Growth progression in trilobites: a preliminary study.

Arthropod evolution has frequently been characterized by marked diversification, be it in the variety of segments or limbs along the body, or in aspects of the developmental schedule. Trilobite post-embryonic development comprised a long series of free-living instars, with transitions between successive instars generally characterized by modest morphological change. Such a condition apparently also characterized basal Crustacea, and may have been the primitive condition in basal euarthropods. Is the increase in structural tagmosis witnessed in trilobite phylogeny matched by increased variety in growth modes, such as the among species variation in average growth rate, or departure from constant growth rate through ontogeny? We approached these questions through the analysis of trilobite ontogenetic growth progression. Over the last 50 years quantitative data on growth increments between putative successive instars have been published for over 65 trilobite species, from strata ranging from early Cambrian to Carboniferous in age. Such instar series have been posited for all of the articulation-based stages of trilobite life history, and for a broad range of environmental settings. A dataset of this information based on published records and our own analyses, accompanied by criteria with which to judge the reliability of the estimates, has been used to review trilobite growth.