A mid-Ordovician brachiopod evolutionary hotspot in southern Kazakhstan

LEONID E. POPOV AND L. ROBIN M. COCKS

Popov, L.E. & Cocks, L.R.M. 2021: A mid‐Ordovician brachiopod evolutionary hotspot in southern Kazakhstan. Fossils and Strata,

Chu‐Ili, now in Kazakhstan, was a substantial independent equatorial microcontinental terrane in Ordovician times, with a small Precambrian core fringed by several island arcs. Its mid‐Ordovician (late Darriwilian to early Katian) faunas were a major evolutionary hotspot within an equatorial archipelago at a period when Palaeozoic sea levels and temperatures were at their highest. As well as reviewing the previously described brachiopods from elsewhere in Chu‐Ili, the mid‐Ordovician brachiopods of the West Balkhash Region, which outcrops west of Lake Balkhash within Chu‐Ili, are newly described here, mainly from the Berkutsyur and Baigara formations. Many represent the earliest occurrence of their lineages, notably the oldest member of the Order Atrypida. More than twelve brachiopod associations are defined, many for the first time and together hosting 73 genera and over 91 species. The new family Kellerellidae is erected within the superfamily Lissatrypoidea. New genera are Aploobolus (Obolidae), Doughlatomena (Rafinesquinidae), and Altynorthis, Lictorthis, and Baitalorthis (all Plectorthidae), Baitalorhynchus (Sphenotretidae), Lydirhyncha (Ancistrorhynchidae) and Costistriispira (Kellerellidae). Eleven new species, including Aploobolus? tenuis, Doughlatomena splendens, Bimuria karatalensis, Apatomorpha akbakaiensis, Lepidomena betpakdalenis, Sonculina baigarensis, Altynorthis betpakdalensis, Altynorthis vinogradovae, Phaceloorthis? corrugata, Batailorhyncha rectimarginata and Costistriispira proavia, and one new subspecies Sowerbyella (Sowerbyella) verecunda baigarensis are also erected. The global palaeogeographical affinities of all the Chu‐Ili brachiopod faunas are discussed, as well as Chu‐Ili's place within the peri‐Gondwanan archipelago. Newly named stratigraphical units are the Berkutsyur (Darriwilian to early Sandbian) and overlying Kopkurgan (Sandbian to Katian) formations within West Balkhash, and the Tastau (Darriwilian) and Takyrsu (Darriwilian to early Sandbian) formations within the northern Betpak‐Dala desert. □ Brachiopoda, Evolutionary hot spot, Kazakhstan, Ordovician.

Leonid E. Popov [leonid.popov@museumwales.ac.uk], Department of Geology, National Museum of Wales, Cathays Park Cardiff CF10 3NP, UK; L. Robin M. Cocks [r.cocks@nhm.ac.uk], Department of Earth Sciences, The Natural History Museum, Cromwell Road London SW7 5BD, UK; manuscript received on 23/04/2020; manuscript accepted on 3/07/2020.

Introduction

During the mid‐Ordovician, sea levels and temperatures were at their highest in the whole Palaeozoic, and second highest only to the mid‐Cretaceous in the entire Phanerozoic, as known from many papers, reviewed by Torsvik & Cocks (2017). In those mid‐Ordovician times (late Darriwilian to early Katian), the Chu‐Ili Terrane formed part of an archipelago which straddled the Equator, comparably to the East Indies today (Popov & Cocks 2017). Some Middle and Late Ordovician brachiopods from several sectors of the Chu‐Ili Terrane have been described, but not from the West Balkhash Region which are thus monographed here. The brachiopods described in previous works from the Chu‐Ili Terrane include the Anderken Formation (Sandbian) (Popov et al. 2002), the Dulankara Formation (early Katian) (Popov et al. 2000; Popov & Cocks 2006), and the Uzunbulak Formation (Darriwilian) (Nikitina et al. 2006). Our aim here is to complete the task for the Middle Ordovician of the entire terrane so as to get a clear picture of its biological and geological significance. We also define more than 12 associations dominated by brachiopods, some previously unrecognised. The freshly described faunas came mostly from the newly defined Berkutsyur and Kopkurgan formations, which outcrop south‐west of Lake Balkhash in central Kazakhstan (Fig. 1) as well as from the Baigara Formation together with the new Tastau and Takyrsu formations exposed between the Karatal River and Baigara Mountain in the inhospitable southern Betpak‐Dala desert, where access is particularly difficult.

Our conclusion is that Chu‐Ili hosted one of the most rapidly‐evolving brachiopod faunas in the world in those times, and fully justifies identification as a ‘hotspot’. The terrane is thus of cosmopolitan importance, and played a key role in the Great Ordovician Biodiversification Event (Webby et al. 2004). The justification for that conclusion is within the ‘Implications for biodiversity’ section below.

Fig. 1. Generalised map of the West Balkhash Region and eastern Betpak‐Dala desert showing the position of the studied areas. Inset, location of south‐central Kazakhstan within Asia. Brachiopod localities: 1, area 6 km south west of Baigara Mountain; 2, River Karatal; 3, area 15 km west of Chimpek Bay (Locality 388); 4, area 4 km south‐west of Lake Alakol; 5, area 7 km south‐west of Lake Alakol; 6, Golubaya Gryada (localities 562, 563); 7, Karakan Ridge (Locality 154); 8, Ergenekty Mountains (Locality 1501); 9, Talapty site.

Geological setting and key sections

The Chu‐Ili Terrane was one of about twenty independent microcontinents and island arcs whose remnants are now preserved in the very large country of Kazakhstan, whose Ordovician rocks were reviewed by Nikitin (1972, 1973), as well as in its adjacent countries of Central Asia, notably Kyrgyzstan, Uzbekistan, and the north‐western part of China including Tarim. In Lower Palaeozoic times they were independent terrane units not far from the vast continent of Gondwana to its north‐west (Popov & Cocks 2017; Torsvik & Cocks 2017) and are together known as the ‘Kazakh terranes’. Chu‐Ili itself is made up of a Proterozoic core with 2.8 Ga zircons and an Early to Late Ordovician accretionary wedge which was caused by the progressive nearing of the adjacent Mynaral - South Dzhungaria Terrane to its north. Chu‐Ili and North Tien Shan merged in mid‐Silurian time (Popov & Cocks 2017), well after the rocks described in this paper were deposited. However, despite differing statements by some authors (Degtyarev & Ryazantsev 2007; Bazhenov et al. 2012; Wilhem et al. 2012), it was not until the Early Devonian that the much larger continent of Kazakhstania, which included Chu‐Ili and North Tien Shan as well as several other terranes, became an entity.

South Betpak‐Dala

The Ordovician stratigraphy of the southern Betpak‐Dala desert between Baigara Mountain at the south‐east and the Karatal River at the north‐east, all situated on the north‐western prolongation of the Chu‐Ili Terrane (Fig. 1), was summarised by Esenov et al. (1971) and Nikitin et al. (1980). The area is on the south‐eastern (modern orientation) margin of the Chu‐Ili terrane facing the Zhalair‐Naiman Fault Zone, which is the Silurian suture (Popov et al. 2009). The Ordovician succession in the area is, in ascending order: (1) a thick succession of graded siliciclastic rocks with subsidiary units of fine rhyolitic tuffs and a few horizons of phosphoritic conglomerates assigned to the Karatal Formation (Floian to early Darriwilian) with an estimated thickness up to 2000 m and whose stratigraphical relationship with the underlying units remains unknown; (2) the Baigara Formation (late Darriwilian to early Sandbian), a transgressive succession of polymict conglomerates, intercalated brownish‐red and greenish‐grey sandstones and siltstones with lingulides, succeeded by intercalating units of siltstones, argillites and nodular limestones with abundant dasyclad algae; (3) a succession of graded siliciclastic rocks with occasional graptolites over 1000 m thick; (4) the Anderken Formation (Sandbian) of mainly sandstones and siltstones with a few units of polymict conglomerates and bioclastic limestones, 1000–1500 m thick; and (5) the Dulankara Formation (Katian) of polymict conglomerates, sandstones, siltstones, and limestones up to 1600 m thick (Popov & Cocks 2006). Most of the fossil lists in previous publications are based on preliminary identifications, and the existing monographic record is poor. The Floian to Darriwilian graptolites from the Karatal Formation are known from Tsai (1974, 1976), and conodonts from a single locality in that unit were made known by Tolmacheva (2014). A few brachiopod species from the Baigara Formation were published by Nikiforova & Popov (1981), Popov et al. (2001), and Bassett et al. (2013), but many more are described in the present paper, while the brachiopod fauna from the Anderken Formation was documented by Popov (1980a, 1980b, 1985), and Popov et al. (2002). The rich brachiopod faunas in the Baigara Formation described here were first discovered by T. B. Rukavishnikova in...