Growth Progression In Trilobites

Trilobite development comprised a long series of free-living instars, with transitions between successive instars generally characterized by modest morphological change. 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. 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. Two metrics, the Average Growth Rate (AGR) and the Index of Conformity to Dyar's Rule (IDC) have been used to facilitate this. We see no striking temporal trends in the degree of departure from Dyar's rule or in the variance in the average growth rate. Our results do not support Chatterton et al's (1990) view that growth increments tended to increase, on average, during trilobite evolution, although particular clades of derived trilobites, such as certain proetoidean species, do have notably high AGR. In fact, AGR and IDC might be more variable in trilobite evolutionary history. We cannot detect any general association between paleoenvironment and either of these measures, although analysis of these issues in phylogenetic context is necessary. As with various and repeated experiments in skeletal tagmosis, it appears that changes in growth rates and degree of conformity to Dyar's rule occurred repeatedly and independently during trilobite evolution.