FOSSIL AND MOLECULAR EVIDENCE FOR THE TIMING OF MILLIPEDE CLADOGENESIS IN THE PALEOZOIC

It was originally thought that most of the high-rank millipede clades originated in the Pennsylvanian. However, new evidence from fossils and molecular phylogenetic analyses indicates that high-rank cladogenesis in Diplopoda occurred significantly earlier. Two new genera of xyloiuloid millipedes are described from the Pragian of Scotland and the Emsian of Québec, significantly extending the stratigraphic range of Xyloiuloidea back from the Pennsylvanian. Xyloiuloidea belong to the superorder Juliformia along with extant ring-forming millipedes. Juliformia is universally recognized as the most derived clade of Diplopoda, thus their presence in the Pragian implies that all of the major clades of Diplopoda had to have arisen by this time. The oldest known millipedes, recently described from the Wenlock of Scotland, belong to basal chilognath orders. Prior to the Wenlock, the only fossil evidence of millipedes consists of Caradoc trace fossils from the English Lake District. Functional morphology dictates that these traces were made by penicillate- or arthropleuridean-like millipedes, providing evidence of the oldest non-chilognathan millipedes. Using the millipede fossil record in combination with evidence from molecular phylogenetic analysis, the following hypothesis is formulated. Rapid evolutionary change in myriapods associated with terrestrialization and diversification into four classes was followed by a period of relatively slow evolutionary change in the Ordovician, mirroring the stasis seen in early terrestrial plants. During this period, the diplopods are represented only by basal lineages (Penicillata/Arthropleuridea). From the Wenlock–Lower Devonian, coeval with the radiation of tracheophytes, millipedes underwent rapid cladogenesis, with the origin of all high-rank clades. Molecular phylogenetic analysis of Myriapoda using nuclear protein-encoding genes, recovered classes and orders with strong to moderate support, but there were relatively few cases in which relationships among classes or orders within classes are recovered. Within Diplopoda only the basal chilognath superorders Pentazonia and Colobognatha were resolved. In combination with the fossil record, these results may indicate heterogeneity in the timing of millipede phylogenetic diversification.