Intra-tooth profiles and ancient seasonality
current as of 25-Feb-07
Two views of a rabbit incisor, an evergrowing tooth that is constantly being worn at the incisal tip (right side) and regenerated at the open root (left side). This tooth preserves a ~3 month record.
Cross-section of a bison molar in a late stage of tooth development.
A test of forward model predictions for an evergrowing tooth with known dietary history.
A test of inverse model estimates of the input signal for the same tooth as figured above.
3D rendering of volumetric microCT data. The tooth with the open root is a rabbit upper incisor; the distal portion of the tooth is to the lower right (the tooth was cut into two pieces to facilitate analysis). The four equant bodies in the middle and lower left are pieces of bison molar that are used as density standards. Field of view is about 1 centimeter. 1.0 MB Quicktime movie
Climate and seasonality Do San Francisco and New York City have similar climates? Both have mean annual temperatures of about 13 degrees Celsius (55 degrees Fahrenheit), and both are in coastal settings, but they differ dramatically in their seasonal marches of temperature. New York in winter is snowy and windy, while San Francisco is rainy but tolerable. In the summer, San Francisco is dry and pleasant, while New York can be hot and muggy.
Seasonality of temperature and precipitation is a key aspect of climate, but most geochemical paleoenvironmental proxies speak only to average yearly conditions. Recently, though, workers have begun to develop methods to provide direct seasonal information about past climates. These include stable isotope profiles in incrementally growing tissues such as shells, corals, otoliths, and teeth.
I have focused on mammal teeth as archives of seasonal information in terrestrial environments. In addition to recording seasonal aspects of climate, mammal teeth may also record seasonality in behavior, including migration and reproduction.
Teeth as isotope tape recorders Like other incrementally growing tissues, teeth have directionality in formation. Mineralization begins at the crown, and proceeds along towards the root. Evergrowing teeth such as rodent incisors are good examples of this directionality: the sharp tip is the oldest portion of the tooth, and the newest tooth material is added at the open 'root' inside the jaw. The tooth is continually worn away at the tip, while it is continually forming at the root, such that the overall length of the tooth remains fairly constant. The tooth, then, contains a record of the animal's isotope chemistry, and the time duration of the record is a function of the growth rate and length of the tooth.
Two stages of mineralization Tooth enamel has a more complex genesis than simple accretionary structures such as tree rings or shell laminae. The first stage of amelogenesis (enamel formation) is accretionary in the sense of tree rings. However, the accretted material—enamel matrix—is soft, porous, and mineral-poor. A second stage of amelogenesis involves gradual replacement of pore space with enamel mineral (bioapatite) until the matrix has matured into hard, crystalline enamel with ~95 volume percent mineral. Maturation takes place over a fairly broad swath of most teeth, and may have considerable duration. The result: isotope signals measured in teeth are time-averaged, or 'smeared', compared to the true pattern of isotope variation experienced by the animal during tooth formation.
Smear prediction: Forward modeling The first step in addressing a problem is understanding the problem. By using chemical methods and X-ray micro computed tomography (microCT) to study developing teeth, we can quantify the temporal and spatial patterns of amelogenesis. This information can then be embodied into a forward model that allows prediction of how an input signal (such as the temporal variation in animal isotopic composition) will appear as spatial isotopic variation along the length of a tooth. Forward models for teeth are similar to running averages, but are specialized cases thereof. The concept of forward modeling was described for evergrowing teeth by Passey and Cerling (2002) pdf.
Recovering the input signal: Inverse modeling Forward modeling shows that the form and amplitude of an intra-tooth isotope profile can be as much an artifact of the maturation process as it is a product of the input signal experienced by the animal. For intra-tooth isotope data to be meaningful, we need a means of reconstructing the primary input signal. We have applied inverse methods, similar to those used by geophysicists interpreting seismic data, to this problem. The methodology is outlined by Passey et al. 2005b pdf. The procedure involves casting the forward model of a tooth in matrix form, and defining the measured isotope signal d as the product of the primary input signal vector m and the forward model matrix A. We utilize a damped, minimum-length solution to estimate m, and use computers solve the system numerous times to generate an array of possible solutions.
Current work My colleagues and I have outlined the theory and application of intra-tooth forward modeling and inverse methods, and have shown with a small number of controlled experiments that these approaches basically work. The question now is how these results will scale to other types of teeth, other species, and real-world situations. We
are now in the process of developing models for non-evergrowing
teeth, using goats, steers, pigs, and other species as study systems. MicroCT work performed on the same tooth positions (e.g. goat upper M2) at different stages in development will enable accurate quantification of the tooth development process. Controlled diet changes recorded in these teeth will allow for evaluation and improvement of modeling techniques.