Biostratigraphical and palaeobiogeographical implications of Lower Silurian Radiolaria from black cherts of the Armorican Massif (France)

A moderately well-preserved radiolarian assemblage was recovered from organic-rich black chert in a Llandovery (Lower Silurian) sequence that crops out in southern Brittany (Chalonnes-sur-Loire section, Armorican Massif, France). The assemblage is composed of two families (Rotasphaeridae and Haplotaeniatidae), four genera (?Diparvapila, Secuicollacta, Orbiculopylorum, Haplotaeniatum) and 13 species. Some were identified from whole specimens preserved in silica and extracted following dilute hydrofluoric acid processing, while others were recognized in thin-section preparations, as they are beautifully preserved as ‘carbonized’ microfossils. The age range suggested by conodonts and chitinozoans yielded after HF processing from one of the 27 studied samples is in good agreement with the previously published age based on graptolites. The recovered radiolarians are discussed, documented and compared with known Rhuddanian, Aeronian and lower Telychian assemblages in the literature. The stratigraphic ranges are extended for the species Secuicollacta bipola, S. hexactinia, S. parvitesta, Orbiculopylorum granti and O. splendens based on our new data. A significant number of radiolarians found in our samples occur in Llandovery sections from Alaska, Nevada, Arctic Canada and Sweden. These similarities are used to discuss the palaeodistribution of Lower Silurian Radiolaria and our observations support the hypothesis of a wide geographical distribution for these Palaeozoic species.

Armorican Massif (Piçarra et al., 2002). This succession was previously dated in detail by the presence of abundant graptolites throughout the sequence (Piçarra et al., 2002). The section exhibits in its northeastern part a c. 30 m thick succession of uppermost Ordovician diamictite (interval 'a' on Figs 1 and 2), followed by c. 2 m thick grey shale (interval 'b'), accumulated respectively during the Hirnantian glaciation and the subsequent deglacial period. Then, c. 10 m of 'phtanite' (cm-to dm-thick black bedded chert rich in organic matter, interbedded with black shale containing pyrite) corresponding to the interval 'c' overlie conformably the grey siltstone and shale (Piçarra et al., 2009). This interval represents the most complete Llandovery succession of the Ligerian domain, recording a continuous sedimentation during the Rhuddanian and the Aeronian, as testified by its graptolitic and chitinozoan content (Piçarra et al., 2009). Interval 'd' corresponds to strata displaying deformed horizons, difficult to access (Piçarra et al., 2002(Piçarra et al., , 2009).

MaterIal and MethodS
Twenty-seven samples were collected at regular intervals (and depending on the accessibility of the outcrop) all along the 'c' interval of the studied sequence, corresponding to the interval of black chert and shale, from the base of the Rhuddanian to the upper Aeronian. Samples containing radiolarians were first selected under a polarizing microscope based on 30 µm-thick thinsection observations. Moreover, the mineralogy of some samples was confirmed by using RAMAN spectroscopy (Jobin Yvon, LabRam HR 800 UV).
Promising samples selected for laboratory processing were chemically treated as follows: samples were first washed, rinsed with distilled water, oven-dried, crushed into smaller fragments and then soaked in numbered plastic beakers containing diluted (4.8%) HF for 24 h. Then, the fragments were sieved with 63 µm and 630 µm sieves and rinsed. Fragments larger than 630 µm were soaked again in renewed 4.8% HF for 24 h, while the fraction   Piçarra et al. (2009) and the stratigraphic position of the radiolarian-bearing samples MT8, MT18, MT19 and MT20 (circles and squares represent SeM and thin section identifications, respectively). comprised between 63 µm and 630 µm was filtered and dried for microscopic examination. This procedure was repeated six times. Residues were examined under a binocular microscope and radiolarians were picked, mounted on carbon stubs, metal coated, scanned and magnified with a scanning electron microscope (SeM; FeI, Quanta 200).

reSultS petrography
In thin sections, we observed two different sedimentary textures; one exhibiting numerous chalcedonic spheres, 200-300 µm in diameter, which are probably very poorly preserved 'radiolarian remains', as mentioned by Piçarra et al. (2009), alternating with a second texture devoid of any spheres. These textures are interpreted to represent alternating radiolarian-rich and radiolarianpoor microfacies. However, both microfacies also exhibit thin laminations rich in quartz silt with a clayey matrix. Samples are also rich in organic matter (matrix as well as some radiolarians) and contain rhombic pyrite crystals. Occasionally, a chalcedonic sphere might contain well-preserved remains of Radiolaria (Tetard et al., 2014, fig. 3.9) that can be identified in thin section (found in samples MT1, MT8, MT18, MT19, MT20). Those well-preserved radiolarians, as well as several others extracted and observed with a SeM (MT8, MT19) seem to be carbonized (RAMAN spectra), such as the Saxothuringian fauna reported by Noble et al. (1998).

radiolarians
The best preserved radiolarians (Pls 1 and 2) come from the sample MT18 which corresponds to a jasper bed. In addition to radiolarians, some conodonts, chitinozoans and sponge spicules were also extracted and picked from the residue recovered from this sample MT18 (Pl. 2). Several radiolarians, although less well-preserved, were also recovered from other samples (e.g. c. 15 specimens from sample MT8, a few others from samples MT19 and MT20). Only the best specimens (i.e. around 150, 6 and 5, respectively, for samples MT18, MT19 and MT8) were mounted on carbon stubs for SeM observation. Regrettably only a fraction of them were identifiable (i.e. around 60 specimens from sample MT18, 2 specimens from MT8 and only one from MT19), a problem attributed to preservation. Some radiolarians were identified based on thin-section observations (samples MT8, MT18 and MT20; Pl. 3).

SySteMatIc palaeontoloGy
Subclass radiolaria Müller, 1858 Superorder polycystina ehrenberg, 1838; emend. Riedel, 1967 Order archaeospicularia Dumitrica, Caridroit & De Wever, 2000 Family rotasphaeridae Noble, 1994;emend. Noble & Maletz, 2000 remarks. While for Dumitrica et al. (2000), the family Rotasphaeridae is a junior synonym of Secuicollactidae, we follow here the opinion of Jones & Noble (2006)  remarks. The specimen is poorly preserved; however, a medullary shell is observed, which may indicate that this is a specimen of Diparvapila. Its medullary shell is a bit larger than that reported for D. larseni MacDonald, 1998, although the conical shape and size of spines are comparable. However, only two primary spines are preserved, probably not intact; the others seem to be broken; pores are not visible, thus preventing species-level identification.
Genus remarks. Some of our specimens exhibit only one major spine, the other one being probably broken, as mentioned by Won et al. (2002). These specimens (e.g. Pl. 1, fig. 2) are tentatively identified as Secuicollacta bipola, because of their poor preservation.
remarks. This species is easily distinguishable thanks to the octahedral arrangement of the primary spines. Won et al. (2002) indicate this species may be synonymous with S. teli MacDonald, 1998 which also has a similar spine arrangement, but since no SeM images showing external details were included by MacDonald, the species are left separate. The presence of byspines and the number, shape and arrangement of principal spines are in good agreement with the description of both species. Secuicollacta glaebosa MacDonald, 1998 differs by the lack of by-spines, a more bladed surface and sometimes more than 6 spines (MacDonald, 1998  2014 Secuicollacta multispinosa (Won, Blodgett & Nestor, 2002); Tetard et al.: fig. 3.3.
remarks. This species is very similar to S. hexactinia, but differs in having more than six primary spines, generally shorter and without any peculiar arrangement. Our specimens display a relatively small and thick spherical shell with a coarse latticed meshwork, without primary spine or with very short ones on some specimens. The identification is doubtful due to poor preservation. This species also shows primary units without primary spine or very small ones (a primary unit with five primary rods is visible in Pl. 1, fig. 10). occurrence. Middle Aeronian (sample MT18), Chalonnes-sur-Loire section, France (this study); lower Telychian, Road River Formation, Tatonduk River area, east-central Alaska (Won et al., 2002); upper Sheinwoodian, Cape Phillips Formation, Nunavut, Arctic Canada (Jones & Noble, 2006).
remarks. Won et al. (2002) established two morphotypes for this species. The first one exhibits a smaller shell, more numerous byspines, fewer and larger pores, while the second one has a denser meshwork. Our specimens exhibit a size difference and may represent examples of both morphotypes; however, poor preservation precludes such detailed discrimination. According to Won et al. (2002), this species is very similar to S. alaskensis Won et al., 2002 which shows a thinner shell wall with a finer structure and shorter spines, and to S. tatondukensis exhibiting a less delicate meshwork and larger pores.  Won et al. (2002) in having straight primary rods. This specimen is also similar to S. parvitesta, but differs in having more numerous and shorter outer spines, and a meshwork consisting principally of straight primary rods.
remarks. Observed only in thin section, this specimen is characterized by a very loose inner meshwork, while the outer part is, on the contrary, very dense.

Integrated biostratigraphy and radiolarian age ranges
The studied sequence at Chalonnes-sur-Loire was previously dated by abundant graptolites collected and identified by Piçarra et al. (2009). Based on their biostratigraphical assignment (compare with their fig. 3), samples MT18, MT19 and MT20 can be assigned to the middle Aeronian (Lituigraptus convolutus graptolite biozone); in the same way, our sample MT8 can be assigned to the lower Rhuddanian (see Fig. 2). The common age range of conodonts and chitinozoans recovered from sample MT18 is in good agreement with the more accurate age provided by graptolites (see Fig. 3).
Silurian radiolarian biostratigraphy has made significant progress over the last 20 years, with the Llandovery being initially characterized by a single zone (Nazarov & Ormiston, 1993), but subdivided later into three (MacDonald, 2006b; see Fig. 3). Sample MT18 contains numerous Orbiculopylorum specimens and it may be assigned to the Orbiculopylorum assemblage of MacDonald's biozonation (2006b), which is an informal taxon range zone whose base and top are respectively defined by the first and last occurrence of the genus Orbiculopylorum. In the Arctic sections studied by MacDonald (2006aMacDonald ( , 2006b, the Orbiculopylorum assemblage was recovered from the lowermost Aeronian Campograptus curtus through mid-Telychian Spirograptus turriculatus Arctic graptolite zones. Orbiculopylorum was not found in material older than Aeronian in the Arctic (MacDonald 2006b).
Based on their occurrences in our section, the stratigraphic range of several secuicollactid and haplotaeniatid species can be extended. Thus, for species Secuicollacta bipola, S. hexactinia and S. parvitesta, for which the oldest previously known level was the lower Telychian (Won et al., 2002), their age range can be now extended down to the middle Aeronian. Concerning the haplotaeniatid species Orbiculopylorum granti, known previously from the lower Aeronian (MacDonald, 2006a), its age range is now extended up to the middle Aeronian. The same holds true for species O. splendens, previously known only from the upper Rhuddanian (Noble et al., 1998). Figure 4 shows the age calibration of all known Llandovery radiolarian assemblages. Those described by Noble & Maletz (2000) and MacDonald (1998MacDonald ( , 2004 will not be considered in the following discussion, as their age is younger than the age of our fauna. The assemblage described by MacDonald (2006a) will not be considered either because it deals only with the family Haplotaeniatidae and not with the entire assemblage, and because the highly dissimilar preservation between well-preserved limestone and moderately preserved chert samples makes it difficult to make any meaningful comparisons of taxonomic composition.
The incursion of tropical radiolarians into polar areas is not rare in modern oceans (Bjørklund et al., 2012), some of them being brought into high latitudes by regular pulses of warm water masses originated from the tropical realm. Figure 5b illustrates a schematic map of global oceanic circulation during the Silurian, reconstructed after Ziegler et al. (1977), Armstrong et al. (2009) and Servais et al. (2014). It displays warm currents circulating towards the south, with western, central and eastern components. The western component is part of a west-Laurentia gyre that drives warm water away from Laurentia and drags plankton along southern Africa into high latitudes, whereas the central and eastern components circulate along the Baltica and Avalonia land masses, within the east Baltica gyre. Thus, planktic populations living in these currents could have been shifted from an equatorial into an austral position and be deposited into the Armorican and Thuringian basins.
Another important factor to take into account is the possible discrepancy between biocoenosis (living assemblage), thanatocoenosis (death assemblage) and taphocoenosis (fossil assemblage) (De Wever et al., 1994). Indeed, these planktic micro-organisms are considerably dependent on the oceanic currents (Bjørklund et al., 2012) and thus skeletons of radiolarians could be possibly transported over long distances even after their death. However, it is usually accepted that the lateral transport of radiolarian shells along long distances is limited. Indeed, the principal transport component is considered to be the sinking through aggregates and fecal pellets (De Wever et al., 1994;Lazarus, 2005). Thus the transport of micro-organisms is very restricted and the presence of foreign taxa in unusual latitudes (e.g. cold-water radiolarians into low latitudes, or reverse) probably shows the tolerance of this species (Lazarus, 2005).

concluSIon
The 'Chalonnes-sur-Loire' section consists essentially of Llandovery (Rhuddanian to Aeronian) black chert alternating with black shale that is rich in organic matter and pyrite and was deposited in an outer offshore environment, probably along a continental slope located on an intermediate-high latitudinal Gondwanan margin. This outcrop produced numerous graptolites and radiolarians, and an assemblage consisting of 13 species is described herein. Based on the composition of our assemblage in the best preserved sample (MT18), the upper part of the section was assigned to the Orbiculopylorum assemblage, which ranges from the base of the Aeronian to lower Telychian in the Arctic where it was originally defined. In the 'Chalonnes-sur-Loire' section, the Orbiculopylorum assemblage is definitively recognized in the middle Aeronian Lituigraptus convolutus graptolite zone, but may extend lower into the middle Rhuddanian, depending on whether specimens identified as ?Orboculopylorum adobensis are, in fact, this species. Comparison with other known lower Silurian assemblages, known essentially from tropical regions, reveals many similarities with our assemblage, which argues for a wide biogeographical dispersal of lower Silurian radiolarian plankton.
acKnowledGeMentS Laurence Debeauvais helped with laboratory processing of cherts, Sylvie Regnier with thin-section preparation, Philippe Recourt with the SeM, Sandra Ventalon with the RAMAN. Thanks to Thjis Vandenbroucke for discussion on chitinozoan identifications. Constructive remarks by Toshiyuki Kurihara and the handling editor (Claire Allen) improved the initial manuscript. The University Lille 1 is acknowledged for sponsoring a visiting professorship to Paula Noble.

Manuscript received 1 May 2014 Manuscript accepted 1 May 2014
Scientific editing by Claire Allen reFerenceS