DISCOVERING PARAMETERS THAT GOVERN VARIATION IN TRILOBITE GROWTH AND BODY SIZE THROUGH MODELING

Like some centipedes and most crustaceans, the final segmental composition of the trilobite body was attained during post-embryonic ontogeny through a series of molts. Trilobites had a hemianamorphic mode of segmentation, meaning they underwent a phase of molts during which new segments were added to the body followed by a phase during which molting continued without further increase in the number of body segments. Previous work has shown that per-molt size increase was constant for some sclerites (particularly the cephalon) at least for some stages of ontogeny. However, detailed data-rich studies of the trilobite Aulacopleura koninckii indicate that the rate of growth in the thorax was governed by a growth field described by a continuous gradient rather than constant per-tergite rates, and that this gradient shifted during the transition from meraspid to holaspid stages. It is possible that for many species, rates of growth varied across different parts of the exoskeleton and changed during ontogeny. Here I use a modeling approach to determine the effect of altering different parameters on trilobite growth and body size. The current model has parameters for setting initial values, transition points, and applying growth gradients. Although the model was calibrated using data from a single trilobite species, a diverse array of observed body sizes and relative proportions of body regions can be attained by altering only a few parameters at a time. Initial results indicate that for a given number of molts, altering growth rates has a greater affect on body size that altering the number of thoracic segments.