Two new xyloiuloid millipedes (Diplopoda: Chilognatha: Juliformia) are described and placed in the new family Gaspestriidae: Gaspestria genselorum n. gen and sp. from the Emsian of Québec and New Brunswick and Sigmastria dilata n. gen. and sp. from the Pragian of the Midland Valley of Scotland. These new millipedes extend the stratigraphic range of xyloiuloid millipedes, which previously were only described from the Pennsylvanian of Europe and North America. All xyloiuloid millipede families are placed within the superfamily Xyloiuloidea which is left incertae sedis within Juliformia due to a lack of preservation of diagnostic characters that would allow placement within an extant order. The presence in the Lower Devonian of the superorder Juliformia, universally agreed among diplopod taxonomists to represent the most derived clade of Diplopoda, indicates that most of millipede cladogenesis leading to high-rank extant clades had to have occurred by this time, much earlier than previously indicated by the fossil record. A stratocladogram for Myriapoda is constructed, and in combination with data from the plant fossil record and nuclear protein-encoding genes, new hypotheses regarding timing of millipede high-rank cladogenesis in the Paleozoic are formulated. These include terrestrialization of Diplopoda no later than the Ordovician along with the origin of the lineage leading to Penicillata and Arthropleuridea, followed by a period of relative stasis until the Middle Silurian, at which point there was a rapid radiation of Diplopoda, producing most of the high-rank clades by the Lower Devonian.