THE RISE OF AN ARTICULATED BODY: INVESTIGATING MORPHOLOGICAL CONSTRAINTS IN STEM ARTHROPODS USING PHYLOGENETIC DATA

The early evolution of the most diverse phylum of eukaryotes on Earth, the Arthropoda, has been debated for a century, but we still lack a consensus on the causality of this event and on the macroevolutionary patterns involved. Focusing on this issue, a certain theoretical emphasis on the respective roles of genes, ecology and development has become central in the synthetic literature dealing with the subject. A preliminary step towards identifying the causes of trait plasticity based on fossil evidence is to investigate character correlation, using a phylogenetic dataset and its most parsimonious tree. Along with our project to provide an updated phylogeny of stem arthropods, given the number of recently published descriptions of new morphospecies, we used a dataset rich in multistate characters to try and detect constrained character changes from basal to most derived taxa and along the antero-posterior axis. Our results, mainly thanks to a new representation of the data called ‘macroevolutionary fingerprint’, provide a first insight into the link between the overall bodyplan and morphological plasticity across fossil taxa during the Cambrian radiation. We describe the coevolution of tagmata (showing that the bipartite homeogene expression pattern in chelicerates is retrievable within character correlation), point out conditions for trunk segmentation variability (notably the increasing plasticity of terminal body segments in trilobitomorphs) and also introduce evolutionary morpho-units in fossils such as [number of trunk limbs; number of podomeres in trunk limbs; reduction of terminal trunk segments]. Used as a feedback on phylogenetic results, the investigation of character correlation also constitutes a testing method for homology hypotheses.