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Discovery of Serratognathus bilobatus in the Early Ordovician Emanuel Formation of the Canning Basin, Western Australia, has regional biogeographic and biostratigraphic implications. Distribution of Serratognathus indicates a close biogeographic link between Australia and adjacent eastern Gondwanan plates and terranes during the latest Tremadocian to early Floian (Early Ordovician), and the formation of the so-called “Australasian Province”, a distinctive biogeographic entity that existed throughout most of the Ordovician. The S. bilobatus fauna from the Canning Basin is much more diverse in comparison with those assemblages bearing Serratognathus from coeval Chinese Lower Ordovician successions and probably represents an assemblage inhabiting relatively deeper water (mid-outer shelf) environments. The Canning Basin fauna contains many pandemic forms, and bridges the gap in the regional correlation of this widely distributed fauna across eastern Gondwana. This well-preserved, diverse fauna includes Serratognathus bilobatus and 23 associated species: Acodus deltatus?, Acodus? transitans, Bergstroemognathus extensus, Cornuodus sp., Drepanodus arcuatus, Drepanoistodus sp. cf. D. nowlani, Fahraeusodus adentatus, Lissoepikodus nudus, Nasusgnathus dolonus, Paltodus sp., Paracordylodus gracilis, Paroistodus parallelus, Paroistodus proteus, Prioniodus adami, Protopanderodus gradatus, Protoprioniodus simplicissimus, Scolopodus houlianzhaiensis, Semiacontiodus sp. cf. S. cornuformis, Stiptognathus borealis, Triangulodus bifidus, Tropodus australis, gen. et sp. indet. A and gen. et sp. B. A P. adami-S. bilobatus Biozone is defined within the middle and upper Emanuel Formation. Correlation of this biozone suggests an early Floian (late P. proteus Biozone to possibly earliest P. elegans Biozone) age for the middle and upper members of the Emanuel Formation. Zhen, Yong Yi, & RobeRt S. nicoll, 2009. Biogeographic and biostratigraphic implications of the Serratognathus bilobatus fauna (Conodonta) from the Emanuel Formation (Early Ordovician) of the Canning Basin, Western Australia. Records of the Australian Museum 61(1): 1–30. 2 Records of the Australian Museum (2009) Vol. 61 Conodonts are abundant, diverse, and well-preserved in the Early Ordovician Emanuel Formation of the Canning Basin in Western Australia. In the nearly sixty years since the discovery of the fauna (Guppy & Öpik, 1950), this rich fauna has formed the subject of several important studies, particularly on the prioniodontids by McTavish (1973) and by Nicoll & Ethington (2004), rhipidognathids by Zhen et al. (2001), Jumudontus brevis by Nicoll (1992), and Stiptognathus borealis (Repetski, 1982) by Ethington et al. (2000). As documentation of the entire fauna represented by Fig. 1. Localities and geological maps of the studied area. (A), map of Australia showing location of the Canning Basin; (B), map of the Canning Basin; and (C), map of the Emanuel Creek area showing location of section WCB705 along Emanuel Creek (modified from Nicoll et al., 1993). a huge collection recovered from several measured sections and subsurface core material is still in progress, the current contribution focuses only on Serratognathus bilobatus Lee, 1970 and the associated fauna recovered from three samples of the Emanuel Formation. The discovery of S. bilobatus in the Emanuel Formation, which was deposited in relatively deep water, mid-outer shelf settings, provides crucial new biostratigraphic data to more precisely date and correlate the Serratognathus faunas widely distributed in eastern Gondwana and adjacent PeriZhen & Nicoll: Canning Basin Serratognathus bilobatus Fauna 3 Fig. 2. Conodont biostratigraphy of the Emanuel Formation (late Tremadocian to early Floian, Early Ordovician) of the Canning Basin, and correlation with coeval successions in other regions in South China (Zhen et al., in press a), North China (An et al., 1983) and western Newfoundland (Stouge & Bagnoli, 1988). Gondwanan plates and terranes. Regional correlation with coeval Serratognathus-bearing faunas from the Arafura Basin and from China, Korea and Malaysia, defines a distinctive Australasian Province in eastern Gondwana (Zhen et al. in Webby et al., 2000) during the latest Tremadocian to early Floian (Early Ordovician). Moreover, as the S. bilobatus fauna from the Emanuel Formation contains a number of pandemic species, it serves as an ideal bridge correlating the S. bilobatus faunas in China and the more intensively studied Balto-Scandian and North American Mid-continent successions. Regional geological setting and lithostratigraphy The Canning Basin is an intracratonic sag basin, bounded by Precambrian rocks of the Kimberley Block to the north and the Pilbara Block to the south, occupying about one-sixth of Western Australia’s onshore area (Fig. 1). Sparsely inhabited and covered largely by flat-topped hills and sand dunes, it is dominated by a Palaeozoic-Mesozoic sedimentary succession more than 15 km in thickness. Extensive geological investigations have been carried out in the region for more than a century (Purcell, 1984), spurred on by its high potential for fossil fuel reserves. The Early Ordovician Prices Creek Group, representing the oldest stratigraphic unit in the basin, is only exposed along its northern margin (Lennard Shelf) where it rests unconformably on Precambrian basement. This basal contact is observable only in drillcore. The Prices Creek Group is overlain paraconformably by the Middle Devonian (Givetian) Cadjebut Formation (Hocking et al., 1996). The group was initially subdivided into a lower Emanuel Limestone (subsequently renamed Emanuel Formation) and an upper Gap Creek Dolomite (later amended to Gap Creek Formation) with a measured thickness of 595 m for the Emanuel Formation in the type section along Emanuel Creek (Guppy & Öpik, 1950; Guppy et al., 1958). Exploration drill holes in the area revealed an additional unexposed section between the Emanuel Formation and metamorphic rocks of the Precambrian basement, comprising a dolomitic interval 88 m thick underlain by 82 m of arkose (Veevers & Wells, 1961; Henderson, 1963; McTavish & Leg, 1976; Towner & Gibson, 1983). Nicoll et al. (1993) formally named the dolomitic unit the Kudata Dolomite and the arkose unit the Kunian Sandstone, and revised the Emanuel Formation as consisting of limestone intercalated with shale and siltstone with a total thickness of 435 m exposed in the type section (Fig. 1). Regionally the Emanuel Formation conformably overlies the Kudata Dolomite and is conformably overlain by the Gap Creek Formation of mid-late Floian age (Fig. 2). The basal arkose of the Kunian Sandstone was deposited in a major transgressive phase flooding the Canning Basin during the Tremadocian (Nicoll et al., 1993). This age determination was based on the regional tectonic framework and event stratigraphy, as well as fossil evidence from the overlying Kudata Dolomite, as no fossils were found in the arkose unit. The Emanuel Formation was subdivided into three informal members (Nicoll et al., 1993). However, in the type area, both lower and upper members with intercalated shale, siltstone and limestone (or nodular limestone in the upper member) are poorly exposed, whereas the limestonedominated middle member with a thickness of 143 m is better exposed forming a more or less continuous section. The Emanuel Formation has not only yielded a rich conodont fauna, but also trilobites (Legg, 1976; Laurie & Shergold, 1996a, 1996b), graptolites (Thomas, 1960; Legg, 1976), nautiloids (Teichert & Glenister, 1954), gastropods (GilbertTomlinson, 1973; Jell et al., 1984; Yu, 1993), brachiopods (Brock & Holmer, 2004) and various other invertebrate and microfossil groups (Guppy & Öpik, 1950; Brown, 1964; Schallreuter, 1993a, 1993b). 4 Records of the Australian Museum (2009) Vol. 61 Age and correlation of the conodont fauna McTavish (1973) described 20 multi-element conodont species and subspecies from the Emanuel Formation and lower part of the Gap Greek Formation; 19 of these were referred to prioniodontid genera, namely Acodus, Baltoniodus, Prioniodus and Protoprioniodus, and one to Periodontidae. On the basis of correlation with Balto-Scandian and North America Mid-continent successions McTavish (1973) determined a Latorpian age (latest Tremadocian to Floian in current terminology) for the Emanuel Formation and lower Gap Creek Formation. In particular, occurrence of Acodus deltatus?, Paroistodus proteus and P. parallelus in the Emanuel Formation allowed its correlation with the Paroistodus proteus Biozone (Hunneberg) of the Balto-Scandian succession, and the appearance of Oepikodus communis in the lower Gap Creek Formation supported correlation with the Ninemile Formation of central Nevada (McTavish, 1973, pp. 31, 32), namely the O. communis Biozone of the North America Midcontinent succession. In our study 24 conodont species are documented from three samples in the middle and upper members of the Emanuel Formation (Table 1). Paroistodus parallelus, Acodus deltatus?, Tropodus australis, Protopanderodus gradatus, Stiptognathus borealis, Bergstroemognathus extensus, and Drepanodus arcuatus dominate the fauna, whereas Serratognathus bilobatus is relatively rare. As shown in Table 2, among the 24 species recorded in this study, 11 species were also recorded in the Honghuayuan Formation in the Yangtze Platform of South China (An, 1987, Ding et al. in Wang, 1993; Zhen et al., in press a), seven in the Liangjiashan Formation of the North China Platform (An Table 1. Distribution of conodont species recovered from three samples in the Emanuel Formation of the Canning Basin, Western Australia. species WCB705/133 WCB705/243 161–166 m Total Acodus deltatus? 14 185 10 209 Acodus? transitans 4 4 Bergstroemognathus extensus 32 71 103 Cornuodus sp. 7 12 4 23 Drepanodus arcuatus 65 19 11 95 Drepanoistodus sp. cf. D. nowlani 30 13 9 52 gen. et sp. indet. A 10 1

records of the australian museum

Biogeographic and biostratigraphic implications of the Serratognathus bilobatus fauna (Conodonta) from the Emanuel Formation (Early Ordovician) of the Canning Basin, Western Australia