Paper No. 5
Presentation Time: 2:45 PM


LEGG, David Alexander, Earth Science and Engineering, Imperial College London, Royal School of Mines, South Kensington Campus, London, SW7 2AZ, United Kingdom, SUTTON, Mark D., Department of Earth Science and Engineering, Imperial College, London, SW7 2AZ, England and EDGECOMBE, Gregory D., Department of Earth Sciences, The Natural History Museum, London, SW7 9BD, United Kingdom,

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.