DEVELOPMENTAL CONSTRAINT AND RELEASE IN THE EARLY CAMBRIAN TRILOBITE GENUS ZACANTHOPSIS

Ancient developmental systems are challenging to study, but they offer unique insight into the ways evolutionary processes may interact with the machinery of growth and development. Allometric shape change along an ontogenetic trajectory is one developmental relationship which offers clear and testable hypotheses of constraint on the evolution of morphology. Ontogenetic allometry may constrain and channel phenotypic evolution along a stereotyped line of evolutionary least resistance, or it may be itself subject to evolution and repatterning. Indeed, highlighting the importance of fossil information, both observations may not be mutually exclusive with development constraining evolution on short timescales and evolution modifying development over longer timescales.

Species of the early Cambrian trilobite genus Zacanthopsis are represented by abundant fossil material from the American Great Basin which allows for detailed morphometric characterization of ontogenies and thus careful quantification of a number of possible developmental constraints on evolution within the group. New and previously described species are included in a novel analysis of phylogenetic relationship, a framework which permits the explicit testing of hypotheses of developmental constraint on phenotypic evolution in this early metazoan radiation.

We find that within this genus of Cambrian trilobites, while ontogenetic allometry may be a factor in the evolution of morphology, it does not function as a hard constraint and has itself measurably evolved. The ways in which both disparity and intrageneric ontogenetic diversity are structured along phylogenetic lines are explored in phylomorphospace and phyloallometryspace. Consideration is made to stratigraphic and geographic contributions to intraspecific variation.