Expanded concept and revised taxonomy of the milliped family Xystodesmidae Cook, 1895 (Polydesmida: Leptodesmidea: Xystodesmoidea): incorporations of Euryuridae Pocock, 1909 and Eurymerodesmidae Causey, 1951, taxon revivals/proposals/ transferrals

Euryuridae Pocock 1909 and Eurymerodesmidae Causey 1951, both endemic to the eastern/central United States (US), are incorporated into Xystodesmidae Cook 1895 and reduced to subfamilies and (sub)tribes n. stats. Euryurina and Melaphina Brolemann 1916, n. stats., are sister-taxa that differ primarily in epiproctal configurations and comprise Euryurini; sister-taxa Eurymerodesmina and Nannariina Hoffman 1964, n. stats., the latter transferred from Xystodesminae, comprise Eurymerodesmini, n. stat., in which plesiomorphic forms exhibit sublinear, "stick-like," and subapically curved/bent gonopodal acropodites with moderately-long to long hairs, often with distal tufts, on their “inner” surfaces. Additional transferrals include Wamokia Chamberlin from the xystodesmine tribe Xystocheirini to Xystodesmini (= Harpaphini), and Macellolophus Attems, from Xystodesmidae to Chelodesmidae. Except for Chonaphini, the term, "prefemoral process," has traditionally been assigned to the secondary and shorter telopodital projection regardless of its position, origin, or configuration. Homology of these different structures has never been demonstrated and requires investigation, but the multitude of differences suggests that they are not such and warrant different names, for example "femoral process," for the similarly positioned branches in Devillea Brölemann and Rhysodesmus Cook (Xystodesminae: Devilleini, Rhysodesmini). The latter tribe may be polyphyletic, and new tribes may be required for components with acicular "prefemoral processes" (Boraria and Cherokia, both by Chamberlin, Gyalostethus and Erdelyia, both by Hoffman, and Pleuroloma Rafinesque) and the southeastern US genera with small-bodied species (Caralinda Hoffman and Gonoessa, Parvulodesmus, and Lourdesia, all by Shelley). Taxonomic value is accorded the "prefemoral extension/elongation," which is absent from Eurymerodesmina; complete, encircles the acropodite, and extends for \~1/3 to 1/2 of the latter's lengths in Euryurini; and incomplete and extends for \~1/4 to 1/3 of the "outer" acropodital surfaces in Nannariina and xystodesmine tribes. Other newly recognized taxonomic characters include the "inner" and "outer" acropodital surfaces/margins, the position on the acropodital stem of the "distal curve/bend," and the length of the "distal zone." Rhysodesmus and Sigmoria (Rudiloria) t. trimaculata (Wood) (Xystodesminae: Rhysodesmini, Apheloriini) are recorded from, respectively, Chihuahua, Mexico, and Québec, Canada, as are Xystodesmidae/-inae and, provisionally, Chonaphini, Montaphe Chamberlin, and M. elrodi (Chamberlin), the only plausible taxa for an unidentifiable juvenile from near Yahk and only 2.5 km (1.6 mi) north of the International Border. The southern periphery of interior British Columbia (BC) thus represents the second xystodesmid faunal region in BC and the third in Canada. While incorporation of Euryuridae does not affect the family's overall distribution, that of Eurymerodesmidae fundamentally alters it by joining the formerly separate East-Nearctic and Meso-American regions into a continuous one extending, north-south, from Montréal Island, Québec, to Santa Ana Department, El Salvador, a distance of around 4,944 km (3,090 mi). Xystodesmidae also inhabit two West-Nearctic regions, one in the interior stretching from southernmost BC to northeastern Oregon and the other running along the Pacific Coast from southern Alaska to southern California. The family also occupies two Palearctic regions, each with three subregions, an eastern one spreading from Hokkaido, Japan, and the southern Maritime Province, Russia, to Taiwan; a point locality in northern Vietnam; and southern/eastern China. The second Palearctic area extends along the Mediterranean and adjoining seas from Morocco, Sardinia, and the southeastern corner of France to Cyprus and southern coastal Turkey. New locality data, references, and maps are provided along with diagnostic accounts of all reconceptualized taxa and new/revived statuses. A simple, sublinear, "stick-like" acropodite with a curve or bend near midlength or subapically and without a secondary telopodital projection is the hypothesized plesiomorphic gonopodal condition in Xystodesmidae. This form has undergone multitudinous modifications/alterations - twists, curls, variably configured thickened and laminate expansions, reductions, bi-/trifurcations, enlargements, ornamentations, etc. that are manifested in today's xystodesmine tribes. When Avalonia collided with Baltica 450 million years ago, ancestral xystodesmoideans on the former dispersed into the latter, penetrated and occupied vacant niches, and evolved into today's Melaphina (Euryurinae: Euryurini) and Devilleina (Xystodesminae). A similar evolutionary burst leading to today's Nearctic and East-Palearctic faunas occurred 10 million years later when Avalonia + Baltica collided with Laurentia to form Euramerica. Ancestral forms of Euryurinae and Xystodesminae again penetrated vacant niches and evolved; the former maintained the general gonopodal structural pattern of Melaphina but changed the epiproct from triangular to broad and spatulate, thereby creating Euryurina. The earliest xystodesmine taxa to evolve in Laurentia were Rhysodesmini and Rhysodesmus, which spread southwestward, penetrated "proto- Mexico, and left relict populations in today's southern Appalachians. Eurymerodesmina and Nannariina arose from ancestral euryurine stock prior to the Cretaceous in western Appalachia in their present area of overlap. The former dispersed to the west and south while the latter expanded to the east and north; consequently, the Western Inland Seaway minimally impacted Nannariina while eradicating Eurymerodesmina from the inundated area. Today's populations in the Plains and south-southeastern states therefore represent secondary dispersion in the past 50-60 million years. The Seaway also eradicated Rhysodesmus from these areas, but enough forms survived in high mountain refugia to replenish the fauna when the embayment receded.