Segment Variation in the Cambrian Trilobite Elrathina Cordillerae in the Context of the Burgess Shale Fauna

Variation in the number of segments in the thorax of mature specimens is a marked aspect of trilobite evolution at a range of taxonomic levels. Likewise, variation in segment number and segment form are key aspects of arthropod evolution as a whole. A trend toward stabilization of mature thoracic segment numbers is evident not only in the evolution of trilobite species, but also at higher taxonomic levels. The examination of species that vary in mature segment number offers potential microevolutionary insight into how trilobites generated the major, macroevolutionary differences in segmentation that characterize the whole clade. The middle Cambrian Burgess Shale offers an extraordinary opportunity to examine this issue by using the abundantly represented trilobite species Elrathina cordillerae. This species, which is known to vary the number of thoracic segments in the holaspid phase, was deposited along with soft-bodied fauna during a series of burial events that make up the Greater Phyllopod Bed. Intraspecific variation in segment numbers has been confirmed at several horizons, and mean numbers may differ among beds. Furthermore, the taphonomic condition of E. cordillerae may provide additional insight into preservational controls on Burgess Shale fauna. By studying the covariation in mature segment number with the soft-bodied faunal assemblages, we are testing whether variation in thoracic segment number was related to repeated paleoecological associations with the soft-bodied fauna. We will use this relationship to explore what inferences may be drawn on the adaptive basis of variation in trilobite segment numbers.