Tyrannosaurs Don’t All Have Bananas for Teeth


One of the things that I’ve been itching to do is generate a database of variation in dental series across a variety of taxa, virtually most if not all variations of archosaurs and beyond. This is to assess the quality of jaw shape with tooth shape, tooth function, jaw function (not always overlapping), and diet. Fun stuff … for me. It would be incredibly repetitive, boring, and time-consuming research, and probably something that would take me years of world-hopping to achieve (although I can work from casts just as well as real specimens).

Why?

Because systematists have often, if not frequently, named taxa based on the barest of scraps based on the inferred distinction of said scrap to another scrap with a different name, under the impression that these scraps preserve a quality of “diagnostic” that has been granted to them from up on high (else where would the definition derive?). Based on scraps is one thing, and requires its own qualifer; based on teeth is another, and requires only that the type, or type-series, be comprised solely of teeth. Loose teeth are often recovered as shed items, rarely as taphonomic artefacts part of other specimens, and often broken (few preserve an intact root). Despite this, they have been used as valuable for recording taxonomic density in a given area or stratigraphy, and for limiting or expanding taxa across regions. They have been used to aid in lumping and splitting. There are over 100 taxa that have been named on the sole basis of a tooth-based holotype in Dinosauria alone.

But despite this level of caution, some workers continue to value teeth as both stratigraphically and taxonomically valuable.

Jack Horner and colleagues in a recent paper in PLoS One have sampled an entire exposure (the Fort Peck reservoir) of the Hell Creek Formation (latest Maastrichtian) in Montana, but included in this a small cautionary tale, showing something that I’ve been wanting to do myself:

Tooth size varies as much as 300% in a single jaw, particularly in hadrosaurids (MOR 1609, Becky’s Giant), ceratopsids (MOR 2574, Quittin Time) and tyrannosaurids (MOR 1125, B-rex). This is one reason the assignment of some dinosaur teeth to ‘‘babies’’ [19] may be incorrect. These teeth are more accurately interpreted as being derived from the anterior or posterior portions of jaws from older individuals (Figure 3). Only the largest and most robust tyrannosaurid teeth are reliable indicators of adults.

– from Horner et al. (20110). (“[19]” refers to Carpenter (1982). Citation is provided below. Figure 3 is reproduced below, with reformatting for this blog.)

Warning! Big!

Teeth of a specimen of Tyrannosaurus rex, MOR 1125. A is a distal maxillary, B is a rostral maxillary. Both preserve complete roots and are recovered in situ from a maxilla.

While cited in caution of ascribing variation among taxa from isolated teeth (especially when distinguishing inferred ontogenetic stages as taxa — I can’t help but think this is specifically being pointed at citing small tyrannosaur teeth as Nanotyrannus lancensis, which the authors {among many others} argue is a juvenile of Tyrannosaurus rex), it serves an additional purpose: Note how the smaller tooth is so very morphologically different from the larger? The smaller tooth has a distinct ch:bl (crown height:basal length) aspect ratio, while showing a higher curvature of the mesial margin compared to the distal margin. Horner et al. (2011) might even have made the same comparison viable had they compared a distal maxillary with any premaxillary crown, as the two may be close in size and even aspect, but differ in morphology.

It is not enough, perhaps, to show that size disparity exists in a maxilla’s crowns. Of course they do.

Tyrannosaur maxillae, showing A-B, Tarbosaurus baatar juvenile and subadult, C, Shanshanosaurus hongshuyanensis (Tarbosaurus juvenile?), and D, Raptorex kriegsteini. All images modified.

It should be required that to infer that any qualified disparity exists, the inferrer must quantify the level of distinctiveness involved. Gross size disparity is not enough; morphological disparity is needed. If these two teeth were found isolated, it might very well have been useful for early discoverers to have named two different taxa from them. One of them would end up being a fairly smaller animal than the other, or perhaps the larger was a short-face, sabretoothed reptile? Whoever would know? We can’t get away with that crap now, of course, because we have things like complete jaws where the level of variation is knowable … it’s just not known.

The need to adequately quantify, to minute levels of detail, factors excluding the size of teeth are important for assessing dental variation in sampling, so that issues such as the dental “bum rush” of the early 1900s do not occur, or that specimen variation can be assessed versus inferred taxa, so that some way to measure diversity (and thus measure a proxy from which to distinguish a “morphological taxon”) can be determined.

I want to take my time with such a study. It would be helpful if others could quit anecdotally hand-waiving these issues for the sake of making taxonomic arguments (either there are fewer or more taxa to account for the relative variation involved). It doesn’t help.

Horner, J. R., Goodwin, M. B. & Myhrvold, N. 2011. Dinosaur census reveals abundant Tyrannosaurus and rare ontogenetic stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA. PLoS One 6(2):e16574 doi:10.1371/journal.pone.0016574
Carpenter, K. 1982. Baby dinosaurs from the Late Cretaceous Lance and Hell Creek formations and a description of a new species of theropod. Contributions to Geology. University of Wyoming 20: 123-134.

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5 Responses to Tyrannosaurs Don’t All Have Bananas for Teeth

  1. Mark Wildman says:

    Good post! Darren Tanke’s recently prepped and well publicised juvenile Gorgosaurus skull demonstrates perfectly the variation in maxillary teeth alone.
    Although a juvenile, a few teeth could easily pass as sub-adult while the smaller could be deemed even younger than that.

  2. “One of the things that I’ve been itching to do is generate a database of variation in dental series across a variety of taxa, virtually most if not all variations of archosaurs and beyond. This is to assess the quality of jaw shape with tooth shape, tooth function, jaw function (not always overlapping), and diet. Fun stuff … for me. It would be incredibly repetitive, boring, and time-consuming research, and probably something that would take me years of world-hopping to achieve (although I can work from casts just as well as real specimens).”

    Well, this is actually exactly the purpose of my thesis. However, I should achieve such exciting work in… three years. I already created a datamatrix of more than 150 characters on teeth and, although I still need to add many new characters regarding the morphology of denticles for instance, and also many other taxa to my matrix(I only coded 35 taxa so far), the results are already quite encouraging.

    • That sounds excellent, Christophe. Of course, the desire for the data matrix as it were is to feed into my exploratory work for diet reconstruction mapped to living animals. I hope, nay desire, that many diverse extant taxa are sampled, from various reptiles through to basal amniotes and up to synapsids. Of coruse, mammalian paleo used to be all about teeth, so much data can be present therein, too!

  3. As I’m sure you know, this is exactly along the lines of what Josh Smith was beginning to do. I certainly needs the kind of expansion that you’re talking about, but there is, at least, a decent foundation of a sort in place!

    Smith, J.B. 2002. An Examination of Dental Morphology and Variation in Theropod Dinosaurs: Implications for the Identification of Shed Teeth. Unpublished Ph.D. dissertation, Dept. of Earth and Environmental Science, University of Pennsylvania, Philadelphia, 617 pp.

    Smith, J.B. 2005. Heterodonty in Tyrannosaurus rex: implications for the taxonomic and systematic utility of theropod dinosaurs. Journal of Vertebrate Paleontology 25(4):865-887.

    Smith, J.B. 2007. Dental morphology and variation in Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar; pp. 103-126 in Sampson, S.D. and Krause, D.W. (eds.), Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar. Society of Vertebrate Paleontology Memoir 8.

    • Ah, yes. I was eventually going to continue citing Smith’s work (a little lazy of late, been doing so anecdotally) when I started making slightly more substantive non-“that should be a paper!” posts, but I’ve been focusing on fleshing out toothLESS study, so it slips my mind when I suddenly delve back into teeth fullbore after a year of hiatus.

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