ONTOGENY AS AN EVOLUTIONARY CONSTRAINT AND SOURCE OF NEOMORPHISM WITHIN THE EARLY CAMBRIAN TRILOBITE GENUS ZACANTHOPSIS

Ontogenetic allometry may act as a line of evolutionary least resistance, constraining and channelling phenotypic evolution to a morphological trajectory established by an ancestral ontogeny. Alternatively, ontogenetic allometry may itself be repatterned, generating neomorphisms and novel allometries. Whether and how macroevolutionary dynamics are influenced, or even driven, by constraint and release of ontogenetic allometries remains incompletely explored.

Here, we characterize the diversity of ontogenies within the Cambrian trilobite genus Zacanthopsis using geometric morphometrics. Ontogenies are examined in the context of a novel phylogenetic hypothesis which includes previously undescribed species. With this hypothesis of evolutionary relationship, morphology can be compared in a phylomorphospace. In turn, this is translated into a phyloallometryspace in which the evolution of ontogeny itself is readily explored. A clear image of the diversity and phylogenetic distribution of both morphology and ontogeny permits powerful insight into the temporal and phylogenetic persistence of allometric patterns as well as any influence they may exert on the evolutionary structure of disparity within Zacanthopsis. The ways in which ontogenetic allometry may serve to constrain phenotypic evolution are contrasted with the modes by which modification to allometry can propel evolution in unexpected phenotypic directions.

Careful understanding of the dynamics of developmental constraint and release in this particular early metazoan radiation contributes to the larger macroevolutionary picture of the evolution of developmental systems through deep time.