Rusophycus carleyi on the Edge of the Siberian Platform
¹ Geological Institute of Russian Academy of Sciences, Pyzhevsky per.7, 119017, Moscow, Russia; dronov@ginras.ru;
² Boryssiak Paleontological Institute of Russian Academy of Sciences, Profsouznaya ul. 123, 117997, Moscow, Russia; vkush@paleo.ru.
1 Introduction
In a working collection of Roman F. Hecker (1900-1991) stored in the Boryssiak Paleontological Institute of Russian Academy of Sciences in Moscow, an interesting specimen was found. The specimen is a fragment of a quartz sandstone layer with ripple marks on the top and a peculiar trace fossil on the bottom bedding plane. A half-decayed label indicated that the specimen (N765) was taken during the summer of 1962 from the Middle Ordovician of the Nizhnaya Tunguska River valley (Siberian Platform). The specimen was collected by a geologist of the Northern Geological Expedition of the Irkutsk State University and sent to R. F. Hecker for identification. On the other side of the label, in the hand of R. F. Hecker, is written:“Rusophycus—resting trace of trilobite”. The Rusophycus is well-preserved and possesses lateral lobes with coxal impressions in the mesial opening.It can be identified as Rusophycus carleyi(James,1885).This is the first report of Rusophycus carleyi from the Ordovician of the Siberian Platform.
2 Location
The trace fossil was collected from an outcrop on the right bank of the Nizhnaya Tunguska River about 200 m upstream from the mouth of its left tributary, the Usolka River. The locality is only 80 km from the southeastern (in present-day orientation) edge of the Siberian Platform (Fig. 1). In terms of modern-day lithostratigraphy, this outcrop belongs to the Surin Formation (Dapingian-lower Darriwilian).
3 Description
The trace fossil is 8 cm long and 5 cm wide, and is elliptically shaped with two distinct convex hyporelief lateral lobes, which are closed posteriorly. No obvious ridges are visible on the lobes. The mesial opening bears eight paired subrectangular coxal impressions flanking the midsagittal line, between the lateral lobes (Fig. 2). The coxal impressions become thinner from anterior (4 mm) to posterior (2 mm).
Figure 1 Geological map of the southern part of the Siberian Platform and the Baikal-Vitim folded belt.The star indicates the Rusophycus carleyi location.
4 Cruziana stratigraphy
The Cruziana stratigraphy was developed in the 1970s and early 1980s based on rocks containing trilobite trace fossils from Wales and Spain, particularly of Furongian (late Cambrian)-Early Ordovician age (Buatois & Mángano, 2011). Subsequently, Seilacher (1970, 1991, 2007) further developed the time range and geographical extension of the model. Additional studies have considerably extended this ichnostratigraphic scheme. According to Buatois&Mángano(2011),Rusophycus carleyi has a time range from the lattermost Early Ordovician until the end of the Late Ordovician: this is consistent with the age of the rocks containing R.carleyi on the Siberian Platform.Recently Gibb&Pemberton (2017) suggested synonymization of R. morgati Baldwin, 1977, R. moyensis Mángano, Buatois,and Muñiz Guinea,2002,R.polonicus(Seilacher,1970),and Cruziana radialis Seilacher,1991 under the heading of Rusophycus carleyi.This synonymization broadens the time range of R.carleyi to Cambrian-Late Ordovician.
Figure 2 Rusophycus carleyi from the Surin Formation(Middle Ordovician)of the Siberian Platform.
5 Cruziana palaeogeography
Rusophycus carleyi has an almost global distribution.It has been reported from the USA/Canada (Osgood, 1970; Brandt et al., 1995), Libya (Seilacher, 1970, 1991), Morocco (Gibb et al., 2010), Australia (Draper, 1980), and Argentina (Buatois & Mángano, 2011) as well as Poland, Wales, France, Spain and Newfoundland (Gibb & Pemberton, 2017) (Fig. 3). It is worth noting,however,that the rocks containing R.carleyi are older on the Gondwanan continent and blocks that were rifted and drifted from Gondwana. The age of these rocks is Cambrian and Early-Middle Ordovician in the Gondwanan realm and Middle-Late Ordovician in Laurentia. The age difference could be explained by the Taconic orogeny and docking of some Gondwanan microcontinents to Laurentia in the Middle Ordovician. The same scenario probably holds for the appearance of R. carleyi on the Siberian Platform.
Figure 3 Distribution of localities with R.carleyi(red stars)in the Late Ordovician.
6 Discussion
Cruziana and Rusophycus trace fossils have been reported recently from the Middle and Upper Ordovician of the Siberian Platform (Kushlina & Dronov, 2011; Dronov &Kushlina,2014;Dronov et al.,2016).However,all the Siberian Cruziana and Rusophycus have no direct analogues among the Middle and Upper Ordovician ichnospecies and seem to represent endemic forms of the Siberian palaeocontinent. In contrast, Cruziana cf. semiplicata and Cruziana cf. rugosa, which represent typical Gondwanan ichnospecies, were reported from the Lower Ordovician of the Kalar Graben on the north of the Baikal-Vitim folded belt, very near the edge of the Siberian Platform (Vilmova, 2016). The highly metamorphosed Baikal-Vitim folded belt was regarded for a long time as a tectonotype for the Baikalian orogeny, but later it was proved to be Caledonian (Belichenko& Khrenov, 1965; Fedorovsky et al., 1995).The appearance of R. carleyi in the Middle Ordovician deposits of the Siberian Platform near its boundary with the Baikal-Vitim folded belt probably indicates the same tectonic event as at the eastern (in present-day orientation) margin of Laurentia, i.e. the Taconic orogeny. On both palaeocontinents (Siberia and Laurentia), the invasion of the Gondwanan ichnospecies R. carleyi in the Middle Ordovician marks the docking of some Gondwanan microcontinents with these landmasses.
7 Conclusions
1)Rusophycus carleyi is reported for the first time from the Middle Ordovician of the Siberian Platform.
2) The appearance of this typical Gondwanan ichnospecies probably indicates collision of some microcontinents that rifted and drifted from Gondwana with the Siberian palaeocontinent during the Taconic orogeny.
3) Finding Middle Ordovician R. carleyi on the edge of the Siberian Platform near its boundary with the Baikal-Vitim folded belt reinforces the striking similarity in the geological evolution of Siberia and Laurentia during the Ordovician.
Acknowledgments Financial support for this research was provided from the Russian Foundation for Basic Research (No. 16-05-00799). This is a contribution to the IGCP Project 653.
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