ARTHROPOD PHYLOGENY, SIMULTANEOUS ANALYSIS, AND MANDIBULATE AFFINITIES OF TRILOBITA

Combination of morphological data and sequences from nine molecular markers under Direct Optimization resolve the interrelationships among extant arthropod lineages in favor of Mandibulata, Chelicerata s.l., and Tetraconata across many more analytical conditions than favor the rival groupings Paradoxopoda, Cormogonida, and Atelocerata. Monophyly of Mandibulata is supported by a substantial body of anatomical evidence, including characters of the mandible, central body of the brain, ommatidia, and serotonin-reactive neurons, as well as Hox expression domains. Despite decades of broad acceptance of a relationship between trilobites and chelicerates (formalized in the Arachnata or Arachnomorpha concepts), head segmentation in trilobites suggests an alternative relationship to Mandibulata. Anatomical, embryological, and gene expression data align the anterior segments of arthropod heads with three main brain components: a protocerebral ocular segment; a deutocerebral antennal/cheliceral/chelifore segment; a tritocerebral second antennal/pedipalp/intercalary segment. Onychophoran antennae are pre-ocular and protocerebral, and align with the ‘primary antennae' of fossil stem-group euarthropods rather than with the deutocerebral ‘secondary antennae' of Mandibulata. In this framework, primary antennae are lost in the euarthropod crown group. Trilobite antennae have the same relationship to the hypostome as have the ‘secondary antennae' of Mandibulata. A position of trilobites on the mandibulate rather than chelicerate stem-lineage as indicated by head tagmosis withstands parsimony analysis of morphological data with fossils included.