A pelagic myodocopid ostracod from the Silurian of Arctic Russia

The Silurian myodocope ostracod Richteria migrans is reported from Arctic Russia, from Kotel’ny Island (New Siberian Islands) and the Taimyr Peninsula in strata of Ludfordian (late Ludlow, Late Silurian) age. These occurrences extend the biogeographical range of R. migrans from tropical to mid latitudes of the Early Palaeozoic Rheic Ocean in the palaeo-Southern Hemisphere, into subtropical regions of the palaeo-Northern Hemisphere on, or adjacent to, the Siberia Palaeocontinent. The new records reinforce the idea that R. migrans had wide dispersal capacity and probably possessed a pelagic lifestyle. It also endorses the use of R. migrans as a biostratigraphical marker fossil for the Ludfordian Stage, Ludlow Series, Upper Silurian.

, Taimyr Peninsula after Metelkin et al. (2005). Section for the Taimyr Peninsula after Tesakov (2009); that for Kotel'ny Island after Kuzmichev (2009). by the Institute of Geology & Geophysics, Novosibirsk. The Taimyr Peninsula lies between the Kara Sea and the Laptev Sea and contains igneous, metamorphic and sedimentary rocks of Proterozoic to Cretaceous ages. Taimyr has generally been divided into three east-west-trending zones, namely North, Central and South Taimyr (Fig. 1c). The Palaeozoic continental margin succession of Central Taimyr represents the stable platform sequence of the northern Siberia passive margin. Silurian strata are well exposed in Central Taimyr, consisting of carbonates and shales up to 500 m in thickness. Zlobin (1962), Tesakov et al. (1995) and Sobolevskaya et al. (1997) established a detailed bio-and lithostratigraphy for this area. The Silurian strata of Central Taimyr comprise the Dvoinaya and Middendorf groups (Tesakov et al., 1995), which are composed of predominantly black to dark-grey shales containing thin lenses and/or interbeds of black limestones. The limestones become thicker and more numerous up the succession.

MaterIal and MethodS
The myodocopes consist of partially flattened and 3D-preserved specimens, now housed in the Palaeontological collections, Museum of Evolution, Uppsala University, Uppsala, Sweden, numbers PMU 27241-54. The morphological terminology for the myodocope valve follows that of Siveter et al. (1987). Rock matrix was removed from the specimens mechanically, using fine needles. Casts of the external moulds of ostracods were made using silicone rubber by the technique of Siveter (1982). Photographs were taken using a Leitz Aristophot mounted with a Canon EOS 5D camera, following the methods outlined in Siveter (1990).

bIoStratIGraphIcal SIGnIFIcance
Perrier et al. (2007) proposed that R. migrans is a marker fossil for rocks of the lower and middle part of the Ludfordian Stage, Ludlow Series across Europe; this can now be extended into Arctic Russia, in the Nikola Formation of Kotel'ny Island and the Middendorf Group of the Taimyr Peninsula. R. migrans was previously recorded in the Kopanina Formation, Bohemia, Czech Republic (Perrier et al., 2007); the Long Mountain Siltstone Formation, Welsh basin, UK (Siveter, 2009); the La Lande-Murée and La Tavelle formations of the Armorican Massif and La Rouquette Formation of the Montagne Noire, France (Perrier et al., 2007); the Fluminimaggiore Formation, Sardinia (Perrier et al., 2007;Gnoli et al., 2009); the Niewachlów (Greywacke) Beds, Poland (Gürich, 1896); the Kurgan Horizon, north Nura-Tau Range, Uzbekistan (Mikhailova & Siveter unpublished data, in Perrier et al., 2007) and possibly in the Ludlow Series of Germany (Perrier & Siveter, 2013) and the Hemse Formation of Gotland (Hede, 1921) (Fig. 2a).
In the Czech Republic R. migrans ranges from the Monograptus fritschi linearis to Bohemograptus bohemicus s. l. Interzone graptolite biozones (Perrier et al., 2007). In Britain it ranges from the Saetograptus leintwardinensis graptolite Biozone to the Bohemograptus proliferation interval (Siveter, 2009). The graptolites associated with some of the French and Sardinian occurrences of R. migrans are insufficiently studied or too illpreserved to provide precise independent stratigraphic control; however, based on the available geological information and its consistent stratigraphic range in the Czech Republic and Britain, Perrier et al. (2007) suggest a mid to late Ludlow Epoch age. Furthermore, although the Polish, central Asian, German and Swedish material remains unrevised, R. migrans was always recorded in these areas from the mid Ludlow Series.
The proposition of Perrier et al. (2007) that R. migrans can be used as a marker fossil for rocks assignable to the Ludfordian Stage of the Ludlow Series (Fig. 2a), is strengthened by the new records in the Kotel'ny Island (Ludfordian, Saetograptus leintwardinensis Biozone) and the Taimyr Peninsula (Ludfordian, Monograptus fritschi linearis Biozone).

palaeoGeoGraphIcal SIGnIFIcance
The widespread palaeogeographical distribution of R. migrans, in southerly tropical to mid latitudes on both sides of the Early Palaeozoic Rheic Ocean, was noted by Perrier et al. (2007). The new occurrences expand this distribution to the subtropical region of the palaeo-Northern Hemisphere on, or next to, the Siberia palaeocontinent (Fig. 2b). These new records therefore reinforce the idea that R. migrans had wide dispersal capacity.
The New Siberian Islands along with the western part of the East Siberian Sea belong to the New Siberian Islands microcontinent (Bogdanov et al., 1998;Kos'ko, 2007;Drachev, 2011;Cocks & Torsvik, 2011;Vernikovsky et al., 2013). During the Late Silurian, the New Siberian Islands Terrane was located in northerly subtropical, 20-25° latitudes. Cocks & Torsvik (2011) placed the New Siberian Islands microcontinent between the Siberia palaeocontinent in the NE, the Kolyma-Omolon Terrane in the NW, and Baltica further south (Fig. 2b).
In contrast, based on palaeomagnetic data, Vernikovsky et al. (2013) and D.V. Metelkin (pers. comm.) place the New Siberian Islands north of the Chukotka Terrane, at about 45°N (Fig. 2b). However, this position seems unlikely because of the Siberian/ Baltic affinities of the New Siberian Islands trilobite fauna in Early Palaeozoic strata (Holm & Westergaard, 1930;Danukalova et al., 2012), contrasting with the mix of Laurentian and Siberian faunas found in the Lower Palaeozoic of the Arctic Alaska-Chukotka microcontinent (Cocks & Torsvik, 2011).
The Taimyr Peninsula samples belong to the Central Taimyr Domain, a microplate that was accreted to the Siberian margin in the Late Neoproterozoic (c. 600 Ma), and covered by Late Neoproterozoic (Vendian) and Palaeozoic successions (Vernikovsky, 1996;Vernikovsky et al., 2004). During the Late Silurian the Central Taimyr Domain was located in subtropical latitudes, at about 20°N, on the southern part of the Siberia palaeocontinent (Metelkin et al., 2005; Cocks & Torsvik, 2011) (Fig. 2b).
Our new observations confirm that, as noted by Perrier et al. (2007), the occurrences of R. migrans are always associated with either black micritic limestones (e.g. Kote'ny Island, Taimyr, Sardinia, Montagne Noire) or shales (e.g. Bohemia, Armorican Massif, central Asia). These lithologies contain cephalopod-, graptolite-, phyllocarid-and bivalve-dominated assemblages. The myodocope-bearing facies typify possible deep-shelf environments or topographic lows on the shelf (Siveter et al., 1991;Perrier et al., 2011) and are characterized by the lack of bioturbation, the presence of lamination and by a low diversity, mostly pelagic, fauna. These facies data, added to the cosmopolitan distribution, suggest that R. migrans probably possessed a pelagic lifestyle, and this is consistent with the timing of a proposed ecological shift in pioneer pelagic (myodocope) ostracods from benthic to pelagic modes of life during the late Early or early Late Silurian (see Siveter, 1984;Siveter et al., 1987Siveter et al., , 1991Siveter & Vannier, 1990;Vannier & Abe, 1992;Perrier et al., 2007Perrier et al., , 2011.

remarks.
Based on what seems to be a more delicate ornament, Kříž & Bogolepova (1995) assigned the Taimyr material to 'Entomis' aff.
lamarmorai Canavari, 1900, andAbushik et al. (2003) created a new sub-species Richteria migrans taimyrica. Based on our evaluation of the global dataset of R. migrans, we consider this ornamental variation to be intraspecific (see Perrier et al., 2007). The largest Kotel'ny Island specimens are larger (3.5 mm vs. 2 mm) than those described by Perrier et al. (2007) from France and the Czech Republic, but of comparable size with the Sardinian specimens (Fig. 3).