|Palaeos: Paleozoic||Guadalupian Epoch|
|Permian Period||Roadian Age|
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Russian terrestrial stratigraphy
Roadian tetrapod faunas
The Roadian is the first of the three ages of the Middle Permian (Guadalupian epoch). It takes its name from the Road Canyon Member, the lower (oldest) part of the Word Formation. It was established in 1968, but only added to the international IUGS timescale in 2001, when the International Commission on Stratigraphy established the Global Stratotype Section and Point (GSSP) in the Cutoff Formation (Guadalupe Mountains of Texas). The base of the Roadian is defined as the appearance of the conodont Jinogondolella nankingensis.. Other important fossils characteristic of the stage include the ammonoids Demarezites and Waagenoceras (right) and the foramnifers Neoschwangerina tenuis and Afganella tereshkovae. The top of the Roadian and base of the Wordian is given by the first appearance of fossils of conodont species Jinogondolella aserrata. (online refs - Britannica, Wikipedia, Upper Paleozoic timescale)
The Russian terrestrial late Permian strata are traditionally divided into the Ufimian (the earliest), the Kazanian, and the Tartarian. There has however been some difficulty mathching these with the International scale. Whereas the Kazanian and the Tartarian are secure, the fate of the Ufimian is uncertain, It is generally considered equivalent to perhaps the lower part of the Roadian, but has sometimes also considered belonging to the next older age (Kungurian) or the next younger, the Wordian (e.g. Gilmour and Morozova 1997). On the basis of conodont zonation, Chernykh 2002 says:
"The upper Kazanian substage is most probably also Roadian, because Merrillina galeata, which is characteristic of higher stratigraphic horizons (Wordian) is absent in the Upper Kazanian of the Mid-Volga Region. Because the lower Kazanian correlates with the lower Roadian and I suggest the upper Kungurian correlates with the Cathedralian, it appears the Ufimian may lose its status as a stage."
However another paper (which considers vertebrates and strata of the South Urals) by Tverdokhlebov et al 2005 p.30 retains the Ufimian, noting that fishes have been found from deposits of this age north of the region being considered, but no tetrapods.
The problem is that the Kungurian, which is one of the stages shared with both the International Scale and East-European Scale, has a different definition with which. Hence (as recommended in the Permophiles Newsletter of the Subcommission on Permian Stratigraphy #46) in order to have uniformity the Ufimian is either removed, or reduced to a substage in the East-European Time Scale equivalent to the upper part of the Kungurian Stage (Latest part of the Early Permian) in the International Time Scale.
Finally, the Kazanian, the middle of these three stages, has been equated with the Wordian, the latest Roadian to Early Capitanian (Lucas 2004), the Middle Roadian to Early Wordian (Benton et al 2004), and the Roadian (Permophiles #46 2005). The Russian commission on Permian Stratigraphy considers that the Kazanian and Roadian Stages are equivalent, based on new discoveries of Roadian conodonts and ammonoids in the lower Kazanian (Kotlyar and Pronina-Nestell 2005).
There is some controversy over the Roadian tetrapod fauna, which is due to the problem of correlating East European tetrapod faunas with the international stratigraphic standard.
Dr Spencer Lucas (2002 and 2004) argues that no tetrapods are known from the Roadian, and thus there is a hiatus between the Pelycosaur dominated Early Permian (up till the Kungurian) and the Therapsid dominated middle and late Permian (beginning in the Wordian). He refers to this as "Olson's Gap", after Everett C. Olson, who investigated these stratigraphic levels in the search for tetrapods intermediate between pelycosaurs and therapsids and wrote a number of important monographs on the subject (e.g. Olson, 1962). Dr Lucas' arguments have been challenged by Reisz and Laurin 2002 who have described a typically Russian middle Permian fauna from North America, and argue that this is Roadian, not Kungurian in age. A recent issue of the Permophiles Newsletter of the Subcommission on Permian Stratigraphy (no. 46, December 2005 - correlation table reference) argues that the the Kazanian and Roadian Stages are equivalent. If so this means that there is no Olson's Gap, because the Kazanian tetrapods that were previously thought to be Wordian are actually Roadian. This does not mean that all the Kazanian faunas are Roadian, since the best known ones are late Kazanian, which may mean they overlap with the Early Wordian. See also Lozovsky 2005
Not to be outdone, Lucas met these arguments with Lucas 2004, Lucas 2005. More recently it has been argued that the very primitive Therapsida of the Xidagou Formation (Dashankou locality) in China are of Roadian age (Liu et al 2009). I have to admit I feel biased towards acknowledging Roadian tetrtrapod faunas, if only because the the morphogenetic (evpolutionary) difference between primitive Chinese and Russian faunas and the more advanced but traditionally Wordian Eodicynodon fauna of South Africa. Here I assume here that the Dashankou, Lower Kazanian, and part of the Upper Kazanian are of Roadian age, with the Russian Ocher fauna on the Roadian/Wordian boundary or thereabouts.
During this period the low diversity and very poorly known amphibian and parareptile fauna that characterized the Kungurian age is supplanted (and for the most part replaced) by a rich range of early therapsids. There seems to be a great morphological gap between even the latest Kungurian caseid fauna, and the Roadian therapsid fauna. The therapsids of this time belonged to several distinct (albeit related) lineages, none with clear antecedents.
One example of this therapsid-dominated fauna tetrapod assemblage is in the Lower Kazanian stage of the Russian Cisurals. The Golyusherma locality is correlated with the Baitugan horizon at the base of the Kazanian, and hence is Earliest Roadian Kotlyar and Pronina-Nestell 2005, reveals a tetrapod assemblage of diverse amphibians: archegosaurid and melosaurid temnosondyls and leptorophid seymouriamorphs; and reptiles: bolosaurid parareptiles, captorhinid eureptiles, and primitive estemmosuchud and brithopodid therapsids. Specimens were collected during mining operations at Bashkirstan (also Russian Ural region) may be just as old. These include fragmentary remains of temnospondyls, the dinocephalian "Brithopus" and phreatosuchids (possibly Caseid pelycosaurs although this is not certain Olson 1962). (Lucas 2004 - note that Lucas considers all these Lower Kazanian faunas no earlier than early Wordian; however according to the contributers of Permophiles (#46) the Kazanian is actually Roadian. See also see also East European Stratigraphy - Notes)
The late Kazanian Belebey Community (also in the Cis-Urals region, Russia) provides another glimpse of the various organisms and trophic interactions of this time (V.P. Tverdokhlebov et. al. 2005 - see diagram below). This fauna would seem to be the same age, or possibly a bit earlier, than the Ocher assemblage. Various species of fishes, especially palaeonisciforms, fed on water plants, insects, and other invertebrates. These in turn were preyed upon by batrachomorph amphibians. Procolophonomorph anapsid reptiles such as nycteroleterids, Belebey, Davletkulia, and Tokosaurus filled the terrestrial small lizard niche. There were doubtless carnivorous biarmasuchians and brithopodids but these are not known from this locality. The largest animal, and the only therapsid, is a medium-sized (see XLS data file - Ecological Sort) herbivorous dinocephalian, listed as Estemmenosuchus sp. (Tverdokhlebov et al 2005 p.45), which at Beleby seems to have been free of predation, although at Ocher it would have faced large Eotitanosuchids. Possibly these animals were here too but have not been preserved.
Of very similar age, if not even slightly earlier, to these Russian faunas is the rich fuana of the Xidagou Formation (Ordos Basin of northern China). This includes the dissorophoid temnospondyl Anakamacops, an Intasuchus-like temnospondyl, the anthracosaurs Ingentidens and Phratochronis, the bolosaur Belebey (as menationed this is also known from the Russian Kazanian - both bolosaurs and dissorophids otherwise occur together only in the Early Permian), a captorhinid, the basal theraspid Raranimus, dinocephalian Stenocybus and anteosaur Sinophoneus, and the anomodont Biseridens. (Lucas 2006 p.81, Liu et al 2009 p.397)
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