|The Vertebrates||Tooth Implantation|
Mammal teeth get all the press, but even the relatively strightforward teeth of reptiles require a little study, since the mode of replacement and implantation is often of phylogenetic interest. Like mammal teeth, reptile teeth seem to have an almost magnetic quality that attracts obscure and inconsistent nomenclature. Interestingly this is also a property shared by pre-dental tooth analogues, i.e. scales, aspidine and so on. Is there something special about hypermineralized tissues that stuns the speech centers of the neocortex, rendering normal communication impossible?
Whatever the magical properties of apatite may be, it is necessary for us to imbibe another tun of terminology. Here we follow the widely-admired system of Motani (1997) for tooth implantation in non-mammals. Truthfully, Motani's brief summary is so succinct and so complete that most of the following is simply cribbed from his article.
Different types of tooth implantation are recognized by a combination of three features: (a) whether or not the tooth is fused (ankylosed) to the jaw (b) whether or not the teeth are set in separate sockets or in a groove; and (c) whether or not they are assymmetrically exposed as in the pleurodont example in the figure. Generally there are two "extremes." Teeth which are ankylosed to the jaw are referred to as acrodont, or pleurodont if fusion is to the side of the jaw bones. At the other extreme, teeth are said to be thecodont if they are set in sockets, without ankylosis. Between these two endpoints, there are a number of other possibilities, which are set out in alphabetical order below.
Acrodont: teeth ankylosed to the jaw bone. There are no dental sockets or grooves. Examples: some lizards, Sphenodon.
Ankylosed thecodont: teeth set in sockets, which may extend to the crown of the teeth. bones of the socket are ankylosed to the jaw. A dental groove is absent. Example: Mixosaurus.
Aulacodont: teeth set in a groove, without ankylosis to the jaw. Example: (probably) Ichthyosaurus.
Labial pleurodont: probably same as pleurodont, but involves a dental groove with a low labial wall, rather than none at all. Thus the teeth are ankylosed to the jaw on the outside (labially) and rest on a bone shelf on the inside (lingually). Example: some lizards?
Pleuroacrodont: same as labial pleurodont.
Pleurodont: teeth ankylosed to jaw. No sockets, and teeth rest in a dental groove with a high labial (outside) wall and a low or no lingual inside) wall. Ankylosis is normally on the labial side of the tooth only, or on the labial side and at the bottom of the groove. Example: varanid and iguanid lizards.
Pleurothecodont: same as subthecodont.
Prothecodont: same as subthecodont.
Subacrodont: same as pleurodont.
Subpleurodont: a variation of pleurodonty in which (as noted above), there is well-developed bone of attachment at the bottom of the tooth in the dental groove.
Subthecodont: both a groove and shallow sockets, within the groove, are present. The groove has a high lbial wall and low lingual wall, as in pleurodonty. Apparantly, there is also ankylosis. Example: Paleothyris, Petrolacosaurus, some non-amniote reptilomorphs.
Thecodont: teeth in deep sockets, deeper than than height of tooth crowns. There is no ankylosis and roots are cylindrical. Examples: most archosaurs, including living crocodilians, mammals.
We may summarize Motani's system in the following table:
|Ankylosis||Dental sockets||Dental Groove|
|Ankylosed Thecodont||socket bones only||yes, shallow||no|
|Subpleurodont||yes, strongly attached in groove||no||yes|