Head anatomy and phylogenomics show the Carboniferous giant belonged to a millipede-centipede group.

The Carboniferous myriapod 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. shares features with both millipedes and centipedes. Total-evidence phylogeny combining morphological and transcriptomic data resolves alone as a stem group millipede, but the inclusion of the highly incomplete Siluro-Devonian draws it deeper into the myriapod stem. suggests transitional morphology between clades united primarily by molecular information and underscores the value of total-evidence phylogenetics to understanding evolutionary history.