Endemic thelodonts (Vertebrata: Thelodonti) from the Lower Silurian of central Asia and southern Siberia

Abstract

New fossil vertebrate microremains from the Lower Silurian of NW Mongolia, Tuva and S Siberia have been discovered, and previous collections of thelodonts (Vertebrata: Thelodonti) from this region re-studied, figured and described, following recent advances in morphology and systematics of thelodont scales. As a result, six thelodont species are described here and attributed to two families and three genera. An emended diagnosis is given for each species. Morpohological scale varieties of each species are revised, and the squamation types are introduced to the species descriptions, resulting in newly grouped morphological sets of scales. The previously monotypic genus Talimaalepis Žigaitė, 2004, is proposed to contain two different species; therefore, two new combinations of morphological scale sets of two species are described for the first time. Both of them show transitional scale structure between the genera Loganellia (family Loganellidae) and Helenolepis (family Phlebolepididae). Two of the thelodont genera, Angaralepis and Talimaalepis, as well as all the six species, are endemic and not known anywhere else in the world. They are accompanied by a variety of other peculiar early vertebrates, such as mongolepids, two endemic genera of acanthodians, and putative galeaspids. This work enhances our knowledge of early Silurian vertebrate diversity, and provides evidence of regional palaeoenvironmental conditions and palaeogeographical relationships of the Siberia and Tuva terranes.

New data on Silurian vertebrates of southern Siberia

“…angachiensis Vladimirskaya, and Pseudocamarotoechia ubsuensis (Tchernychev). This "Tuvaella" or "Tuva-Mongolian" type of fauna is characterized by endemism in benthic invertebrate groups especially brachiopods, bryozoans, and corals (Modzalevskaya, 1968(Modzalevskaya, , 1977Modzalevskaya, 1969;Chekhovich, 1972a,b;Vladimirskaya, 1978;Kulkov et al, 1985;Vladimirskaya et al, 1986), as well as in pelagic ichthyofaunas (Obruchev, 1956(Obruchev, , 1958(Obruchev, , 1960Obruchev, 1967;Karatajute-Talimaa, 1973Talimaa, 2000;Kurilenko et al, 2002;Karatajute-Talimaa and Ratanov, 2002;Žigaite, 2004, 2013Žigaite and Blieck, 2006;Žigaite and Karatajute-Talimaa, 2008). The Tuva-Mongolian Silurian is inferred to have accumulated in a shallow-water basin (or several basins) with carbonate-terrigenous and mainly redbed sedimentation (Vladimirskaya, 1967(Vladimirskaya, , 1973Vladimirskaya et al, 1986).…”

“…Analysis of vertebrate ranges in the studied Tuva Silurian sections (Bazhin-Alaak, Elegest, Ulug-Khondergei) along with revision of published data (Obruchev, 1956(Obruchev, , 1958(Obruchev, , 1960Karatajute-Talimaa, 1973Vladimirskaya et al, 1977Vladimirskaya et al, , 1986Kulkov et al, 1985;Karatajute-Talimaa and Ratanov, 2002;Smith, 2003, 2004;Žigaite, 2004, 2013Žigaite and Blieck, 2006;Žigaite and Karatajute-Talimaa, 2008) from the other Tuva sections (Chaakhol, Kadvoy, KuzulChiara) enables us to trace their integrated ranges within horizons (regional stages) (Fig. 7).…”

“…Collections of vertebrate remains (agnathan and fish scales) were obtained by acid-leaching of limestones, limy mudstones, limy siltstones, and limy sandstones in dilute acidic acid. Published data on ranges of the Silurian vertebrates in Tuva sequences used for this summary include those of Karatajute-Talimaa (1973,1978), Vladimirskaya et al (1977Vladimirskaya et al ( , 1986, Kulkov et al (1985), Karatajute-Talimaa and Ratanov (2002), KaratajuteTalimaa and Smith (2003),Žigaite (2004, 2013, Žigaite and Blieck (2006), andŽigaite and Karatajute- Talimaa (2008).…”

“…Vertebrates have been reported from the Tuva Silurian sequences in several papers (Obruchev, 1956(Obruchev, , 1958(Obruchev, , 1960Karatajute-Talimaa, 1973Vladimirskaya et al, 1977Vladimirskaya et al, , 1986Kulkov et al, 1985;Karatajute-Talimaa and Ratanov, 2002;Smith, 2003, 2004;Žigaite, 2004, 2013Žigaite and Blieck, 2006;Žigaite and Karatajute-Talimaa, 2008). New vertebrates from the Upper Baital Subformation of the Taungateli Horizon (Bazhin-Alaak/Chadan section) are represented by Helenolepsis obruchevi Karatajute-Talimaa, Helenolepsis sp., Apalolepsis sp., and elasmobranchs (Fig.…”

“…Siberian Platform Moskalenko (1968), Karatajute- Talimaa and Predtechenskyi (1995) andŽigaite (2013) Tuva Karatajute-Talimaa (1973), Vladimirskaya et al (1977Vladimirskaya et al ( , 1986, Karatajute-Talimaa (1978), Kulkov et al (1985), Karatajute- Talimaa and Ratanov (2002), Karatajute- Smith (2003, 2004), Zigaite (2004Zigaite ( , 2013, Žigaite and Blieck (2006) anď Zigaite and Karatajute-Talimaa (2008) Severnaya Zemlya Matukhin and Menner (1999), Karatajute-Talimaa and Märss (2002) and Märss and Karatajute-Talimaa (2002) Trans-Baikal Vladimirskaya et al (1986), Rodina et al (2002) and Karatajute-Talimaa and Smith (2003) foldbelts of subduction-accretionary nature, the Middle Paleozoic Anui-Chuya marginal sea trough, the Late Paleozoic-Early Mesozoic Kuznetsk Basin, which is a Hercynian continental intermountain depression, and the Zaisan collisional belt (Dobretsov, 2003;Sennikov et al, 2008;Vladimirov et al, 2008). The complicated structure of the ASFA is formed by gradual northward accretion of Late Neoproterozoic-Paleozoic oceanic, island-arc and active margin terranes and Precambrian microcontinents to the Siberian continental margin (Dobretsov et al, 1995;Dobretsov, 2003;Windley et al, 2007;Safonova et al, 2011).…”