Darriwilian Saucrorthis Fauna : implications for the Great Ordovician Biodiversification Event ( GOBE )

The Saucrorthis Fauna is a brachiopod-dominated shelly fauna developed in relatively deeper-water benthic regimes of a few peri-Gondwana terranes (e.g. northern Iran) and palaeoplates (South China and Sibumasu) during the Darriwilian (late Middle Ordovician). It has its longest geological range, widest palaeogeographic distribution, most abundant and diversified brachiopod taxonomic composition and most complex palaeoecological differentiation in South China, where it concentrated in the centre of the Upper Yangtze Platform. The generally low similarity coefficients (< 0.3) between any two representatives of the Saucrorthis Fauna firmly support a statement about the Great Ordovician Biodiversification Event (GOBE) that strong endemism or provincialism made a great contribution to the second pulse of the Ordovician brachiopod radiation in South China, which might also be true for the international trend.


INTRODUCTION
The Darriwilian (late Middle Ordovician) Saucrorthis Fauna is a brachiopod-dominated normal marine shelly fauna characterized by the small-to medium-shelled, costate eponymous genus and the predominance of some common orthides (e.g.Orthambonites and Parisorthis) and strophomenides (e.g.Leptellina, Calyptolepta and Bellimurina) (Fig. 1).The genus Saucrorthis of the subfamily Productorthinae was named 40 years ago from the calcareous mudstone of the Shihtzupu Formation (Darriwilian) at Shizipu of Zunyi, northern Guizhou Province, southwest China (Xu et al. 1974), but the Saucrorthis Fauna as a whole was first recognized by Zhan et al. (2007) as a unique benthic shelly fauna representing the second diversity acme of the Ordovician brachiopod radiation in South China.Saucrorthis is now known to be widely distributed on the Upper Yangtze Platform of the South China palaeoplate, and also at some localities in the Shan States (Myanmar, Sibumasu palaeoplate) and the Alborz Mountains (northern Iran, peri-Gondwana terrane), but the taxonomic composition of the Saucrorthis Fauna at different localities varies substantially.Another Darriwilian brachiopod fauna, the Aporthophyla Fauna, has a much wider distribution than the Saucrorthis Fauna but shows a similar or slightly lower latitudinal affinity.They rarely occur together in the rocks with just one exception at Wudang of Guiyang, central Guizhou, southwest China.This paper is to document the occurrences of the Saucrorthis Fauna and to investigate its implications for the Great Ordovician Biodiversification Event (GOBE) by comparing its taxonomic composition and palaeogeographic distribution with those of the Aporthophyla Fauna.The specimens studied are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, with a prefix NIGP for each number.

STRATIGRAPHICAL AND GEOGRAPHICAL DISTRIBUTION OF THE SAUCRORTHIS FAUNA
Since its establishment in the 1970s (Xu et al. 1974, p. 150), Saucrorthis has been found at almost all localities of the Shihtzupu Formation on the Upper Yangtze Platform covering a large area in Guizhou, Yunnan, Sichuan and Hubei provinces and Chongqing Municipality (Zhan & Jin 2005b;Liang & Zhan 2012) (Fig. 2A).Besides, it is also known from the Dashaba Formation (Darriwilian) of Shuanghe, Changning County, southern Sichuan Province (Zhan & Jin 2005a), the upper Houping Formation (middle-upper Darriwilian) of Houping, Chengkou County, northern Chongqing Municipality (Zhu et al. 1977;Chen et al. 1995), and the Kuniutan Formation (Darriwilian) of Wudang, Guiyang, central Guizhou (Rong et al. 2005), all of which were on the Upper Yangtze Platform.Although the Darriwilian rocks are also well developed and widespread on the Lower Yangtze Platform and even the Kuniutan Formation could extend to parts of the Lower Yangtze Platform (Chen et al. 1995;Liang & Zhan 2012), there are no documentations on the occurrences of Saucrorthis and the Saucrorthis Fauna yet.Such a phenomenon may have been caused by the local environment where the water depth was too great for Saucrorthis to survive in the Lower Yangtze Platform.On the regional scale, the Saucrorthis Fauna is most abundant in the central part of the Upper Yangtze Platform in the area of the Shihtzupu (e.g.CWX in Appendix) and the Dashaba (CCN in Appendix) formations and decreases sharply in diversity in nearshore and more offshore localities (Fig. 2A).
After careful taxonomic revision and redescription in the light of modern brachiopod studies conducted by Cocks & Zhan (1998), the brachiopod fauna from the Naungkangyi Group and its equivalents of southern Shan States, Myanmar, is proven to be very close to that  that of the Shihtzupu Formation (Cocks & Zhan 1998).A new association of the Saucrorthis Fauna, the Saucrorthis-Leptestiina Association, has been reported in the Sibumasu palaeoplate from the Shihtien Formation (Darriwilian) in the Baoshan-Shidian area, western Yunnan, southwest China (Zhan et al. 2014a).It also has a pretty high brachiopod diversity including 27 genera such as Antigonambonites, Atelelasmoidea, Baoshania, Calyptolepta, Dedzetina, Glyptomena, Glyptorthis, Horderleyella, Leptastichidia, Leptellina, Leptestiina, lingulid  Also marginal to the Gondwana supercontinents, the Alborz Mountains of northern Iran were reported to yield some brachiopods apparently attributable to the Saucrorthis Fauna from the lower Lashkarak Formation (middle Darriwilian) (detailed faunal list of IDM in Appendix) (Ghobadi Pour et al. 2011) (Fig. 2B).Based on both outdoor and indoor preliminary investigations, only five brachiopod genera are known to occur, dominated by two local endemics Bastamorthis and Semnanostrophia (Ghobadi Pour et al. 2011).Besides, some other groups of fossils also occur in the rocks, such as, e.g., echinoderms, trilobites and conodonts.
Up to now, South China is the region where the Saucrorthis Fauna has its longest geological range (extending for the entire Darriwilian), the widest geographic and palaeogeographic distribution (several hundreds of kilometres long and wide), the most abundant constituents (the amount of fossils) and the most complicated palaeoecological differentiation from nearshore shallow-water to offshore deeper-water benthic settings.There is another interesting phenomenon about the Saucrorthis Fauna.The trilobite Neseuretus, a cool and relatively deeper-water environment indicator (Ghobadi Pour et al. 2011), is found to occur together with the Saucrorthis Fauna in Myanmar and western Yunnan, southwest China (Sibumasu palaeoplate) and northern Iran (peri-Gondwana terrane), but never in South China where it was also widespread, flourished and highly diversified on the species level before the appearance of Saucrorthis during the late Floian and Dapingian (Zhou & Dean 1989;Zhou & Zhou 2008;Zhou Zhiyi pers. comm. 2014).

IMPLICATIONS FOR THE GOBE
In order to analyse the relationships between different representatives of the Saucrorthis Fauna, we selected six communities or associations plus one outgroup association from southern Tibet for comparison: four from different lithological units on the Upper Yangtze Platform of the South China palaeoplate and one from Myanmar of the Sibumasu palaeoplate and northern Iran of the peri-Gondwana terrane, respectively (see Appendix for detailed faunal lists).
Among 53 brachiopod genera involved in the numerical analyses, 37 (70%) are known from only one locality, 15 (28%) from two or three localities (amongst which most from localities of the South China palaeoplate), and only the eponymous genus is common to all localities.The cluster analysis (CA, Fig. 3) shows that the similarity coefficient between any two representatives of the Saucrorthis Fauna is always less than 0.3 although several similarity measures are tried under the algorithm 'paired group'.The association from the Lower Formation of the Chiatsun Group of southern Tibet (CTJ in Appendix) has a very low similarity coefficient (less than 0.1) with any other associations or communities with Saucrorthis, indicating the substantial difference between the Aporthophyla Fauna and the Saucrorthis Fauna.The late Middle Ordovician has been shown to be an interval when strong endemism and provincialism occurred in marine shelly faunas (Williams 1973;Jaanusson 1979), and when the GOBE got its first diversity acme on a global scale for all marine organisms (Webby 2000), the brachiopods in particular (Harper et al. 2004).The case study conducted in South China further indicates that the appearance and flourishing of typical regional shelly faunas made an essential contribution to the GOBE in each particular palaeoplate or terrane, e.g. the Saucrorthis Fauna in the Sibumasu palaeoplate in South China and the peri-Gondwana terrane in northern Iran.
Compared with another widespread Darriwilian shelly fauna, the Aporthophyla Fauna (CTJ in Appendix) (Zhan et al. 2014a), the Saucrorthis Fauna is rather limited in its palaeogeographic distribution confined to peri-Gondwana.Both of them share very few common constituents of brachiopods (Zhan et al. 2014b).There is only one locality where Saucrorthis occurs together with Aporthophyla and some other shelly fossils constituting a unique brachiopod-dominated association, the Yangtzeella-Orthambonites Association from the Kuniutan Formation at Wudang of Guiyang, central Guizhou (South China palaeoplate) living in a shallowwater benthic environment corresponding to BA3 (Rong et al. 2005).The Aporthophyla Fauna was flourishing in nearshore shallow-water benthic regimes (corresponding to lower BA2 to BA3) of many palaeoplates or terranes during late Middle Ordovician time (Darriwilian), all of which were with a medium or low latitude affinity (Neuman & Harper 1992;Rong et al. 2005;Zhan et al. 2014a).

CONCLUSIONS
The Darriwilian (late Middle Ordovician) Saucrorthis Fauna, a brachiopod-dominated shelly fauna in normal marine (relatively deeper) benthic regimes with muddy or calcareous muddy substrates, was a typical regional fauna confined mainly to the South China palaeoplate, together with some sporadic occurrences in the Sibumasu palaeoplate and a peri-Gondwana terrane (i.e.northern Iran), all of which are of peri-Gondwana affinities.The fauna represents the second pulse of the brachiopod GOBE in South China, coinciding with the first diversity acme of the GOBE globally not only for the brachiopods but also for all fossil groups.The generally low similarity coefficients between different Saucrorthis Faunas of different localities and regions, and between all Darriwilian shelly faunas indicate that the diversity acmes of the GOBE were always manifested by (a) the flourishing of local or regional faunas and (b) strong endemism or provincialism in the epeiric seas during the Ordovician.

Fig. 2 .
Fig. 2. Geographic and palaeogeographic distributions of the Saucrorthis Fauna.A, Darriwilian lithological units developed on the Upper and Lower Yangtze Platform of the South China palaeoplate, and the diversity change of the Saucrorthis Fauna from the Shihtzupu and Dashaba formations to the Kuniutan, Qiaojia, Fengdonggang and Siliangssu formations (modified from Liang & Zhan 2012).Numbers 1-19 in the map represent the number of brachiopod genera of the Saucrorthis Fauna at each particular locality on the Upper Yangtze Platform.Hollow circles indicate the cities and solid circles the section sites we investigated.B, regional distribution of the Saucrorthis Fauna (indicated by red solid triangles) showing its marginal Gondwanan affinity and possible migration routes (base map after Torsvik & Cocks 2013).

Fig. 3 .
Fig. 3. Cluster analysis (CA) of six representatives of the Saucrorthis Fauna in the South China palaeoplate (4), Sibumasu palaeoplate (1) and peri-Gondwana terrane (northern Iran) (1) with the contemporaneous association from the Lower Formation of the Chiatsun Group of southern Tibet as an outgroup, showing the strong endemism and provincialism during late Middle Ordovician time.