ASSESSING THE ROLE OF DEVELOPMENTAL CONSTRAINTS IN TRILOBITE EVOLUTION (Invited Presentation)

Rates of taxonomic and—for at least some traits—morphological evolution generally declined through trilobite history. Such a pattern might have resulted from decreasing ability to generate phenotypic novelty and/or decreasing opportunity for any such novelty to become ecologically established. Intraspecific variation represents a raw material upon which natural selection operates, so it is intriguing that the frequency and magnitude of intraspecific variation have also been demonstrated to have been higher in Cambrian versus post-Cambrian trilobites. However, such an observation is silent as to underlying cause(s) because phenotypic variation is influenced by genetic, developmental, and ecological constraints. Resolving the relative contributions of these constraints to macroevolution is an important but nontrivial task.

Developmental constraints are biases on the production of phenotypes imposed by the developmental system. Not all types of developmental constraint can be identified in the fossil record, but for those that can it is desirable to assess their relative importance to macroevolution. With a series of case studies, I show how geometric morphometric analyses can rigorously test whether and how trilobite disparification was influenced by various types of developmental constraint. These tests exploit the fact that different types of developmental constraint (e.g., homeorhesis, developmental integration, and ontogenetic allometry) make unique predictions regarding how a particular aspect of phenotypic variation (e.g., static or dynamic trait [co]variance; symmetric or asymmetric components of trait variance) should relate to the direction of evolution.

Analyses conducted to date indicate that, among Cambrian trilobites, developmental constraints were generally weak. Both the structure of integration and the pattern of ontogenetic shape change were evolutionarily labile even over small phylogenetic distances, and neither would have served as a long-term constraint over the direction of phenotypic diversification. This is consistent with the dramatic disparification seen among early trilobites. With adequate sampling of appropriately preserved material, the timescale over which developmental constraints operated can be investigated.