Tooth-Based Taxonomy

Taking abreak here to discuss something more germane to this blog’s theme. We’ll get back to oviraptorosaurs and their nearly utter lack of teeth shortly.

Tooth-based taxonomy (or TBT) is something you will see frequently from here on in this blog. It is fundamental to my primary direction of research, in that species named based solely on teeth are limited in their comparability to species not based on teeth or species for which no teeth are known. To resolve this issue, the topic of the importance of isolated teeth (the basis of TBT) will fill this (dense) post. More below.

“What’s in a tooth?”— not Shakespeare

In many cases, teeth enjoy a special place in the taxonomic scheme. They are used predominately to identify mammal and fish fossils, and are very useful in biostratigraphic work when comparing marine and land faunal ages. Fossil sharks and some fairly extensive mammal clades are known solely on the basis of teeth. For mammals, a domineering amount of research has gone into defining and associating tooth types and associating each dietetically and phylogenetically [2,3].

This utility is often anecdotal, however, and has led to some misidentifications when more complete material has come up. In fact, much of the early erroneous identification of mammalian teeth is cleared up almost solely through the recovery of a set of associated teeth and/or a jaw with teeth in situ.

[Note: A section appeared here describing dental morphologies. I removed this because I realized it deserves a more specific treatment; dropping a small section about morphology of teeth into a topic about the messiness of TBT seems clunky and unfocused, and this post is already too much of that. Rest assured, it will come back in a more discussable form.]

Unlike mammals, in reptiles more complex morphologies are less common, and when they crop up, they are distinctive. Especially peculiar and distinctive teeth occur in some crocodilians (such as the multi-cusped, multi-rowed teeth of Chimaerasuchus [1]), hadrosaurian dinosaurs (enamel on one side, tiered and ranked like a Roman phalanx, and bearing articulate ridges, lophs, and marginal and lophal denticulation). Blade-shaped teeth (called ziphodonty) is the most common morphology among crocodilians, and indeed among archosaurs in general, as well as various carnivorous lizards, including monitors; snake teeth are generally cones (termed conodont, although as some note, this term is clunky and confusing), and lizards in general vary a good deal between ziphodonty (not common) and phyllodonty (common), although the difference between these may be so blurry that the term “phyllodont” (leaf-tooth) may be useless. This makes associating tooth morphology to a clade difficult, to say the least. Historically (and [2] & [3] are good examples of this, dentition alone has provided the systematist with the idea to refer a taxon to a given clade: many ziphodont crocs were considered theropod dinosaurs, or the reverse. So why is a seemingly unstable system still being used?

The clue in which TBT has been most useful has been biostratigraphy, associating faunal compositions to a fine degree across formations, countries, and continents (as well as through time), as well as assessing ecomoprhologies (knowing your prey and your predator species, and their relative numbers). Collapsing taxonomy has a related affect in reducing (although sometimes increasing) the comparability of two different faunas or formations, allowing the stratigrapher or the ecologist the ability to simplify his or her argument when comparing two groups. It is arbitrary, but at least the data is all there to assess.

For the most part, TBT is predominately a match game within a broader systematic puzzle: Teeth are referred to one another on a “best-fit” paradigm, in which gross morphology is used to assign specimens to varying types of taxa. Traditionally, these taxa are defined by their rank: It is easier to place varying teeth in families, but not so much genera, and certainly more difficult into species. The scale of the rank of the taxa the teeth are placed into are categorizations of how clear-cut the “best-fit” model works: Conodont teeth with fluting in theropods are generally assigned to Spinosauridae (and even closer, can be placed into Spinosaurinae), and tiny, low-aspect, “coarse” denticulate teeth are referred to Troodontidae. It takes more specific criteria to begin assigning these to genera and species, but for the most part, this is not at all morphological but stratigraphic (see above, although there are exceptions). This renders much of the broader concept of TBT to a parataxonomic process, by which form taxa are used to represent proxies to real taxa.

In the end, theoretical arguments need to be made which regard the morphology of teeth, and questions about their applicability to taxonomy. This requires a framework under which the parataxonomic process can be assessed to “actual” taxonomic processes, and if the two are compatible in the case of teeth.

So where am I going with this? I want to ask some important questions, and certainly the biggest among them are: Does the variation in teeth represent actual taxonomic variation, and if it does, how significant is this? If this variation is not significant what do we do about TBT?

In the general scope of science, comparability among morphological schemes is important. In systematics, for example, the referral of specimens to taxa are taken under certain rules, although most of these are soft and not enforceable. One of the more important of these, and it’s the most by a wide margin, is that when two specimens are not comparable, one should not consider them identically. We have a tendency to overlook this when there are other considerations, and in the hopes of ansering the above questions, I am tempted to simple state that when all is said and done, when two specimens with different names are not comparable to one another, do not refer them to the same taxon. The systematist who does this is taking a leap of faith with little grounding on the principle of assumptiosn that have nothing to do with comparison of material. This will be my primary rule when comparing tooth-based taxa, and my yardstick for segregating material for data points in order to answer the above questions.

[1] Wu X.-c, Sues, H.-D. & Sun A.-l. 1995. A plant-eating crocodyliform reptile from the Cretaceous of China. Nature 376:678-680.
[2] Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co., New York. xiv+698 pp.
[3] Romer, A. S. 1962. The Vertebrate Body. W. B. Saunders Co., Philadelphia. vii+627 pp.

Update: I’ve edited this post slightly to reflect removal of a section concerning general morphology of teeth.