CONSERVATION AND CHANGE IN SEGMENT NUMBER IN TRILOBITES OVER THE PALEOZOIC (Invited Presentation)

Arthropods are invertebrate animals with an exoskeleton, paired jointed appendages, and a segmented body. The final segmental composition of the body in many arthropods is attained during post-embryonic development through a series of molts. Trilobites, in particular, have been useful for studying post-embryonic segmentation patterns because their exoskeleton was highly biomineralized from an early post-embryonic stage, and thus they have a rich fossil record that includes complete developmental series for many species. The trilobite exoskeleton was composed of a cephalon (head shield comprised of fused segments), thorax with repeated, articulated tergites, and a pygidium (tail shield comprised of fused segments). Anecdotally, it has been proposed that the average number of thoracic tergites decreased over the evolutionary history of the clade, and that the number of segments in the pygidium increased over time. The latter has been referred to as “caudalization”, an idea which also encompasses an apparent trend towards increased differentiation in the morphological expression of segments in the pygidium relative to the thoracic tergites. Together these observations predict long-term trends in shifting patterns of segment identity and expression in the trilobite body. Using images from museum repositories and primary and secondary literature, we tabulated the number of segments in the post-cephalic body for over 1100 trilobite species, selected to maximize taxonomic, geographic, stratigraphic, and environmental representation. We found a long-term decrease in the maximum number of thoracic tergites and evidence of increased conservation of thoracic tergite number within groups over the Paleozoic. The mean number of pygidial segments increased throughout the Paleozoic, significantly from the Cambrian to the Ordovician and in the late Paleozoic. The average total number of post-cephalic segments was constant through most of the Paleozoic, but increased in the latest Paleozoic with the extinction of all but proetids. Not only do these results broadly confirm earlier observations, they indicate long-term shifts in segment identity within a broader constraint in total segment number in the trilobite body.