Head anatomy and phylogenomics show the Carboniferous giant <i>Arthropleura</i> belonged to a millipede-centipede group

The Carboniferous myriapod <i>Arthropleura</i> is the largest arthropod of all time, but its fossils are usually incomplete, limiting the understanding of its anatomy, ecology, and relationships. Micro-computed tomography applied to exceptionally preserved specimens from the Carboniferous Montceau-les-Mines Lagerstätte (France) reveals unprecedented details of its functional anatomy, such as the head and mouthparts. <i>Arthropleura</i> shares features with both millipedes and centipedes. Total-evidence phylogeny combining morphological and transcriptomic data resolves <i>Arthropleura</i> alone as a stem group millipede, but the inclusion of the highly incomplete Siluro-Devonian <i>Eoarthropleura</i> draws it deeper into the myriapod stem. <i>Arthropleura</i> suggests transitional morphology between clades united primarily by molecular information and underscores the value of total-evidence phylogenetics to understanding evolutionary history.