Atlas of Myriapod Biogeography. I. Indigenous Ordinal and Supra-Ordinal Distributions in the Diplopoda: Perspectives on Taxon Origins and Ages, and a Hypothesis on the Origin and Early Evolution of the Class
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The biogeographic significance of Diplopoda is substantiated by 50 maps documenting indigenous occurrences of the 16 orders, the three Spirostreptida s. l. suborders – Cambalidea, Epinannolenidea, Spirostreptidea – and all higher taxa including Diplopoda itself. The class is indigenous to all continents except Antarctica and islands/archipelagos in all temperate and tropical seas and oceans except the Arctic; it ranges from Kodiak Island and the northern Alaskan Panhandle, United States (USA), southern Hudson Bay, Canada, and near or north of the Arctic Circle in Iceland, continental Scandinavia, and Siberia to southern “mainland” Argentina, the southern tips of Africa and Tasmania, and Campbell Island, subantarctic New Zealand. The vast, global distribution is interrupted by sizeable, poorly- or unsampled areas including the Great Basin, USA; the Atacama Desert region of Chile and neighboring countries; southern South American islands; the central Kalahari and Sahara deserts; the Gobi Desert, Mongolia, and all of north-central and western China; from north of the Caspian Sea, Russia, to central Kazakhstan; and the “Outback” of central Australia. Five Arabian countries lack both samples and published records of indigenous diplopods – Bahrain, Kuwait, Oman, Qatar, and United Arab Emirates – as do Turks and Caicos, in the New World, and Mauritania and possibly Egypt, Africa. New records, including the first for Chilognatha from Botswana and the first specific localities from Northern Territory, Australia, are cited in the Appendix. Increased emphasis on mappings in taxonomic research is warranted along with investigations of insular “species swarms” that constitute a microcosm of the early evolution of the class. The largest “species swarm” in the Diplopoda is Diplopoda itself!
Four taxa – Glomerida, Platydesmida, Julida, and Callipodida – occur exclusively in former Laurasian Territory, and seven – Glomeridesmida, Sphaerotheriida, Siphonophorida, Spirobolida, Epinannolenidea, Spirostreptidea, and Stemmiulida – all absent from Europe, are primarily southern/Gondwanan except for secondary dispersals in Mexico/Central America by all but Sphaerotheriida, which are absent from the New World. Siphoniulida and Siphonocryptida, known from only two and four areas, respectively, are declining towards extinction; the former may constitute a relictual intermediate between Colobognatha and Eugnatha. Polyxenida, Polyzoniida, Chordeumatida, and Polydesmida occur on nearly all continents, while Cambalidea, extinct in Europe, inhabit North/Central America and southeast Asia with an isolated area in Iran. Southeast Asia, from southern China to Sumatra, harbors all 16 orders plus Cambalidea and Spirostreptidea. Southern taxa were passively transported to Asia beginning in the Silurian on terranes that rifted from the “proto-Australia” area of the Gondwana deriving from breakup of the supercontinent Pannotia (hereafter “Gondwana I”); they drifted northwards and accreted to Siberia+Kazakhstania/”Euramerica,” and later the “proto-Laurasia” part of Pangaea, from the Permian to the Jurassic. Laurasian taxa could not penetrate southeastern Asia until those terranes had accreted and the region was available for colonization; before this, they evolved, differentiated, and dispersed east/southeastward from source areas in Euramerica, as evidenced by detached faunal remnants in present-day Central Asia and the Himalayas. Southeastern Asia is thus a “mixing area” for northern and southern diplopods as is Mexico/Central America, which Gondwanan forms entered in the Late Carboniferous, ~ 306 ma, when Euramerica collided with the “proto-South America/Africa” region of Gondwana I, thereby forming Western Pangaea. Closure of the Panama Portal in the Pliocene, ~ 5 ma, allowed northward dispersals of South American forms but is too recent to account for occurrences throughout the Central American land bridge and even into the USA, though it probably explains northward spread of Epinannolenidea and the polydesmidan family Paradoxosomatidae to Costa Rica. Occurrence of the latter in Dominica, Lesser Antilles, is regarded as indigenous rather than introduced and probably represents occurrence in the “Proto-Antillean” area before it rifted from northern South America in the Cretaceous/ Paleocene, ~ 66 ma.
As the earliest Paleozoic fossil is from Scottish Silurian deposits, an operative hypothesis explaining early diplopod evolution requires origin far enough before then for major dichotomies to have taken place and for ancestral forms to have dispersed and become established relatively simultaneously on both Gondwana I and the northern “micro-continents” (Baltica, Laurentia, and Siberia). Only one source area meets these requirements, the Avalonia terrane of Gondwana I before it rifted in the early Ordovician (~ 480 ma) and drifted to and combined with Baltica in the mid-Ordovician (~ 450 ma); 10 my later, Baltica+Avalonia merged with Laurentia to form Euramerica. Presence on Avalonia and neighboring parts of Gondwana I prior to rifting mandates at least Mid- to Late Cambrian origin ( < 524 ma) on or near this terrane with rapid divergence and dispersal onto Gondwana I proper, such that ancestral stock was partitioned when Avalonia rifted. Forms remaining on Gondwana I continued to evolve, differentiate, and disperse, eventually reaching the “proto-east/southeast Asia” terranes before they rifted, while those on Avalonia were confined to this terrane until collisions with Baltica and Laurentia allowed them to colonize these unoccupied lands with numerous vacant niches, which drove evolution in different directions from that taking place simultaneously and in “parallel” on Gondwana I.
Relative ordinal-group ages are postulated as Polyxenida > Polydesmida > Siphoniulida > Siphonocryptida > Spirostreptida s. l./Cambalidea > Chordeumatida > Polyzoniida > Glomeridesmida > Sphaerotheriida > Epinannolenidea > Stemmiulida ~ Siphonophorida > Spirobolida > Spirostreptidea > Glomerida ~ Platydesmida > Callipodida > Julida.
136 pages, 26 Mbytes