Calcareous fossils from the middle-upper part of the Weiba Formation (grainstone) at Dagala, Zhenxiong County, northeastern Yunnan have been extensively silicified and are extracted through glacial acetic acid (5-12%) dissolution, after high temperature deacidification from carbonate matrix. More than 2000 specimens are obtained (Fig. 1). The Weiba Formation is conformably overlain by the Lungmachi Formation (Rhuddanian) and is disconformably underlain by the Daduhe Formation (upper Katian). The basal part of the Lungmachi Formation yields graptolites suggesting the Akidograptus ascensus Biozone. Thus, the Weiba Formation is postulated to be late Hirnantian in age (Wang et al., 2022).
The brachiopods dominate the benthic biota, making up approximately 95% of all specimens in this formation (Fig. 2). A preliminary investigation of the silicified brachiopods is herein presented for the first time. Based on this study, more than 20 taxa are recognized, including Eospirigerina putilla (Hall and Clarke) (predominant), Hesperorthis, Dalmanella (relatively common), Cathaysiorthis, Mendacella, Epitomyonia, Brevilamnulella, Hindella (rare), along with Paracraniops, Fardenia, Triplesia, Leptaena, Eostropheodonta, Eoplectodonta, Platystrophia, Glyptorthis, Thebesia, Hallina? and Eospiirier (very or extremely rare), as well as a few undetermined taxa of orthide, rhynchonellide and atrypide (Figs 3, 4). These taxa belong to 9 orders, 15 superfamilies, and 20 families, suggesting a relatively abundant and highly diverse brachiopod assemblage established in the soon aftermath directly following the Late Ordovician mass extinction (LOME), representing far more genera than expected.
Extensive groups of marine benthic organisms are associcated with the brachiopods that signify both autotropic and heterotrophic organisms, including calcareous algae, bryozoans, crinoids, gastropods, rugose corals, stromatoporoids (sponges), tabulates (auloporids and halysitids), tube worms (such as Cornulites), and a few others. Hence, evidence indicates that diversity of the overall biota living in a shallow-marine setting was relatively high immediately after the LOME in South China.
In terms of the palaeosynecology, the brachiopods studied in this paper are designated as members of the Eospirigerina putilla Community. It may have occupied a near-shore, shallow, well-oxygenated water environment, and is assigned to upper Benthic Assemblage 3. Herein, this community of late Hirnantian age is compared with a similar Dalmanella-Plectothyrella Community of the early-middle Hirnantian Hirnantia Fauna from nearby Bijie and Renhuai counties, in northwestern Guizhou (Rong and Li, 1999). The comparison shows that there occur significant differences in many aspects. The Dalmanella-Plectothyrella Community registers a low diversity, which followed after a disaster crisis during the first phase of the LOME (Fig. 5). Numerous declining genera (including their extreme type, Lazarus taxa), such as Hesperorthis, Glyptorthis, Platystrophia, Katastrophomena, Eospirigerina during the crisis, were common in the late Katian and recovered in the late Hirnantian. They became a symbol of brachiopod survival and recovery during the early Silurian (Figs 6 and 7).
The Eospirigerina putilla Community is found to be related to the brachiopods from upper Hirnantian-Rhuddanian rocks occurring in the Meitan, Zunyi and Shiqian counties of northern Guizhou, Southwest China (Rong, 1979; Rong et al., 2011) and Yushan County of northeastern Jiangxi, as well as the Jiangshan and Chun’an counties of western Zhejiang, East China (Rong and Zhan, 2006; Rong et al., 2008, 2013; Huang, 2008) (Fig. 8). These brachiopods previously were named the Cathaysiorthis Fauna (Rong and Zhan, 2006; Huang, 2008; Rong et al., 2013; Huang et al., 2019), and later the Edgewood-Cathay Fauna in the light of a combination of both the Edgewood Fauna typically developed in Mid-Continent of USA (Amsden, 1974) and the Cathaysiorthis Fauna in Zhe-Gan border area, East China in latest Ordovician and earliest Silurian (Rong et al., 2020).
The Edgewood-Cathay Fauna is known from the uppermost Ordovician and Rhuddanian rocks in the North American Mid-Continent (Amsden, 1974), Manitoulin, East Canada (Stott and Jin, 2007), Sweden (Jin and Bergstr?m, 2010), southern and central Norway (Cocks, 1982; Heath and Owen, 1991; Baarli, 2019, 2021, 2022), Kolyma, Russia (Koren et al., 1983), Chingiz of Kazakhstan (Nikitina et al., 2015), Gorny-Altai (Kulkov and Severgina, 1987), and Tasmania, Australia (Laurie, 1991). It is noteworthy that in aspects of lateral profile, ornamentation, muscle fields of both valves, and cardinalia, Hirnantia enorme Laurie, 1991 from lower Westfield Sandstone in Tasmania is attributed to the genus Cathaysiorthis because it was thought to be very similar to Cathaysiorthis yushanensis (Zeng and Hu, 1997) (Rong et al., 2013). It is likely that a close paleobiogeographic relationship existed between South China and Tasmania, judging from the presence of Cathaysiorthis. A recently proposed new term, “Cathay-Tasman Province” shows similar recognition of a faunal province in view of the Ordovician brachiopods and trilobites (Cocks and Torsvik, 2021), suggesting an extension of a faunal relationship shared between the two regions through the LOME.
The strata yielding the Edgewood-Cathay Fauna in South China are generally underlain by Katian rocks with the absence of the typical Hirnantia Fauna and the presence of a disconformity in between, as recorded in Mid-Continent, North America (Amsden, 1974, Bergstr?m et al., 2012a, b). The two phenomena as narrated above may have been consistent with a dramatic global sea-level lowering starting in terminal Katian time (the first episode of the LOME) and a substantial post-glacial sea-level rising in the late Hirnantian (the second episode) (Fig. 9).
Comparison of the Hirnantia Fauna and the Edgewood-Cathay Fauna indicates significant differences in taxonomic composition, biodiversity, predominance, ecological differentiation, biogeography and their symbionts. The major faunal turnover during the late Hirnantian occurred not only in shallow-water, but also in deep-water environments. This suggests an evolutionary process of early Paleozoic brachiopods and their concomitants subjected to cataclysmic shocks (Fig. 10). As supplemented by new data from this paper and based on the data of Rong et al. (2020), the global occurrences of the Edgewood-Cathay Fauna are shown in Fig. 11.
During the time interval between the two episodes of the LOME, the absence of many warm-preference benthic genera (e.g., tabulates, stromatoporoids, and some bryozoans) (Wang et al., 2018; Jeon et al., 2021 Zhang et al., 2018; Ma et al., 2022), the extreme poverty of conodonts (Zhang and Barnes, 2004; Wang, 2013), and nearly disappearances or absence of warm-preference brachiopods (e.g., pentamerides, atrypides, or trimerellides respectively) in South China are in accordance with the data from carbon and oxygen isotopes in North America and elsewhere (Finnegan et al., 2011; Bergstr?m et al., 2019) (Fig. 12). Turnover of the Hirnantian brachiopod faunas has been thought to have been controlled by the coincidence of major perturbations, particularly the intensification of a deteriorating climate (Harper et al., 2014). Major changes of marine surface water temperature may have been one of the negative influences on shallower-water brachiopods and widespread anoxia for deeper-water brachiopods (e.g., Aegiromenella). When the second episode came, brachiopods with a preference for colder water (e.g., Hirnantia, Kinnella, Paromalomena, Draborthis, Dysprosorthis, Trucizetina) were unable to migrate in time into shallow, well-oxygenated, relatively warm-water, into to avoid widespread anoxic environments, whereas many taxa of the Edgewood-Cathay Fauna, mostly eurytopic ones (e.g., Hesperorthis, Glyptorthis, Brevilamnulella, Stegerhynchus, Eospirigerina, Eospirifer), along with those eurytopic taxa from the Hirnantia Fauna (e.g., Dalmanella, Cliftonia, Fardenia, Hindella, Leptaena, Triplesia) did so in order to survive in the then near-shore waters of the late Hirnnatian-Rhuddanian (Fig. 13).
The Akidograptus ascensus Biozone has been ratified as the basal biozone of the Silurian System (Melchin and Williams, 2000; Rong et al., 2006), recognized mainly in graptolitic facies with black shales. Meanwhile, it is possible to roughly delimit the O-S boundary in shelly facies but difficult to define precisely, particularly as bio-and chemo-stratigraphical data are insufficient or contradictory. The investigation in this paper demonstrates that the replacement of the Hirnantia and Edgewood-Cathay faunas in the late Hirnantian, indicating LOME ended slightly before the O-S boundary, as recorded by Stott and Jin (2007).