EARLY EVOLUTION OF DEVELOPMENT IN THE EDIACARAN MACROBIOTA

The generation of morphology is controlled by development and its underlying molecular tools as they interact with the environment. Although crown metazoan marine bodyplans appear in the Cambrian, it is now apparent that many components of animal developmental genetic programs originated much earlier in the Ediacaran and Cryogenian, long before the appearance of the phenotypes with which they are now associated. The less familiar Ediacaran macrobiota is now believed to represent multiple independent higher-order clades, indicating that these groups may have possessed defining developmental strategies and molecular toolkits analagous/similar to metazoan bodyplans. To understand how the developmental process evolved during this time, as well as where these clades may fall within the tree of life, we have conducted a phylogenetically unconstrained evaluation of Ediacaran taxa to determine the minimum developmental toolkits required to build Ediacaran architectural elements. Organismal morphology provides both direct and indirect evidence for a wide array of developmental traits, and using a multidimensional developmental framework we have compared the developmental tools of select Ediacaran and modern taxa. Objectively measurable characters used for developmental categorization included body-patterning strategies such as presence/number of axes, axial (anteroposterior, dorsoventral) symmetry, morphological differentiation along axes, segmentation, or repetition of modular functional units; and ontogenetic parameters such as growth by addition of units (terminal or intercalary), growth by increase in size, or growth by fractal-like branching. These analyses suggest that an important change from Ediacaran to more metazoan-like development involved a transition from “2D” epithelial-style inductive development to a “3D” spatially-specified process. As Ediacaran preservation is as varied as it is exceptional, we have examined how taphonomic biases may have influenced our developmental analysis. Finally, this work also demonstrates the utility of first considering fundamental developmental strategies as a basis for constraining Ediacaran phylogenetic hypotheses.