Lucas, S.G., et al. eds., 2013, The Carboniferous-Permian Transition. New Mexico Museum of Natural History and Science, Bulletin 60. 1 THE CONODONT NEOGNATHODUS BOTHROPS MERRILL, 1972 AS THE MARKER FOR THE LOWER BOUNDARY OF THE MOSCOVIAN STAGE (MIDDLE PENNSYLVANIAN) ALEXANDER S. ALEKSEEV1 AND NATALIA V. GOREVA 2 1 Department of Paleontology, Geological Faculty, Moscow State University, Russia, email: aaleks@geol. msu.ru; 2 Geological institute of Russian Academy of Science, Moscow, Russia, email: [email protected] Abstract—The Moscovian Stage constitutes the Middle Pennsylvanian Series of the Carboniferous System, but a biostratigraphic marker and GSSP for it have not yet been designated. The exact position of the Moscovian boundary cannot be defined properly because in the type area the basal Vereian unconformably overlies the Mississippian limestone or the alluvial and lagoonal Aza Formation of the uppermost Bashkirian. The Task Group to establish a GSSP close to the existing Bashkirian-Moscovian boundary suggested several potential markers among foraminifers and conodonts, but the search for a marker near traditional base of the global Moscovian Stage has stalled. It may be more productive to search for FADs in the lower Moscovian, above the traditional base, to designate the lower boundary of the stage. Relatively rich Vereian and Kashirian conodont assemblages have been recovered from the southwest Moscow Basin, as well as from the Oka-Tsna Swell. The most complete information on the distribution of conodonts in the Vereian- Kashirian boundary interval was obtained from the Yambirno section (Oka-Tsna Swell). The greatest change in conodont assemblages does not occur near the level of the traditional base of the Moscovian, but stratigraphically higher. There is a sharp change at both the species- and genus-levels in conodont assemblages at the boundary between the Vereian Ordynka Formation and the overlying the Kashirian Tsna Formation. The FAD of Neognathodus bothrops Merrill, 1972 has the greatest potential for correlation of a new position of the lower boundary of the Moscovian near the base of the Kashirian. INTRODUCTION Bender, 1980 (Qi et al., 2007). Diplognathodus ellesmerensis is an easily identifiable species and occurs near the base of the Moscovian Stage in The Moscovian Stage constitutes the Middle Pennsylvanian Se- the Aljutovo Fm. of the Vereian Substage (Makhlina et al., 2001) and in ries of the Carboniferous System (Gradstein et al., 2004). However, a limestone K3 in the Donets Basin (Nemyrovska, 1999). However, the biostratigraphic marker and GSSP for it have not yet been designated. species has a very long stratigraphic range, for it been found significantly The Task Group to establish GSSP close to the existing Bashkirian- higher in the upper Moscovian Podolskian Substage in the Moscow Moscovian boundary has suggested several potential markers among Basin and in the Arkhangelsk Region and in the southern Urals (Dalniy foraminifers (first occurrences of Aljutovella aljutovica, Depratina prisca, Tyulkas section). In addition, it occurs only sporadically and its ancestor or the genus Eofusulina). However, foraminifers are restricted geographi- is not well defined. All these factors reduce its correlation potential. cally and have less potential than conodonts for international correlation. Subsequently, the FAD of advanced forms of Streptognathodus Among conodonts three potential events were originally proposed to expansus Igo and Koike, 1964 or S. suberectus Dunn, 1966 was pre- designate the base of the Moscovian (Groves and Task Group, 2006): (1) sented as a potential marker for the Bashkirian-Moscovian boundary (Qi The first appearance datum (FAD) of Declinognathodus donetzianus in et al., 2010). The primitive forms of S. expansus were re-named as “S. the evolutionary lineage D. marginodosus (Grayson, 1984) – D. praexpansus,” and were considered as an ancestor of S. expansus (Qi et donetzianus Nemirovskaya, 1990. (2) The FAD of Idiognathoides al., 2010, 2011). However, typical examples of S. expansus occur in the postsulcatus in the lineage I. sulcatus Higgins and Bouckaert, 1968 – I. undoubtedly older upper Bashkirian limestones G1 to H6 in the Donets postsulcatus Nemyrovska, 1999. (3) The FAD of Neognathodus nataliae Basin (Nemyrovska, 1999). Use of this maker is unacceptable because it Alekseev and Gerelzezeg, 2001. would lower the base of the Moscovian almost to a level within the Of these three, the most promising marker for the establishment middle of the Bashkirian. of the boundary appears to be the FAD of Declinognathodus donetzianus, Thus, the search for the base of the global Moscovian Stage marker which occurs at the base of the Moscovian Stage (Aljutovo Formation of near its traditional position has stalled and the most recent proposals the Vereian Substage, Makhlina et al., 2001). The evolutionary lineage D. have attempted to shift this boundary to a level within the middle or marginodosus – D. donetzianus is well documented in the Donets Basin upper Bashkirian. In contrast, we suggest that it may be more productive (Nemirovskaya, 1990; Nemyrovska et al., 1999). However, this species to search for FADs of conodont species within the lower part of the has a limited geographic distribution. To date, it has only been found near traditional Moscovian that may have greater potential for boundary the Donets Basin in the Moscow Basin (Makhlina et al., 2001), in the designation and international correlation. Aegiranum Marine Band of the basal Bolsovian in Great Britain (Boogard and Bless, 1985), and in the Southern Urals (Basu section, Pazukhin et LOWER MOSCOVIAN CONODONTS OF al., 2006). Recently it was identified in the lower Atokan of the Appala- THE MOSCOW BASIN, RUSSIA chian Basin (Work et al., 2012), but the illustrated specimen possesses only one incurved node on the platform. This North American occur- The base of the Moscovian Stage in the Moscow Basin coincides rence supports the possibility of using D. donetzianus as the marker with the lower boundary of the Vereian Substage (Fig.1), which corre- species, but a better specimen has to be illustrated. sponds to the base of the ammonoid Diaboloceras-Winslowoceras Zone Based on the study of the Naqing (Nashui) section in South China, and fusulinid Aljutovella aljutovica-Schubertella pauciseptata Zone. Most it was proposed to use the FAD of Diplognathodus ellesmerensis in the of the Vereian is a siliciclastic succession, and carbonate marine units lineage D. coloradoensis (Murray and Chronic, 1965) – D. ellesmerensis occur only at its base and top (Alekseev et al., 2004). The younger 2 section (Fig. 2), in a quarry located in the Oka-Tsna Swell along the left bank of Tsna River (Ryazan Region). The original quarry section was studied in detail, and reliable data on lithology and biostratigraphy were obtained. The correlation between conodont and fusulinid zones was established. Detailed cyclostratigraphic research was conducted recently (Kabanov and Alekseev, 2011). Unfortunately, the exploitation of this quarry ceased during recent decades, which made some of the previously well-represented outcrops in the quarry bottom unavailable for addi- tional study. A total of 4796 conodont specimens were extracted from more than 80 samples weighing more than 100 kg. Based on newly obtained data, the distribution of conodont species has been significantly updated (Fig. 3). In the Ordynka Formation (Vereian) two assemblages were estab- lished – one for beds with Idiognathoides ouachitensis (beds 1–5) and the second for the Streptognathodus transitivus Zone (beds 6–9). For this interval, in addition to the index species, Idiognathoides ouachitensis, Neognathodus atokaensis, N. nataliae, Idiognathodus aljutovensis Alekseev et al., Diplognathodus ellesmerensis Bender, D. ohioensis (Merrill) are represented. Significantly, first occurrences of Idiognathodus praeobliquus, Neognathodus bothrops and N. tsnensis are at the base of the Tsna Formation, bed 10. Neognathodus nataliae and N. atokaensis also occur in bed 10 and range higher (Fig. 3). In the middle part of the formation (beds 25–30), the assemblage is enriched by the appearance of FIGURE 1. Moscovian Stage of the Central East European Platform N. colombiensis (Stibane) and Idiognathodus izvaricus Nemyrovska. At (modified from Atlas litologo-paleogeographicheskikh kart SSSR, 1966). 1, the top of the Tsna Formation, together with the species listed above, the Speculative Moscovian time shoreline. 2, Moscovian isopachs in the first Idiognathodus obliquus is present (bed 30). Characteristic species subsurface. 3, Moscovian isopachs in the outcrop belt. 4, Approx. limit of for the Ordynka and Tsna formations are shown on Figure 4. Oka-Tsna Swell. B, Location of the Yambirno (YA) section (from Kabanov Declinognathodus marginodosus and Neognathodus atokaensis and Alekseev, 2011). are the most characteristic conodont taxa of the Vereian, and both these Kashirian Substage is represented mainly by shallow water marine car- species have their FADs in the Bashkirian. Representatives of the genus bonates with an abundant and diverse marine fauna. Several subordinate Idiognathoides are common in the Vereian assemblage, but they first siliciclastic units occur as well. Relatively rich Vereian conodont assem- appear in the low in the Bashkirian. Only two species of this genus (Id. blages have been reported from the southwestern Moscow Basin, from ouachitensis and Id. tuberculatus) appear later, but they still