This is a short post; I’d write more about denticles, but I’d want to prepare more material to do so (it’s a detailed subject).
Most carnivorous archosaurs have denticles on their teeth. These are small, nearly microscopic nodules that persist on the carina of teeth (or sometimes off of it). They are serial (i.e., found in a line), and generally occur on the mesial and distal sides of a crown. They occur in non-carnivorous archosaurs, but are less recognized as such. Their shape can be diagnostic, but their presence has allowed systematists to confuse what are dinosaurs and what aren’t, up to including denticulate, blade-shaped crowns that are actually rauisuchian crurotarsans (croc-line archosaurs that include the awesome Postosuchus kirkpatricki and the stranger Effigia okeefeae). What is most odd is that, in the specific cases of some teeth, the identification to group for denticulate teeth can be confused on the morphology of the denticles themselves. Here’s an example:
Variation is not limited here, but it’s instructive. One of these is NOT a theropod dinosaur. Can you guess which is which?
After a few weeks, giving this a close:
A is Goyocephale lattimorei, an ornithischian. Expect similar morphology from Heterodontosaurus tucki. More specifically, they correspond to the first premaxillary “fang,” which I discuss here.
B is a generic troodontid.
C is Richardoestesia gilmorei. The rectangular, closely-spaced denticles are a dead giveaway, although they occassionally come with hooks (like in A).
D and E are both therizinosauroids. D represents a morphology known for derived “therizinosaurid” forms like Erlikosaurus andrewsi, Segnosaurus galbinensis, and Nothronychus mckinleyi. E is Alxasaurus elesitaiensis.
So what’s F? F is odd; while you might expect the common form of theropod — especially highly predaceous theropod — teeth to have hooked denticles, slightly hooked and certainly perpendicular (rather than apically inclined, as in all of the others shown here except C). F, of all things, is from a tooth referred to Saurornitholestes langstoni. It’s Judithian, from the US, and the morphology of the crown is very Saurornitholestes-ish. But that’s a subject I can get into elsewhere, and have already touched on on this blog.
None of these teeth, with the exception of C, which is diagnostic of “richardcoestesian” teeth but also found in some tyrannosauroids, are immediately discernible from the others. Some, perhaps due to the quality of the art produced (my fault), show that mere shape of the denticles, hooking, the quality of the sellae between teeth, the extent of the sulci that pass from the sellae onto the crown surface, their inclination, the degree of “hooking,” etc. If these features are diagnostic, they are hardly used (by any specific quality) to discriminate teeth. One alternate, more conservative approach, is to be less judgmental on their morphology, that they can vary crown to crown in a single dental series. This is less well-known, but demonstrable in some dentitions. We’ll get to that eventually on this blog.
Hmm… I’m guessing B, D or E, so I’ll say… E.
You should at least be familiar with these, Mickey; care to name them?
I’m guessing A is Richardoestesia (though on second thought, they don’t seem to be to scale, so maybe not), B is Alxasaurus, C is Dromaeosaurus, maybe D is the non theropod, E could be Troodon, F could be a tyrannosaurid with the blood grooves. But this is just based on my memory…
I recently started a PhD on the evolution of teeth and tooth-bearing bones in nonavian theropods at the New University of Lisbon and one of the main goals of this project is to find as much anatomical characters as possible on teeth in order to identify them (ideally) at the genus level. Of course, the denticles are essential anatomical features and I am currently examining them in some theropod teeth from Portugal. Hopefully, their shape, size and orientation, combined with other anatomical characters in teeth, will be useful enough to allow me to achieve such objective. I just started to look at the literature (which is quite scarce on that specific topic. Way too few detailed illustration of denticles alas) and examined several teeth from the Upper Jurassic of the Lourinhã and the Morrison Formation. So far, I’m not yet able to answer to that question with certitude, unfortunately. But I think I saw denticles C in the dromaeosaurine Dromaeosaurus (Currie et al., 1990) and denticles D in the troodontid Troodon (Currie, 1987 ; Currie et al., 1990). Denticles B and E are extremely hooked and separated and resembles those of the therizinosaurid Alxasaurus (Russel and Dong, 1993), as well as some Troodontidae. I saw almost similar denticles as denticles of picture F in the indeterminate dromaeosaurid described by Van der Lubbe et al. (2009). I haven’t seen any denticles that look like the very minute, sharply hooked denticles of picture A, so I would say without any certitude that denticles A may belong to a non-theropod dinosaur.
It’s certainly true that limited discussion has occured on theropod tooth denticles; most of it is limited to Campanian dinosaurs. Little analysis at all has occured on NON-theropod tooth denticles, and arguably none at all. It has yet to be resolved that ornithischian tooth “cusps” are the same as denticles, which is odd because ornithischian teeth have them too.
Troodontid and therizinosaur denticles are amazingly similar, as are richardoestesian and tyrannosaurian denticles.
All of the above are distal denticles drawn from photos of material. I think I should add a few taxa to it to round out some phylogenetic space. I will resolve the answer when I publish a post on the non-theropod, which is coming soon.