Tagmata and segment specification in trilobites

As in some Recent arthropods, the link between segmentation as expressed in the dorsal exoskeleton of trilobites and that of ventral appendages is not always exact, suggesting that specification at a cellular level and the morphological appearance of segments may have been significantly dissociated during trilobite ontogeny. It is possible that the cellular primordia of all segments to be expressed later morphologically were specified during a short sequence of events early in ontogeny. Such a mechanism is invoked in the multiplicative model of segmentation suggested for centipedes, and could apply to all arthropods, trilobites included. This model contrasts with the additive model for trilobite postcephalic segmentation, which suggests that segments were both specified in and budded from a posterior generative zone in a gradual manner that extended late into ontogeny. Discriminating between these ideas in trilobites will require careful exploration of the dynamic relationship between the size of individual segments and the size of the entire organism. The trilobite postcephalic region also shows a dynamic exchange of segments between the transitory pygidium and the thorax. This contrasts with how tagmata are defined among living arthropods, and thus the adult pygidium of trilobites is a kind of ‘frozen’ postcephalic growth zone, following the cessation of the appearance of new segments in the dorsal exoskeleton. As such, we suggest that the trilobite exoskeletal bodyplan should be regarded as consisting of only two tagmata – the cephalic and postcephalic tagma.