Cephalopods of the Early Permian Shakh-Tau Reef
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©Filodiritto Editore – Proceedings Cephalopods of the Early Permian Shakh-Tau reef LEONOVA Tatiana1, SHCHEDUKHIN Alexander2 1 Borissiak Paleontological Institute of RAS, Moscow, (RUSSIA) 2 Moscow State University, Moscow, (RUSSIA) Emails: [email protected], [email protected] DOI: 10.26352/D924F5026 Abstract This paper presents the first results of the study of the cephalopod collection from the Early Permian Shakh-Tau Reef. This Late Asselian-Early Sakmarian ammonoid assemblage contains nine species of nine genera, some of which are identified in open nomenclature. In the Early Permian (Asselian-Sakmarian and Late Artinskian) assemblages of non-ammonoid cephalopods, about 30 species of coiled nautilids, four species of straight nautiloids (Orthocerida and Pseudorthocerida), and two species of Bactritida have been identified. In addition, two species belonging to the order Oncocerida were found in the Asselian-Sakmarian assemblage. The Asselian-Sakmarian reef community of non-ammonoid cephalopods differs significantly from the Sakmarian communities known from the basinal sediments of the South Urals, both taxonomically and in prevailing morphotypes. The Late Artinskian post-reef assemblage is closer in all respects to the Artinskian South Uralian assemblages, but also has its own characteristics. Keywords: Ammonoids, bactritids, nautiloids, oncocerids, Early Permian, Urals, Shakh-Tau reef Introduction The Early Permian Shakh-Tau reef, one of the four Sterlitamak reef mounds (“shikhans”), has now been almost completely excavated by a limestone quarry and has ceased to exist. The history of the study, geological structure, and components of the bioherm facies have been the subject of a number of works, and were studied in particular detail by Korolyuk [1]. During the quarrying of the limestone, layers containing the richest invertebrate fauna, including cephalopods, were opened. The fossils were sampled in 2015-2019 by a team from the laboratory of molluscs of the Paleontological Institute (Russian Academy of Sciences), which included A.V. Mazaev, M.S. Boiko, and in the last year, A.Yu. Shchedukhin. Thanks to the enthusiasm and efforts of A.V. Mazaev, a unique collection of early Permian cephalopods, including the Asselian-Sakmarian and Late Artinskian assemblages of ammonoids, straight and coiled nautiloids and bactritoids was assembled in the laboratory. Results and Discussion In the collection studied, the Late Asselian-Early Sakmarian ammonoid assemblage includes nine species of nine genera: Neopronorites tenuis (Karpinsky), Sakmarites postcarbonarius (Karpinsky), Medlicottia subdorbignyi Gerassimov (Fig. 1, H), Agathiceras uralicum (Karpinsky), Somoholites shikhanensis Ruzhencev (Fig. 1, G), Paragastrioceras aff. sterlitamakense Gerassimov, Uraloceras aff. limatulum Ruzhencev, Properrinites sp., and Propopanoceras sp. Previously, seven species of seven genera were recorded in Shakh-Tau [2]. In addition to the representative of the family Perrinitidae Miller and Furnish, 1940 [3], which was first discovered in the Urals, it was possible to establish the presence of Medlicottia 155 ©Filodiritto Editore – Proceedings in the Asselian-Sakmarian sediments, based on well-preserved material. Thus, the validity of Medlicottia subdorbignyi, a species established by Gerassimov [2] is confirmed. This means that the genus Medlicottia appeared a whole stage earlier than had previously been thought [4], [5], [6], [7]. Uraloceras is found in this locality for the first time. The newly obtained and refined data made it possible to compare this assemblage with the Early Sakmarian assemblage from Yukon, which also contains a primitive perrinitid and ancient Medlicottia [8]. The Canadian assemblages are less taxonomically diverse, but 80% of their genera (five of seven) are the same as in Shakh-Tau. These are the genera Properrinites (Subperrinites), Medlicottia, Prothalassoceras, Uraloceras, and Somoholites. The Pamir Asselian-Sakmarian ammonoid assemblage described by Ruzhencev [9] and Bogoslovskaya [10] is much richer than that of the Urals, out of 16 genera, four (25%) are shared: Agathiceras, Prothalassoceras, Somoholites, and Properrinites. In addition, the Pamir assemblage contains Prolecanitida: Boesites, Metapronorites, Vanartinskia; Goniatitina: Glaphyrites, Eoasianites, Svetlanoceras; Adrianitina: Emilites and Cyclolobina: Almites, Cardiella, Tabantalites, Prostacheoceras, and Martoceras. The Late Artinskian ammonoid assemblage is studied by Boiko; it includes about 20 species of 15 genera and is a good source of data for a comparative analysis of the reef and basinal communities of the Early Permian ammonoids of the Urals. A paper on the results of this study is being prepared for publication. The richest occurrence of nautiloids from the Asselian-Artinskian beds of Shakh-Tau is of particular interest. Even though many researchers pointed to the findings of non-ammonoid cephalopods on the shikhans, to date they have not been systematically studied. In recent years, based on material collected by A.V. Mazaev and M.S. Boiko, this unique nautiloid assemblage was preliminarily characterized by I.S. Barskov, who established in its representatives of the orders Pseudorthocerida, Orthocerida, Bactritida, and Nautilida [11]. In that collection, he found four specimens of curved shells, which, according to external morphology, corresponded to representatives of the order Discosorida (known from the Devonian) or Oncocerida [12]. Further study and the finding of another specimen with a preserved narrow marginal siphuncle confirmed the presence of Oncocerida (Fig. 1, A), which according to the most recent data became extinct by the Early Carboniferous. Altogether, around 30 nautilid species have been recorded from Shakh-Tau. The Asselian-Sakmarian coiled Nautilida include representatives of the following genera: Endolobus, Stenopoceras, Rhiphaeoceras, Pararhiphaeoceras (P. aktastense (Fig. 1, B), P. sp. nov.), Temnocheilus, Domatoceras, Liroceras, Koninckioceras, Megaglossoceras, Permonautilus. The Artinskian beds include species of the genera Parastenopoceras, Sholakoceras, Condraoceras, Neothrincoceras, Metacoceras (M. subquadratum (Fig. 1, C)), Neodomatoceras, Pseudotemnocheilus, Liroceras, Hemiliroceras, Gzheloceras, Lophoceras, and also Dentoceras (D. latum (Fig. 1, D)) with a straight shell lacking a phragmocone. In addition, straight Artinskian cephalopods are represented by Pseudorthocerida (species of Uralorthoceras (U. tzwetaevae (Fig. 1, E, F)), Shikhanoceras, Dolorthoceras) and Bactritida – Hemibactrites and Parabactrites. Some of these taxa are species described by Shimansky [13], [14], other species and three genera are new. No such taxonomically rich nautiloid assemblage is known from any other Permian site in the world. Usually one location contains no more than 2-5 genera and species. The richest assemblages of the Early Permian cephalopods (Sakmarian and Artinskian) were described by Shimansky [13], [14] from the basinal facies of the South Urals (several localities). Nautiloids from Shakh-Tau differ significantly in taxonomic and morphological composition from these assemblages, especially the Asselian-Sakmarian. 156 ©Filodiritto Editore – Proceedings The Late Artinskian cephalopod assemblage, formed after the end of the life of the reef, is much more similar to that of the South Urals, although it also has its own characteristics. As noted by I.S. Barskov [11], the differences in the Asselian-Sakmarian reef assemblage mainly consist of the prevalence of coiled taxa over straight ones. In the basinal assemblages of the South Urals, the ratio is reversed. Fig. 1. Some Early Permian cephalopods from Shakh-Tau Reef. A – Oncocerida, gen. et sp. nov., specimen PIN no. 5668/2, Upper Asselian-Lower Sakmarian; B – Pararhiphaeoceras aktastense Ruzhencev et Shimansky, specimen PIN no. 5668/29, Upper Artinskian; C – Mosquoceras sp. nov, specimen PIN no. 5668/30, Upper Asselian-Lower Sakmarian; D – Dentoceras latum Ruzhencev et Shimansky, specimen PIN no. 5668/61, Upper Artinskian; E, F – Uralorthoceras tzwetaevae Shimansky, E – specimen PIN no. 5668/10, Upper Artinskian; F – specimen PIN no. 5668/64, Upper Artinskian; G – Somoholites shikhanensis Ruzhencev, specimen PIN no. 5615/7, Upper Asselian-Lower Sakmarian; H – Medlicottia subdorbygni Gerassimov, specimen PIN no. 5615/3, Upper Asselian-Lower Sakmarian According to the ratio of morphotypes, the Asselian-Sakmarian assemblage of nautiloids is comparable to the Roadian (Kazanian) community described from the quarries of the Kirov region and Mari-El, from sediments of reef origin [15]. In the Middle Permian reef community, 157 ©Filodiritto Editore – Proceedings coiled forms also predominate, but, unlike the Shakh-Tau, they are represented by extremely smooth-cut forms, whereas in the Shakh-Tau ribbed shells make up about 50% of the coiled nautiloids. Conclusions The data obtained expand our understanding of the biological and morphological diversity of Early Permian cephalopods, their stratigraphic and geographical distribution, and also allow us to carry out paleobiogeographic and paleoecological studies. They open up new opportunities for a detailed study of the Permian reef buildups in the east of European Russia. Acknowledgments The authors express their deep gratitude to I.S. Barskov, the greatest specialist in cephalopods, who began this work. To our deep regret, illness and death prevented him from continuing his research. We, his students, consider it our duty