Recent advances in development genetics and cell biology are revolutionizing understanding of the developmental basis of animal form. The mechanisms by which body segments, the modules from which arthropod bodies are constructed, are specified and differentiated are becoming known across members of the crown group Euarthropoda, which contains all living arthropod groups. The extinct clade Trilobita also belonged to the Euarthropoda but with respect to the control of its body patterning did not depart markedly from what was apparently the ancestral euarthropod condition. Hence the exceptionally well-preserved record of trilobite ontogeny and phylogeny recorded in early Paleozoic rocks assumes special significance as a chronicle of early euarthropod experiments in body patterning. The Silurian aulacopleurid trilobite Aulacopleura konincki has proved to be an exceptional model system for exploring how trilobites controlled the number of segments assigned to the trunk region and the controls on their allocation of trunk segments to the thorax or to the pygidium - the terminal shield consisting of conjoined segments. Aulacopleura konincki shows unusual variation in the number of thoracic segments during the mature, segment-invariant growth phase. Previous work by the PI Hughes has tested hypotheses concerning control of the mature segment number, with results favoring the Early Determination Hypothesis (EDH), in which the ultimate number of segments was determined early in ontogeny. This suggests highly regulated developmental control among early arthropods, even those apparently plastic in aspects of body construction. However, determining the controls of development in a species 435 million years old presents a severe challenge. This project will explore ontogenetic trajectories of sclerite shape change to provide additional tests of the EDH, based primarily on the pattern of shape change in the cephalic (head) region that is independent of trunk segmentation. The paleoenvironmental context of changes in segmentation will also be explored through a geochemical proxy approach. The project will involve graduate student training and extensive collaboration with scientists in the Czech Republic, Italy, and Canada, and will involve the development of new statistical software that will be made freely available to the scientific community.
