Making horseshoe crabs crabs again: Integrating data classes in the phylogenetic inference of ancient radiations

The backbone phylogeny of Chelicerata–the subphylum of arthropods that includes spiders, mites, and horseshoe crabs–remains intractable to phylogenetic resolution, owing to a confluence of conflicting phylogenetic signal, long branch attraction artifacts, limitations in taxonomic sampling, and episodic extinction at the ordinal level. Salient sampling omissions in modern chelicerate phylogenies include the miniaturized orders Palpigradi and Schizomida, as well as Opilioacariformes, the sister group to the remaining Parasitiformes. To redress these gaps, I review three solutions to resolving the chelicerate tree of life: (1) increased taxonomic sampling of poorly studied groups, (2) phylogenomic subsampling of data matrices, and (3) integration of morphology and rare genomic changes (RGCs) as arbiters of topological conflicts. The integration of these approaches, in a dataset that samples all living arachnid orders, recovers three wholly unanticipated outcomes: horseshoe crabs are derived arachnids; pseudoscorpions are the sister group of true scorpions; and Palpigradi is the sister group of Solifugae (camel spiders).

Contact: Dr. Kevin Kocot