Conodonts from Late Devonian island arc settings (Baruunhuurai Terrane, western Mongolia)
Introduction
Previous studies of Upper Devonian deposits from NW-China (Suttner et al., 2014; Carmichael et al., 2014, Carmichael et al., 2016) and western Mongolia (Kido et al., 2013; Ariunchimeg et al., 2014), have highlighted the lack of knowledge regarding the biodiversity of conodont faunas in island arc settings and other less common palaeoenvironments. This realization led us to our primary question, whether particular palaeoenvironmental settings could be validated as refugia or radiation centres of marine faunas, both during and in the aftermath of global extinction events. Previous work on echinoderms and bryozoans in northwestern China (Xinjiang Autonomous Region), indicates that the Central Asian Orogenic Belt (CAOB) acted as a diversity hot spot during the Famennian (Waters et al., 2003; Waters and Webster, 2009; Ernst, 2013). Biodiversity analysis and palaeobiogeographic relations of ammonoids (Zong et al., 2015) and ostracods (Song et al., 2017) from the Xinjiang region likewise suggest high Famennian diversity. An overview of marine macrofossils from the Baruunhuurai Terrane in western Mongolia (Ariunchimeg et al., 2014) documents the high diversity and abundant occurrence of flora and fauna during the Frasnian–Famennian interval.
Fossil taxa that are important biostratigraphic indicators, such as conodonts, carry the assumption of a cosmopolitan palaeogeographic distribution occurring in a broad range of ecologically or climatically defined microfacies. Moreover, conodonts that are geographically or ecologically endemic (e.g., Klapper, 1995; Bultynck, 2003) can form significant elements of global biozones. Some index taxa even show a slight temporal asynchronous first occurrence datum (FOD) when occurrences are compared at high-resolution scale (compare Spalletta et al., 2017, and references therein) allowing us to not only reconstruct faunal provinces during different time intervals, but to track palaeogeographic radiations with appropriate datasets.
Only a few conodont studies are known from the Upper Devonian succession of western Mongolia (Kurimoto et al., 1997; Ruzhentsev, 2001). Outcrops of Upper Devonian sediments in northwestern China, which are palaeobiogeographically analogous to those from western Mongolia, have produced a quite diverse conodont assemblage (e.g., Xia, 1997; Wang et al., 2015, Wang et al., 2016), particularly in sections near the Boulongour Reservoir, at Wulankeshun and in the Halayemen coal mine.
Here we provide a detailed conodont biostratigraphy and biofacies analysis of taxa obtained from the Hushoot Shiveetiin gol section (Baruunhuurai Terrane, western Mongolia) and discuss the global distribution and palaeobiodiversity patterns of Ancyrognathus, Pelekysgnathus, Mehlina and Icriodus that occurred during the early Famennian from the Palmatolepis subperlobata until the Palmatolepis gracilis gracilis biozones.
Section snippets
Geological setting
The Palaeozoic fold belts of Central Asia, located between the Siberian platform and the Cathaysian terranes (Fig. 1) are part of the Central Asian Orogenic Belt (CAOB). Formation of the CAOB is associated with the closure of a part of the Palaeo-Asian Ocean prior to the end of the Carboniferous and consists of a complex amalgamation of intra-oceanic island arcs and continental fragments (Xiao et al., 2010; Metcalfe, 2011; Choulet et al., 2012; Yang et al., 2013; Li et al., 2017).
The deposits
Material and methods
Seventy rock samples that weighed between 0.5 and 4.5 kg were collected for biostratigraphic analysis from the Samnuuruul section (SAM/01, SAM/02/23–66, SAM/03; Fig. 2: locality 1) and sixteen rock samples from the Hushoot Shiveetiin gol section (HS-cono-1–16; Fig. 2: locality 2) during fieldwork in 2012 and the IGCP 596 field conference in 2014.
Conodont extraction from limestone and marls followed standard processing methods (Jeppsson and Anehus, 1995). Rock material of each sample was cleaned
Biostratigraphy and regional correlation
Our original goal in this fieldwork was to identify Late Devonian anoxia and extinction events previously recognized in northwestern China (Carmichael et al., 2014, Carmichael et al., 2016) in a coeval Mongolian terrane in the Central Asian Orogenic Belt. Tectonic displacement and diagenetic overprint, particularly at the Samnuuruul locality, made this goal difficult. Generalized correlations between the Samnuuruul (SAM) and Hushoot Shiveetiin gol (HS) sections (Fig. 2) are possible based on
Discussion
Results from conodont biofacies and microfacies analyses suggest slightly changing bathymetric conditions from the Palmatolepis minuta minuta to Palmatolepis rhomboidea biozones. Generally, a polygnathid-palmatolepid conodont assemblage is dominating the sequence, which is interrupted by a short interval of an icriodontid-polygnathid biofacies just before the beginning of the Palmatolepis rhomboidea Biozone. According to biofacies distribution models for Belgian palaeoenvironments (Dreesen and
Conclusions
A Famennian age of the Upper Devonian deposits from the Baruunhuurai Terrane in western Mongolia has been confirmed by conodonts. The conodont assemblage from the Hushoot Shivetiin gol section consists of 30 taxa and spans a part of the early Famennian from the Palmatolepis minuta minuta through the Palmatolepis rhomboidea biozones. The conodont assemblage includes taxa of Ancyrognathus, Icriodus, Mehlina, Polygnathus and Palmatolepis. Based on significant morphological differences of some
Acknowledgments
The study was supported by grants of the FWF (P23775-B17) and the OEAW/ESS (subproject of IGCP 596). Waters' fieldwork was supported by an EAGER grant from the U.S. NSF. Cameron Batchelor's participation in the fieldwork was supported by a grant from the Explorers Club, New York. The OEAD is thanked for granting a 3-month-scholarship of Ariunchimeg Yarinpil (EURASIA PACIFIC UNINET), who completed all laboratory work during her stay at Graz University in 2015. The research was conducted within
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