SIMPLE METRICS CHARACTERIZE TOOTH FORM AND EVOLUTION OF ITS COMPLEXITY OVER TIME AT TWO LEVELS OF ANALYSIS IN THE JAW APPARATUSES OF CONODONTS AND ARCHOSAURS

Existing approaches to the characterization of complexity of organic form are limited by the fact that quantitative analyses have been undertaken at a single, common level of structural organization. Our objective is to circumvent this limitation by employing several approaches to the measurement of complexity in tandem. This enables us to study patterns of evolution simultaneously at different hierarchical levels of organization.

We have established a theoretical morphospace, variants of which enable us to explore patterns in the evolution of the teeth of different groups of chordates. This morphospace has elements in common with the Skeleton Space of Thomas and Reif (1993). In this case, we are able to quantify the variables in such a way as to generate simple information functions that reflect tooth complexity. Our conodont and archosaur element/tooth design spaces and the metrics derived from them enable us to analyze patterns of evolution in tooth form, both within specific clades and amongst higher taxa in these two groups of chordates. We have chosen to investigate this system in particular because the forms of living and extinct vertebrate teeth are well documented, they are morphologically tractable, and teeth are sensitive indicators of vertebrate evolution.

We have also devised measures of the degree of differentiation of elements and teeth in the apparatus or jaws of the same taxa. Complexity at this level of skeletal organization is necessarily correlated with the complexity of individual elements or teeth, since primitive conodonts and archosaur teeth were both simple and little differentiated within the apparatus or jaw. However, this correlation weakens in more specialized taxa. Either more complex individual elements and teeth or more strongly differentiated functional complexes emerged in response to the demands of distinct feeding or other adaptations. Here, there is a trade-off, based on functional and constructional constraints, between the complexity of structure at one level or organization and the next.

Thomas, R. D. K. and Reif, W.-E. (1993) The skeleton space: a finite set of organic designs. Evolution 47: 341–360.