THE ROLE OF HETEROCHRONY IN THE EVOLUTION OF CAMBRIAN TRILOBITES

1. Renewed interest in the role of changes to developmental regulation in organisms has highlighted the importance of heterochrony in the evolution of the Metazoa.
2. Beecher's interpretation of the evolution of the Trilobita as having been principally by peramorphosis is examined, as is the view of later workers, principally Stubblefield and Hupé, that paedomorphosis was a dominant factor in trilobite evolution.
3. Both peramorphosis and paedomorphosis are considered to have been important in trilobite evolution.
4. The role of paedomorphosis in the evolution of major morphological novelties is critically examined. Its importance in changes to the structure of the glabella is discussed and new terms proposed to describe the ontogenetic and phylogenetic state of the glabella.
5. The highly variable nature of early Cambrian trilobites, in particular the large degree of ontogenetic change, is considered, along with possible poor developmental control of the growth and moulting hormonal systems, to have been significant in providing a high degree of intrapopulational morphological variability. Selection of these heterochronic variants was responsible for the rapid diversification of the Trilobita during the Cambrian.