Thursday, 15 September 2011

Fossil Teeth Reveal So Much Data


I’ve blogged about fossil teeth before and, more recently, referred to Eagle et als paper (2011) which demonstrated how dinosaur body temperature could be determined by analysis of tooth enamel. Teeth are able to provide an amazing amount of data and some of it is often surprising.

The best thing about teeth is that they fossilise quite readily. They are tough and reliably tolerant of distortion and they can be found in such pristine condition that it looks like they may have been shed only yesterday. And they are relatively commonplace in many localities, even when skeletal remains are rare. Chief amongst the providers of teeth are vertebrate microsites and these provide a valuable window into the make up of a vertebrate palaeocommunity with not only dinosaurs but also fish, amphibians, other reptiles, mammals and birds sometimes all represented.

Identifying dinosaur species at family, generic and species level has represented an ongoing challenge for several authors over the years. Teeth, even those that are stunningly preserved, can often only be diagnosed to family level since, beyond this point, it is common for taxon specific teeth to be almost identical, thus harder to diagnose. Those teeth that are naturally worn, broken and weathered are even harder to assign.

So there is always an element of error in identifying scattered isolated teeth unless they are associated with skeletal material and it is very likely that some isolated teeth probably represent new taxa yet to be discovered. And yet today there is more and more detail being extrapolated from fossil teeth than ever before.

The aforementioned tooth enamel is now also being used as a tool to aid identification. Hwang (2010) has been examining the microstructure of tooth enamel using scanning electron microscopy and the details revealed have proven illuminating. Of particular interest is the fact that teeth, despite looking morphologically similar, can be differentiated because of the make up of the enamel. This is really useful with groups such as hadrosaurs, ankylosaurs and tyrannosaurs.

However, although this technique enables teeth to be identified more readily, it still is not always possible to provide diagnosis at the generic or species level. But even here, there has been a modicum of success and a few specimens have indeed been identified to the genus level. This research is yet another tool helping to identify isolated teeth and will be especially useful in identifying those that are too damaged or worn and unidentifiable.


Theropod teeth are always impressive to look at and as such are intensely researched and the chief interest around their morphology usually focuses on their serrations. The most obvious analogy that everybody is familiar with, especially in the popular media, is that serrations on teeth made them akin to a steak knife of today and aided the animal in cutting through flesh and slicing and dicing meat. But things are seldom as simple as this.

Serrations, or denticles to give them their correct term, have been described as helping the tooth penetrate flesh and bone but with less force than would be required with a tooth that had none (Abler 1992). Generally speaking, theropod teeth are more densely serrated distally than they are mesially, troodontid teeth not withstanding. But teeth need to work in conjunction with a number of other factors to make them truly efficient.

D’Amore (2009) conducted experiments that demonstrated that the curvature of teeth increased distally in the dentary which made for a very efficient jaw mechanism. Its simple mechanics when you think about it in as much as the tooth that is closest to the hinge will pass through the greatest amount of rotation, thus needing to be more recurved so that it penetrates the substrate efficiently whilst those at the front of the dentary tend to be straighter because they do not go through such an acute angle (see below).


From D'Amore 2009
Tooth serrations are remarkably efficient since so many taxa have developed them, both extinct and extant. Experiments using finite element analysis (FEA) have also been utilised to determine their functionality and how they may have evolved, not only in dinosaurs, but also in sharks and mosasaurs. Various elements have to be taken into consideration when utilising FEA in this form of study such as variation in denticle composition, the overall gross morphology of the teeth and how the distribution of pressure and different stress levels affects these different morphologies.

Miriam Reichel of the University of Alberta has been performing this research over the last few years as part of her PhD project and has found that tooth function dictates how teeth are serrated and affects their physical properties. To emphasise the efficiency of serrations, all teeth subjected to the pressure of estimated bite forces performed admirably with very little stress displayed.

So it can be seen that teeth, even today, are still yielding a wealth of information and will, no doubt continue to do so. The fact that there are so many teeth to work with is obviously a contributing factor. Theropods replaced their teeth continuously throughout their lives and, asides from the previously mentioned microsites, are found in association with skeletal remains, often scattered throughout the carcass and the immediate surroundings.

Strangely though, when theropod skulls and jaws are recovered, they seldom have vacant tooth sockets displayed and, when this is compared with the amount of shed teeth that are found, suggests that tooth replacement in theropods was indeed rapid. And, luckily for us, this provides a wealth of fossil teeth that will continue to satisfy our continual thirst for knowledge in the foreseeable future.

References

Abler, W. L. 1992 The serrated teeth of tyrannosaurid dinosaurs, and biting structures in other animals. Paleobiology 18, 161–183.

D'Amore DC. 2009. A functional explanation for denticulation in theropod dinosaur teeth. Anatomical Record 292: 1297–1314.

Eagle, R.A., Thomas Tütken, Taylor S. Martin, Aradhna K. Tripati, Henry C. Fricke, Melissa Connely, Richard L. Cifelli, John M. Eiler. Dinosaur Body Temperatures Determined from Isotopic (13C-18O) Ordering in Fossil Biominerals. Science, 2011; DOI: 10.1126/science.1206196

Hwang, S.H., (2010) The evolution of dinosaur tooth enamel microstructure. Biological Reviews Volume 86, Issue 1, pages 183–216, February 2011 Online publication date: 1-May-2010.

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