TRILOBITE LIFE-HISTORY STAGES IN THE CONTEXT OF ARTHROPOD SEGMENTATION AND SEGMENT DIFFERENTIATION

Trilobite life history phases are recognized on the basis of the development of exoskeletal segmentation of which there are several different aspects. Exoskeletal segments may differ in shape, size, and articulation state, and different intervals of growth may have either increasing or stable numbers of trunk segments. The phaselus, protaspid, meraspid, and holaspid phases are here specified explicitly in terms of exoskeletal articulation. Other life history phases are here based on patterns of segment expression and include an early cephalic epimorphic phase, a trunk anamorphic phase during which new segments were expressed, and a trunk epimorphic phase that was terminal. Where the trunk was divided into two batches of similar segments a growth phase boundary can be recognised at the first appearance of posterior trunk segments. Comparison of the coincidence of the phases boundaries of these different aspects of segmentation highlights rich variation in segmental development across Trilobita, and provides a basis for assessing trilobite life history phases in the context of those known among modern arthropods. Trilobites displayed a hemianamorphic growth pattern accomplished over an extended series of instars that spanned free-living juvenile stages to mature individuals.