A Little About Interdental Plates

Interdental plates represent another feature of variation in the dental row, and one generally taken for granted by most researchers, and the following illustration represents a crude way to encapsulate some of the variation that occurs in archosaurs. Because interdental plates appear virtually exclusively in crurotarsans and dinosaurs, I focus my attention on the subject to those groups where present.

Both clades convergently acquired interdental plates from a common ancestor whose various ancestors had a solid plate lingual to the dental row (which I call the lingual dentigerous wall, or LiDW). First, an interdental plate is a relatively simple structure. It’s T-shaped in section, and comprises three general laminar features (because the structure is associated with teeth and the jaw, I use the dental orientation terms used by [1], only I push tooth-bearing bones further to all associated structures): the broad flat surface of the plate facing lingually is comprised of a mesial and a distal lamina, and where they converge a labial lamina sites between the individual sockets. The labial lamina not only divides adjacent tooth sockets, it contacts a comparable lingual lamina from the labial dentigerous wall (or LaDW).

Terminology of the interdental plate.

Evolution of the interdental plate. Explanations below.

This illustration is not intended to be comprehensive, and is fairly representative of a “napkin doodle” for me (it was actually done on a piece of lined paper shortly after hashing an even worse earlier version [on a napkin of sorts, no less] with Jon Wagner at the ’05 SVP). Some of the conditions shown represent secondary loss of distinct plates, while in others the plates are lost. A term for section or perforated thecodonty in this manner could be useful, as such a system of terminology may assist the delineation of features of the interdental plates in archosaurs, a topic that has so far been relatively underscored in systematics and anatomical description.

Let’s examine these in detail:

A. Typical thecodont implantation, with large nutrient foramina which are connected by a longitudinal nutrient/nervous groove. The groove is positioned at the base of the LiDW.
B. Initial segeregation of the LiDW into discrete interdental regions, although the LiDW remains intact.
C. The interdental plates have fully divided the LiDW, although they may still be bound to the labial dentigerous wall (LaDW).

Sometimes, however, the development or appearance of interdental plates is only a portion of the story, as they also tend to disappear in archosaurs. D, E, F, and H represent a series of progressive interdental loss. G, inserted within, represents an alternate development of interdental plates.

D. Interdental plates reducing in size, increasing the space between them, but the base of the remainder of the LiDW expands apically, perhaps due to the loss of a supportive effect of the interdental plates (a research topic, of course, would be to assess the effect on the jaw in biting of interdental plates, which would require artificially adding and removing said plates without affecting the tissues in the remainder of the socket).
E. Loss of interdental plates entirely with further expansion of the basal LiDW.
F. Loss of teeth occurs, and the LiDW remains.
H. Loss of the LiDW entirely.
G. Alternatively to D-H, slots between interdental plates are lost, and the plates fuse into a secondary LiDW above the basal remains of the first.
I. A three-quarters view of C, combining the lingual and sectional views.

There’s more than a little more to say about this figure, but that will have to wait. There’s a whole system of relationship of tooth, bone, and soft-tissues that many people are already working on, and certainly this represents a somewhat dated view of a topic I’ve never been able to fully explore. Some of this represents things that I arrived at convergently with what I am sure are other peoples’ research topics (that I’ve never seen nor heard of, I assure you).

Terminology for the bone-tooth system in archosaurs receives far less attention than most people realize, and when there is an extensive set of information that can be derived from this well of material, such as the form of the dental platform, evolution of the LiDW, development and eventual loss of the interdental plate, etc., one can only hope for more, much more on this subject to come.

[1] Smith, J. B. & Dodson, P. 2003. A proposal for a standard terminology of anatomical notation and orientation in fossil vertebrate dentitions. Journal of Vertebrate Paleontology 23(1):1-12.

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2 Responses to A Little About Interdental Plates

  1. The fact Euparkeria has interdental plates (Senter, 2003) suggests they are primitive for archosaurs as opposed to convergent in dinosaurs and crurotarsans.

    • This post actually represents one of the major sidelines of my interest in the evolution and loss of dentition in archosaurs, because the loss or modification of the lingual dentigerous wall in archosaurs appears to be somewhat plastic: segmentation may be convergent, as it is certainly convergently lost among taxa, versus reduced, or fused; this is especially important when in Dinosauria, some basal sauropodomorphans have interdental plates (although they may be “guaibasaurs”) but no later ammosaurians and “true” sauropodans appear to; similarly for ornithischians, the lingual dentigerous wall does not segment into plates even when the anatomy implies it could (as in hadrosaurids).

      In some cases, the lingual dentigerous wall may be identified by the anterior coronoid/pronoronoid ossification, which may fuse to the dentary itself, and thus to any potential interdental ossifications. Development of these structures is only testable in well-studied and heavily numerous fossil taxa, or in crocodilians. All else requires speculation of the various reasons why segmentation or nonsegmentation is identified.

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