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Ceratosaurus Part 2: What do we actually know about Ceratosaurus teeth?

December 30 , 2016:

by Robert Gay

What you'll notice about both of these diagnoses, however, is that they contain little in the way of dental characters. Sure they both say the premaxilla only contains three teeth, but that is of little help if you are trying to ID isolated teeth. This brings me to the crux of this post in the first place

As I said in a previous post, "Teeth attributed to Ceratosaurus do turn up in the field, however, and are usually distinguished by the presence of ridges near their bases."

Madsen and Welles (2000) found "longitudinal grooves" in the anterior dentary teeth of Ceratosaurus magnicornis (this is not listed in their diagnosis. Another tooth character is though: "the teeth are longer and stouter."), and also cited "persistent parallel grooves on the medial surfaces of the premaxillary teeth and the anterior three teeth of the dentary," in the etymology for C. dentisulcatus (this morphology is not listed as an autapomorphy in their diagnosis - though "teeth more massive" is). Later in their publication, however, Madsen and Welles (p. 35) do state that, "lateral grooves are diagnostic of the premaxillary and anterior three teeth of the Ceratosaurus dentary," when discussing Ceratosaurus sp. from Tanzania. In the next paragraph they discuss "lingual grooves" as being characteristic of Ceratosaurus. They point out that these grooves are not diagnostic to the species level. There are two problems with this: there is now only one recognized species of Ceratosaurus, and the holotype lacks premaxillary and anterior dentary teeth.

While Gilmore does use some dental-based characters, they are not tied to a specific tooth morphology and instead deal with the number of teeth in the various tooth-bearing bones.

So what do we actually know about Ceratosaurus teeth?

The holotype lacks the apparently diagnostic teeth of Madsen and Welles' C. dentisulcatus. As you can see from Gilmore, the left hand side of the skull features no anterior teeth. The right side is equally bleak when it comes to anterior dentition. So we are now faced with a tooth morphology that may be diagnostic (but not used in a formal diagnosis) of a junior synonym that cannot be directly be compared with the holotype.

Continuing to muddy (or clarify?) the waters is C. ?meriani, from the Late Jurassic of Switzerland. Madsen and Welles (2000) figure it and refer it to Ceratosaurus sp., and Mickey Mortimer over at the Theropod Database says, "It differs from Genyodectes and Ostafrikosaurus in lacking mesial serrations. As it is of identical size and found in temporally equivalent beds, I believe it should be called Ceratosaurus meriani." Well there are other teeth from the Late Jurassic that have fluting and lack mesial serrations. Specifically some teeth referred to Ceratosaurus from the Morrison Formation of the American West. But why are they referred to Ceratosaurus? It is the right size and shape and comes from the right aged beds...but that's usually not good enough to assign to a highly exclusive clade.

So let's sum up what we can say about the teeth of Ceratosaurus (and please chime in if I've goofed somewhere):

Ceratosaurus nasicornis has no known premaxillary or anterior dentary teeth
C. magnicornis has both premaxillaries preserved but, again, no teeth preserved in situ. It also lacks a dentary
C. dentisulcatus preserves both an toothed premaxilla and dentary with the lingual surfaces of the premaxillary and anterior dentary teeth preserving apicobasal grooves.
C ?meriani lacks mesial serrations and appears to preserve apicobasal fluting
C. "stechowi" from Tendaguru has apicobasal fluting
C. "sulcatus" from Como Bluff, Wyoming preserves apicobasal fluting
Several teeth from the Mygatt-Moore Quarry have been referred to Ceratosaurus on the basis of apicobasal fluting.
So it seems that people are treating these apicobasal flutes (and in some cases the lack of mesial serrations) as diagnostic to Ceratosaurus. But apicobasal flutes are known in more taxa than just Ceratosaurus; several Triassic archosauriform tooth morphotypes possess them, as do temnospondyls, some phytosaurs, some early Jurassic dinosaurs (WFtP), spinosaurs, crocs, and gators. That's just tetrapods; I haven't wanted to dive deep into what sort of crazy dentition fish might have. As I understand it (though I don't have any sources right next to me at the moment) this adaptation would be useful for resisting strain on the tooth; basically the animal was punching through tough, wriggling stuff with its face. Understandable then that many things that eat tough prey would have this sort of adaptation.

Where does that leave us with Ceratosaurus? Well it leaves us with the possibility (not the certainty) that Ceratosaurus had apicobasal fluting on its anterior teeth. Certainly specimens with non-dental remains seem to suggest at least one species referred to Ceratosaurus previously had these flutes. I would urge caution, however, in assigning all Late Jurassic stout, fluted theropod teeth to Ceratosaurus. First off, the holotype doesn't have them, so the referral is based on previously referred specimens. That sort of secondary referral is done relatively frequently but it should give one cause for consideration. The second point is that we really don't have a good idea of the dental variation in other Late Jurassic theropods. We know the teeth of Allosaurus fairly well, but what can we say of the tooth structure at all major tooth positions in Torvosaurus, for example (especially considering derived megalosaurs, the spinosaurs, developed fluted tooth crowns as well)? Do we really know what the teeth were like in some of the mid-sized Morrison taxa as well? Since most aren't represented by good cranial remains we don't know what tooth variation is out there.

In closing, we can say that it is likely Ceratosaurus had fluted front teeth but because of problems with the type specimen and the plesiomorphic nature of dental fluting, the rampant homoplasy in tetrapod dentition in general, and the lack of information about other Morrison theropods' teeth it is unwise to assign isolated fluted teeth to Ceratosaurus at all. Hopefully descriptions of other Ceratosaurus specimens with preserved anterior teeth along with a better understanding of Morrison theropod tooth diversity will lead to more accurate diagnoses of these fluted theropod teeth.

Minor aside
Life happens. I've been off the blog game for almost the whole year but I am hoping to be back at it through 2017, especially in terms of blogging about our next publications we have coming out. I've also brought a couple amigos on board here at the Prehistoric Pub so we plan on at least not letting you all down too much this next year. Onward to the next paleo discovery!
Works Cited
Gilmore, Charles W. 1920. "Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus." Bulletin of the United States National Museum 110: 1–154.

Madsen, James H.; Welles, Samuel P. 2000. "Ceratosaurus (Dinosauria, Theropoda) a revised osteology." Utah Geological Survey Miscellaneous Publication 00-2. pp. 80



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