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

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?

Plate 17, Figure 1 from Gilmore (1920), showing the right side of the holotype of Ceratosaurus

Plate 17, Figure 2 from Gilmore (1920), showing the left side of the holotype of Ceratosaurus

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

"Why does science have to name every little thing?"

One of the most common questions I got when I was teaching as some variation on that theme. "Why do scientists have to make things so complicated?" "Why can't they just call it something simple?" It is a question bigger than just high school level as well; one of the barriers to effective science communication and education seems to be the general "fear" of overly technical language . The general public seems to view scientists as speaking in convoluted and complex terms.

It is true that scientists have devised some very complex ways of describing things that might seem simple to a layman. And sometimes we can get wrapped up in using the terms we are familiar with when talking about our research, to the detriment of any non-technical audience. The media is also partially to blame as well, with perpetuating ideas such as all prehistoric reptiles are dinosaurs. But why do these terms exist in the first place?

This brings me to Hendrickx et al. (2015). Hendrickx and his coauthors published a paper last year breaking down theropod dinosaur teeth and analyzing many different aspects of their morphology. In addition, the authors created a standardized terminology for future paleontologists to use when describing their dinosaur teeth. Hendrickx and his coauthors explain why creating such a terminology is needed succinctly; in effect answering the question of why scientists create names for so many things.

...several pivotal theropod taxa with well-preserved dentitions still lack a thorough dental description...leading numerous authors to identify isolated theropod teeth to broad clades with uncertainty...isolated teeth are key pieces of evidence to assess vertebrate paleoecological diversity and are often used for stable isotopic studies with various applications...A better understanding of theropod anatomy and morphological variation is therefore central to help resolving systematic relationships and to provide paleoecological clues. Tooth morphology is tied to diet, which has extensive evolutionary repercussions, such as morphological convergence, more than other parts of the skeleton. Yet, theropod teeth have been shown to possess many diagnostic features of taxonomic value...Although theropod teeth seem simple at first sight, this is effectively a result of the absence of comprehensive studies on tooth anatomy and morphological variation among theropods, as well as the lack of a uniform anatomical nomenclature.

What does the wall of text mean? Basically, theropod dinosaur teeth can be used to study evolutionary relationships, paleoecology, and several other important things in paleontology, but no one has bothered to come up with a good way to talk about them.

That's the crux of scientific terminology; coming up with a good way to talk about things. Good, in this case, means usable. Terms should describe well-defined parts of an organism's anatomy. If we say, "the tip of the tooth," on a tooth that has multiple "tips", how are we to know which specific tip we're talking about? Are there differences between "wrinkles" and "grooves?"

Figure 1 from Hendrickx et al., 2015

Without understanding the distinctions between subtle anatomical differences in different taxa, how are we to find out if these features are actually taxonomically important? A quick example from the Morrison Formation. Here in western Colorado we have basically two relatively common large theropods from the Morrison: Allosaurus and Ceratosaurus. Skeletal remains of Allosaurus, including teeth, seem to dominate in the Morrison making up 3/4 of all the theropod remains (Foster, 2007). Teeth attributed to Ceratosaurus do turn up in the field, however, and are usually distinguished by the presence of ridges near their bases. Now that we have Hendrickx et al.'s paper, we can go into a bit more depth. We can say, for instance, that the teeth attributed to Ceratosaurus have basal fluting, and these flutes are not seen in the contemporaneous Allosaurus. So this may help us distinguish between these teeth in the field and keeps us from mistaking Ceratosaurus teeth (with their flutes) with wrinkled or ornamented teeth (or tooth fragments).

The description of Ceratosaurus teeth by previous authors, however, has been lacking in detail and confusing, often using different terms for the same anatomical feature. As Hendrickx et al. note, having their framework in place will help facilitate such a description and they specifically mention Ceratosaurus as being in need of such a redescription. Hopefully such a project will be forthcoming. This topic will also be the focus of my next blog post!

Going forward I am hoping to see a theropod-wide tooth catalog. While Hendrickx et al. do point out that teeth are quick to change, evolutionarily speaking, to changes in diet and feeding behavior, they also note the taxonomic utility of teeth. While many theropod teeth can't be narrowed down to a genus or species, being able to address higher-level taxonomic questions with teeth is important. In addition, some taxa appear to have diagnostic dental modifications. Doing systematic studies and descriptions of theropod teeth may yield more information on what characters are taxonomically useful and potentially add autapomorphies to established genera.

My biggest complaint is that the authors did not examine what a theropod tooth is. They identify problems with past work, the utility of teeth, and the need for a framework but there is no way to determine if this framework is applicable to a given tooth. Obviously for teeth attached to theropod jaws this isn't a problem, but the majority of the dental fossil record for archosaurs consists of isolated shed teeth. While workers in the Cretaceous and Jurassic strata have this problem to a lesser degree (though it is possible that some crocodylomorphs developed similar tooth morphologies), those of us working in the Triassic are confronted with a host of dental convergences! One need look no further than the saga of Revueltosaurus to find examples of teeth that look similar between widely divergent clades. In the Triassic there are plenty of carnivorous reptiles, many with laterally compressed teeth. While in truth the terms developed by Hendrickx et al. (2015) are likely to be broadly applicable, a brief discussion of what synapomorphies exist among the dentition of theropods would have been appreciated, so that those of us working under all that overburden could sort our rauisuchian teeth from our dinosaur teeth just a little easier.

Works Cited

Foster, John. 2007. "Allosaurus fragilis". Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Bloomington, Indiana: Indiana University Press. pp. 170–176

Hendrickx, C., Mateus, O. and Araújo, R., 2015. A proposed terminology of theropod teeth (Dinosauria, Saurischia). Journal of Vertebrate Paleontology,35(5), p.e982797. 

Brink et al. 2015 - What does it tell us about phytosaurs?

So new tooth news seems to be coming at a rapid pace this year! Several tooth-related papers have come out already and we're barely past the halfway mark. Most recently we have Brink et al. (2015) discussing funky features in meat-eating dinosaurs and, tangentially, other archosaurs. While I obviously love theropods and dinosaurs in general my interest at the moment is with non-dinosaurian archosaurs like our strange Chinle friends from Comb Ridge.

Duane Nash already did a good breakdown of what the article means in terms of theropod dinosaurs and how to relate the findings of Brink et al. to modern correlates as well as exploring what they could mean in terms of feeding and prey capture methods in various dinosaurs. If you haven't read his blog I'll wait.

Okay. Back? Good. As you can tell from both the article and the blog Brink et al. reject the stress-induced formation hypothesis for these interdental folds, as has been suggested previously. Instead they find that these structures are present even before stresses are placed on the teeth - while the unerupted teeth are still in the alveoli. So what does that have to do with Triassic teeth?

If you read the article you will see they sampled a few non-dinosaurian taxa (a phytosaur and an indeterminate Cretaceous croc) as well as the Triassic theropod Coelophysis. We have an abundance of phytosaur teeth at Comb Ridge and have picked up a few teeth we have tentatively IDed as theropod. So not only is Brink et al. a cool paper, it deals with some of our Triassic friends too!

Two views of phytosaur teeth in SEM and thin section, both from Brink et al. (2015), CC-BY
Image C shows mesial denticles under SEM and thin section. D shows a thin section with enamel, globular dentine, and primary dentine.

Brink et al. are looking mainly at the evolution and development of the structures with limited discussion on how the structures would have directly influenced prey capture and processing (though Nash, linked above, goes into that more). One of the more interesting things to me to come out of this is that phytosaurs have interdental fold structures like theropods and unlike crocs, Spinosaurus, ominivorous animals like Troodon and pure herbivores like ornithischian dinosaurs. Brink et al. further state that these adaptations are best interpreted as ways to capture large prey and crush bone. When we talk about phytosaurs, though, most people tend to interpret them as crocodile analogs. Sometimes this means perhaps ambushing large prey, other times preying on fish. This second option has been especially favored for the narrow-snouted forms, viewed by some as not robust enough to deal with large struggling prey.

A Redondasaurus attacks a decent sized prey item - a silesaurid. From Edyta Felcyn: go support her art!

There is some other evidence to suggest that phytosaurs were not just meekly eating fish and moderate-sized animals like dinosauromorphs (see image above). Coupled with their teeth that were perfectly adapted to ripping up large struggling prey items and mashing their bones, we have trace behavioral evidence to indicate this is exactly what happened. Last year Drumheller et al. documented a phytosaur attack on a living rauisuchian. You can read PastTime Podcast's take on the paper if you don't want to read through the paper itself. In short, though, they find evidence that a phytosaur tried to wreck shop on a rauisuchian, an animal that was basically a cross between a tyrannosaur and a crocodile. Wreck so much shop, in fact, that the phytosaur tooth went almost completely through the femur of the rauisuchian. This unfortunate fellow was then attacked by another rauisuchian and finally scavenged by a smaller phytosaur. Times were rough in the Triassic, even if you were the biggest, baddest fellow on the land.

Damaged psuedosuchian femur. The phytosaur attack is represented by the embedded tooth in Box A. Image from Drumheller et al. (2014).

If encounters like this were rare and the exception to the normal behavior of phytosaurs then the fossils described by Drumheller et al. are truly remarkable. Between the marked heterodonty found in adult phytosaurs described by Hungerbühler (2000) and the new evidence that they possessed dental adaptations that enabled them to capture, kill, and process prey larger than them it seems unlikely that this was a one-off chance encounter.

Ventral view of phytosaur snouts from Hungerbühler (2000). Note the different size and shapes of the teeth in this view.

 Instead our view of phytosaurs as fish-eaters occasionally attacking small-to-medium-sized land prey needs to be challenged. Phytosaurs were equipped with a dental battery that enabled them to routinely tackle large, dangerous, struggling prey as adults. This would include animals that were significantly larger than them. While juvenile phytosaurs seem to lack these dental adaptations (see, for example, my earlier post on this topic) and likely pursued prey smaller than themselves, adults would have been terrifying creatures to behold.

An interesting point to consider too: if phytosaurs were more like Nile Crocodiles than gharials, why don't we see ziphodont dentition in crocs? Certainly wildebeast and zebra don't give up after a fight. Brink et al. note that their Cretaceous croc also lacks ziphodont dentition, suggesting the behavior of crocs and their prey haven't changed much. Modern crocs are obviously capable of tackling large prey (though usually not larger than their own body). If they have gone hundreds of millions of years without the interdental folds and can eat large land prey, what were phytosaurs doing different?

Crocodiles and their prey in Africa - 2:57 from National Geographic

We don't have the fossils to answer that definitively but it would appear that modern crocodiles are not as good of an analogy for phytosaurs as has long been supposed. Hopefully future work at Comb Ridge and across Triassic collections will lead to new insights, clarifying what this unique clade was doing.

As an end note, Brink et al. suggest that ziphodont dentition with interdental folds is basal to all theropods, even thought phytosaurs possess the same tooth structure. It would have been nice to look at things like pseudosuchians from the Triassic to see if similar dental structure existed. If so, perhaps this sort of adaptation dates back to the rise of archosaurs in general. I guess that's another paper for another time.

Next up from me: a return to the lighter side. I'm going to be reviewing Richard Delgado's new Age of Reptiles comic series, Ancient Egyptians!

References:

Brink, K. S., Reisz, R. R., LeBlanc, A. R. H., Chang, R. S., Lee, Y. C., Chiang, C. C., ... & Evans, D. C. (2015). Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs. Scientific reports, 5.

Drumheller, S. K., Stocker, M. R., & Nesbitt, S. J. (2014). Direct evidence of trophic interactions among apex predators in the Late Triassic of western North America. Naturwissenschaften, 101(11), 975-987.

Hungerbühler, A. (2000). Heterodonty in the European phytosaur Nicrosaurus kapffi and its implications for the taxonomic utility and functional morphology of phytosaur dentitions. Journal of Vertebrate Paleontology, 20(1), 31-48.

My Students Need Your Help

I'm going to cut right to the point here (in case the title didn't give it away). My students need your help. I know I am the goofy tooth blogger who writes about "Indominus rex" and little tiny teeth from the Triassic but the plain fact is that those little tiny teeth don't find themselves. In fact they like to stay hidden, the sneaky little fellows. My high school students in my paleontology program find the majority of them.

In case you didn't read my introduction post, I teach at Mission Heights Preparatory High School and run the nation's only paleontology program at a public high school. I am pretty dang proud of it and my students. We have been doing this since March of 2014 and we already have one publication with one of my students, another pre-print ready to go to review with student lead authors, and a student-led abstract accepted for presentation at the 75th Annual Society of Vertebrate Paleontology Annual Meeting in Dallas this year. All very exciting stuff and it has all been predicated on the field work we have been doing in the Late Triassic Chinle Formation at Comb Ridge, Utah.

In order to do this fieldwork, we have been relying on two things: rented vehicles, paid for by student fees and my personal truck. These two elements have allowed us to access field sites and bring back fossils to MHP but they have their drawbacks. Student fees create a burden on our students, especially in our low-income, rural community that we serve. Some of these students are classified as homeless. Most of our students are on free-and-reduced lunch, meaning that their annual family income qualifies them for government-provided school lunches. These are generally not kids who can afford a $75 fee to rent vehicles, get gas, and buy food. Several promising young scientists had to miss out on trips (and have since moved on from science all together) because their family lacked the means to support them in pursuing our fieldwork. This is a huge concern for me! I have trimmed by budget as much as possible but with the huge cost of renting vehicles for multiple days I cannot get my per-person cost down any lower for our regular spring fieldwork trips. If we care about having scientists accurately represent our society we should be concerned that low income students are dropping out of science because they feel they cannot participate.

The second drawback is in relation to my personal vehicle. It is a 2004 Ford Explorer Sport Trac. It is a great vehicle and has served me well but it has 186,000 miles on it. I was working at a quarry in Utah with the Natural History Museum of Los Angeles County last week and my truck malfunctioned. Now the fix was easy and relatively inexpensive, and since the crew had multiple vehicles they were not out of work while my truck was in the shop. My truck, however, was out of commission for a day and a half. On a multi-week trip this is not a huge deal but when we are working at Comb Ridge we are typically out for only two days. That is a huge blow! I was lucky enough to have my truck break down in Moab - that would certainly not happen with our Comb Ridge work since we rarely go into town. The nearest town, Bluff, also does not have a full-service auto repair facility and parts stores. In addition the NHMLA had multiple field vehicles available to get in and out of locations and haul gear. For our program we use my truck to haul all the gear and rented vehicles to haul all the students. If we had a breakdown with my truck in the field with students it could be a real disaster with no easy fix.

That is why I am asking for help. I am trying to raise $8,000 for a field vehicle for MHP. We have raised a bit over 1/8th of the total goal but we have a long way to go. I am hoping that if you care about students, paleontology, getting students involved in paleontology, or just creating a more science-literate society you will consider donating to our fundraiser. All money raised will go directly to the cost of a field vehicle or, if we are unable to purchase one, into renting field vehicles until the funds are depleted.

If you like learning about teeth, that is where the teeth come from. And I have more tooth posts coming up soon!