Segment Development in Silurian Trilobite Aulacopleura Konincki

Studies of development of trilobite segmentation provide potential insight into the mechanisms controlling variation in segment numbers. This information is important because variation in segment numbers is a characteristic at higher taxonomic levels among trilobites, but usually constant within species. The ontogeny of trilobites is characterized by a juvenile phase of segment accretion, followed by a mature stage in which the organism achieved a stable number of thoracic segments despite continued growth. The middle Silurian Aulacopleura konincki displays intraspecific variations in numbers of thoracic segments within maturity, ranging from 18 to 22 segments. Two hypotheses have been suggested to explain the variations: the Early Determination Hypothesis (EDH) states that final segment number was determined early in ontogeny (genetically or environmentally) and the Later Determination Hypothesis (LDH), which suggests that the final number was determined when a certain critical threshold, such as size or shape, had been reached. Previous studies solely based on size data suggested that EDH was a better fit to the observed size-frequency distribution than LDH, favoring a scenario in which the number of thoracic segments was determined earlier in the ontogeny. However, preliminary analysis of cranidial ontogeny among the different morphs apparently conflicts with the predications of the EDH. A new dataset of the highest taphonomic quality specimens is required for detecting the subtle shape changes during growth necessary to explore this conflict. Preliminary results from the new dataset confirm the results of prior analysis but at higher resolution: variances within each juvenile developmental stage are lower. In addition, ratios between trunk segment lengths apparently behave in a predictable manner at the juvenile-mature boundary period, and segment length ratio data has potential to aid greatly in further evaluating the two hypotheses.