THEORETICAL DEVELOPMENTAL MORPHOLOGY AND MORPHOSPACE OCCUPANCY. AN EXAMPLE WITH TRILOBITE CRANIDIA

Two major outcomes of studies in theoretical morphology are the heterogeneity of morphospace occupancy and the discrepancy between the empirical diversity of forms and the realm of the theoretically possible. Potential explanations for these patterns range from the role of functional requirements to the effects of 'intrinsic' constraints of development.

Here, we focus on the potential role of development in generating uneven filling of morphospace. Morphospaces are usually constructed using only adult forms. However, organisms may be better understood when considered as dynamical and modular systems with 'form histories'. Within morphospaces, this information can be included through the analysis of ontogenetic trajectories of shape change.

In this context, both evolutionary developmental changes and phenotypic modularity can influence the evolution of the filling of morphospace through time. The former concept deals with the evolvability of ontogenetic trajectories, while the latter deals with the way organisms are composed of evolutionarily dissociable parts. Both constrain phenotypic evolution by acting on the magnitude and direction of shape change in the morphospace, and by imposing variable degrees of covariation among the organism’s constitutive modules. For instance, evolutionary developmental change via heterochrony implies the conservation of the ancestral trajectory of shape change, while change in the evolutionary pattern of modularity via integration and parcellation alters the dimensionality of morphospace but also likely influences the filling of morphospace in terms of patchiness and directionality.

We empirically illustrate these ideas with analyses of several species belonging to the trilobite genus Zacanthopsis. A previous study recognized the modular nature of the cranidium and the role of heterochrony in the evolution of its constitutive modules. We use properties of the modules’ ontogenetic trajectories to build theoretical developmental morphospaces and investigate the role of heterochrony and modularity in leading to irregularities in morphospace occupation.