century, the bony periodontium of many other
amniotes has been described as a single tissue, "bone of attachment”,
causing confusion over dental tissue homology. Conventional definitions of
“thecodonty” exclude species with fused teeth (“ankylothecodonts”),
implying a fundamental difference between mammals, crocodylians, and most
other vertebrates. However, the stereotypically “thecodont” attachment
tissues have been discovered in all major amniote clades, showing that
gomphosis and ankylosis likely stem from heterochronic changes in the
timing and extent of cementum and alveolar bone mineralization. This
challenges (1) previous hypotheses regarding the evolution of the amniote
periodontium, (2) the “bone of attachment” paradigm, and (3) the
significance of “thecodonty”. We suggest a new nomenclatural approach that
incorporates recent histological and evolutionary research and divides
thecodonty into anatomical categories to clarify their origin and
evolution. We propose the terms anisothecodont and isothecodont to denote,
respectively, asymmetric and symmetric implantation of teeth in their
sockets. Regardless of the geometry of the connection, we propose using
ankylosis and gomphosis to denote the mode of tooth attachment.