HUGHES, Nigel C.; Univ. of California, Riverside: Historical Experiments in Terminal Growth, Segmentation and Body Plan Regionalization

Terminal growth is apparent in those fossilized organisms in which progressive incremental change in the morphology of the posterior region occurred during development. It is recognized in fossils via the pattern of appearance of discrete structures, such as scales, plates, or segments. It can be determined with most confidence when the anamorphic addition of terminal structures matched other proxies for growth, such as increased size of pre-existing body elements. Several early metazoan clades, including some members of the late Proterozoic Ediacaran assemblage likely grew by the anamorphic addition of body elements, at least for portions of their ontogenies. Terminal growth is most readily evident among ontogenetic series of early arthropods, in which the appearance of segments of unique identity clearly indicates a sub-terminal zone in which individual trunk segments were first expressed. The Trilobita displayed variations in trunk segment numbers, sizes, shapes, and regional differentiation that may suggest evolutionary trade-offs between flexibility in overall numbers of trunk segments and the advantages of regionalization (i.e. increased tagmosis) within the trunk. They offer insight into the historical steps through which a distinct posterior tagma arose within a major arthropod clade. In some trilobite clades exoskeletal segments served as a primary constructional scaffold upon which regional differentiation was later imposed.