What Is an “Arachnid”? Consensus, Consilience, and Confirmation Bias in the Phylogenetics of Chelicerata

The basal phylogeny of Chelicerata is one the opaquest parts animal Tree Life, defying resolution despite application thousands loci and millions sites. At forefront debate over chelicerate relationships monophyly Arachnida, which has been refuted by most analyses molecular sequence data. A number phylogenomic datasets have suggested that Xiphosura (horseshoe crabs) are derived arachnids, refuting traditional understanding arachnid monophyly. This result regarded as controversial, not least paleontologists morphologists, due to widespread perception unambiguously supported morphological Moreover, some able recover monophyly, galvanizing belief any challenges artefactual. Here, we explore problems distinguishing phylogenetic signal from noise through a series in silico experiments, focusing on recently We assess claim filtering saturation rate valid criterion for recovering Arachnida. demonstrate neither rate, nor ability assemble dataset supporting given outcome with maximal nodal support, guarantor accuracy. Separately, review empirical examine characters Arachnida downstream implication single colonization terrestrial habitats. show support contingent upon small ambiguous or incorrectly coded characters, these tautologically linked adaptation