INCLUSION OF FOSSIL DATA IN PANARTHROPOD PHYLOGENY INCREASES CONGRUENCE WITH MOLECULAR HYPOTHESES

The interrelationships of the major extant arthropod groups are equivocal, with various conflicting hypotheses being proposed. Recent large-scale molecular analyses have produced partly equivocal, sometimes novel and often surprising hypotheses of relationships. Some proposed relationships, such as a sister-taxon relationship between chelicerates and myriapods, seemingly lacked morphological support, thus prompting a reevaluation of character homologies and a search for potential synapomorphies. Despite this the novel molecular clades remain elusive in large-scale morphological phylogenies, which tend to favor “traditional” groupings. The causes of this phenomenon are poorly understood. Here we present a comprehensive phylogeny of 307 ecdysozoan taxa (96 extant, 211 extinct) representing all major panarthropod groups and two cycloneuralian outgroups and utilizing data from morphology, development, behavior and gene-order. We recovered strong support for the inclusion of hexapods within a paraphyletic Crustacea as sister-taxon to remipedes and the Silurian crustaceomorph Tanazios. A remipede-hexapod relationship is consistent with recent molecular analyses but had not been previously recovered in a morphological phylogenetic analysis. This relationship is shown to be the consequence of the inclusion of fossil data in our analysis, reducing the effect of long-branch attraction between extant myriapods and hexapods that result from convergent adaptations to terrestrialisation. Fossil evidence also indicates that the arthropods have an extensive and diverse stem-group which demonstrates the gradual acquisition of typical arthropod features. The diversity of this long stem serves to better polarize characters within Euarthropoda, providing a more accurate picture of basal arthropod features and reducing the effects of long-branch attraction amongst extant arthropod clades.