Tooth Implantation – PiT#2


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:

Tooth implantation stylized; A) acrodonty, B) pleurodonty, C) subthecodonty, D) thecodonty. Black represents the substrate (usually bone or cartilage), gray represents the anchoring material (cementum or soft tissue, or even more bone), and white represents the tooth.

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.

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11 Responses to Tooth Implantation – PiT#2

  1. Nick Gardner says:

    Sphenodontians are rhynchocephalians. Rhynchocephalians are lepidosaurs… not basal diapsids.

    I thought the primitive lepidosaur condition was pleurodonty and that acrodonty is convergent between agamids and most rhynchocephalians.

    • Thanks for the comments, Nick.

      I will have to double check on some of my refs, but for now: acrodonty is apparent in several “amphibian” taxa, rather than pleurodont; I take this to be the condition apparent in the first diapsids, especially when basal synapsids (such as “sphenacodonts”) have shallow, shallow sockets of the acrodont type (convergently acquiring thecodonty with archosauromorphans). Basal reptiles (and basal lepidosaurs like sphenodontians) have what appears to be a consistent ankylosed thecodonty, which may just be fused acrodonty in some conditions [the term’s use is often focused on phylogeny, than morphology).

      I explicitly intended to separate Sphenodontia and Rhynchosauria, and thus should have used the latter term instead of Rhynchocephalia, and will fix that; however, when referring to them as basal diapsids, I am positioning them relative to derived diapsids, and it is not meant to infer they were outside any other clade. This then does not mean I think sphenodontians are not lepidosaurs, although some phylogenies muck with where Rhynchosauria lies and pulls Sphenodontia with it, or leaves it in Lepidosauria.

  2. Nick Gardner says:

    …. I don’t know how out of date your references are on that but most people still use Rhynchocephalia for Sphenodon et al. alone. Rhynchosaurs are archosauromorphs, rhynchocephalians (sphenodon et al.) are lepidosaurs. Rhynchosaurs are hardly basal diapsids. :/

  3. Nick Gardner says:

    Captorhinids (except Acrodonta) and Youngina are subthecodont.

    Which phylogenies are you talking about? All of the relevant diapsid matrices I’ve handled don’t seem to find a strong pull between Sphenodon et al. and rhynchosaurs.

    • I may need a double check. With acrodont and pleurodont basal lepidosaurs, acrodont or ankylosed subthecodont sphenacodonts, procolophonoids, etc. There is a confusion in the literature between ankylosis of the “subthecopdont” condition and acrodonty, as the latter does not require fusion, but the former may exhibit it. In procolophonoids, acrodont dentition is fused to the dentition-bearing bone while there is also a labial wall of the jaw margin on both maxilla and dentary which brace the dental row and, in part, expand between the teeth. This is not subthecodont, and several “subthecodont” taxa exhibit this condition, including some captorhinomorphans.

      And just to be clear … I am not arguing necessarily for a phylogenetic signal in regards to the persistence or lack thereof in dental implantation, especially since it is reversed or altered repeatedly in the lineage from basal Diapsida towards Mosasauroidea alone. While using the phylogenetic signal to support my use of assigning taxa to general positions in the overall Tetrapod groups, this is not intended to imply that these descriptions are relevant (or not).

    • Nick Gardner says:

      So there is no uncertainty over my comment above, when I wrote “Acrodonta”, I was referring to the captorhinid Acrodonta irerhi, not the squamate clade.

      • Yeah, I figured. I refreshed checking on Acrodonta along with captorhinomorph implantation, just to affirm. I will eventually be putting out more data on implantation variation, but that’s a large-topic study and very much down the road.

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