This is the second “Precision in Terminology” post
Tetrapods generally have only a few ways to affix teeth to their tooth-bearing bones. While most of you readers may be familiar with socket-toothed implantation (thecodonty), the range of dental attachment varies greater than that. Generally, then, there are three types:
1) Acrodonty. Rather than being implanted, save for a shallow pit or divot, the tooth is rootless and may be fused (ankylosed) to the underlying substrate. The term “acrodonty” (apex tooth) applies generally because the teeth appear to be set on the rim of the jaw, the dorsal (or apical)-most margin. Common examples include sphenodontians, such as the tuatara Sphenodon and Pleurosaurus, many snakes and some non-sphenodontan lizards (but not all), and the basal diapsidan group Rhynchosauria.
2) Pleurodonty. As in 1, the tooth is generally set in a shallow pit or divot, but rather than being on the edge of the jaw, pleurodont (“side tooth”) settings occur on the lingual surface, forming a short of shelf. When pleurodont teeth are fused to the jaw, true bone can still be distinguished histologically to one side. Examples include most lizards, especially varanoids, and some snakes. A slight lingual small may be formed, and is termed subpleurodont.
3) Subthecodonty. As in 1 and 2, the teeth are set in slight pits and divots, but these in turn are set within a channel, so that a lingual wall parallels the labial wall. This condition is fairly common in basal diapsids and basal reptilians/sauropsidans. Teeth set in grooves do not always correspond to subthecodonty, such as the aulacodont (“groove tooth”) condition in some ichthyosaurs, nor do they seem to be the same in taxa which form small but distinct sockets as in troodontid and alvarezsaurid theropod dinosaurs, while the groove is produced by reduction of the interdental walls between sockets (among other losses). Fusion of the tooth to the groove, often with the labial wall intact, is termed ankylosed thecodonty.
4) Thecodonty. The teeth are divided into deep sockets, and this corresponds to a large root anchored well below the toothed margin of the jaw. This condition is pretty much the normal state for implantation in Archosauria (including birds).
Implantation may be reversed in clades, such as thecodonty transitioning to pleurodonty (in theropod dinosaurs), or acrodonty producing pleurodonty (as in lizards). However, in some taxa, the implantation model has become elaborated, and conditions such as the “stacked” condition of ceratopsian and hadrosauroid ornithischian dinosaurs. This condition may be called seriodonty (from the Greek series, meaning … series, or row), defined by the presence of fully germinated but unerupted teeth ventral (or basal) to the erupted crown, or with multiple erupted crowns exposed above the bone surface that participate in dietary processes.
Implantation can occur in any of the tooth-bearing bones: Pharyngeal bones, the pterygoid, palatine, ectopterygoid, and vomers of the palatal complex, and the premaxilla and maxilla of the upper jaw, and the dentary and various coronoids (generally in non diapsids, and up to three of them), and the symphyseal of the lower jaw. When assessing variation of several types of implantation versus bones (it happens), the use of the adjective for the implantation type preceeding the bone is preferred, but that’s a adjective-noun couplet, which may be older than religion (e.g., thecodont palatine, acrodont maxilla, etc.).
I will continue this discussion when I discuss interdental plates themselves.