Bryozoans from the lower Silurian (Telychian) Hanchiatien Formation from southern Chongqing, South China

Abstract. Eight bryozoan species are described from the Hanchiatien Formation (lower Silurian, Telychian) of southern Chongqing, South China. Four species are new: the trepostomes Asperopora sinensis n. sp., Trematopora jiebeiensis n. sp., and Trematopora tenuis n. sp., and the fenestrate Moorephylloporina parvula n. sp. One species, the cystoporate Hennigopora sp. indet., is described in open nomenclature. Moorephylloporina parvula n. sp. is eurytopic, occurring in all types of facies within the bioherms. Erect Moorephylloporina Bassler, 1952, Trematopora Hall, 1852, and Leioclema Ulrich, 1882 formed pioneering communities on weakly cemented substrata, whereas encrusting Fistulipora M‘Coy, 1849, Hennigopora Bassler, 1952, and Asperopora Owen, 1969 occurred on hardgrounds and formed densely compact framestones. Robust branched Trematopora and Leioclema tend to occur out of the reef core (framework) where they could have formed reef-flank thickets in more agitated conditions. The generic composition of the studied fauna correlates with other localities in South China, and they show general paleobiogeographic relations to Siberia and Indiana, USA.


Introduction
Lower Silurian (Llandovery) marine sediments are distributed along a NNE-SSW trend on the South China plate, which was largely occupied by the Yangtze region in the northwestern sector. During the Llandovery, the Yangtze region has been interpreted to represent a tropical or subtropical 'epeiric platform' in western Gondwana (e.g., Rong and Cocks, 2014). In the earliest Silurian (Rhuddanian stage), black shales with flysch are dominant sediments in the Yangtze region. Due to the rapid rise of sea level, late Aeronian limestone facies became widespread in the epeiric sea of the Yangtze region, however, the early Telychian is dominated by terrigenous sediments with minor calcareous interbeds due to the expansion of the Yangtze Uplift (Rong et al., 2003(Rong et al., , 2012Deng et al., 2012). The early Telychian rocks are represented by the Hanchiatien (or Majiaochong) Formation, usually over a kilometer in thickness, spanning from northern Guizhou to southern Sichuan provinces and the southern Chongqing District. Li et al. (2018) provided an in-depth study of the metersized, bryozoan-bearing bioherms, described preliminarily earlier in Chinese by Li et al. (2012). The bryozoan fauna in those bioherms have not yet been given a systematic treatment. Previous studies of the early Telychian bryozoan fauna of the Yangtze region are scarce (Hu, 1982;Xia and Qi, 1989), thus, a detailed description of the Hanchiatien bryozoans can provide valuable insights into the faunal composition in the region.
Although the Hanchiatien Formation is poorly fossiliferous, diagnostic chitinozoans and graptolites were reported at the top of the formation in the Northeast Guizhou: Ancyrochitina brevicollis Geng, 1986 and Streptograptus plumosus Baily, 1871 suggest the formation should be correlated to early Telychian (Chen, 1986;Geng et al., 1997). Some carbonate interbeds (or marls) occur in the Hanchiatien Formation in a few localities (e.g., Li et al., 2012). Macrofossils (e.g., brachiopods, trilobites, echinoderms, bryozoans, and corals) are abundant in some of the carbonate interbeds. To date, no systematic study has been done on the bryozoans from this formation. Here, we provide the first taxonomic description of the bryozoans from the lower Hanchiatien Formation at the border of Guizhou Province.
The Hanchiatien Formation is thought to have been deposited in a subtidal setting, mainly based on the distributional pattern of the brachiopods (Rong et al., 2003). However, different facies of the Hanchiatien Formation display a fluctuation of marine environments (Li et al., 2018). The paleogeographical setting of the Hanchiatien Formation indicates a seaward ramp oriented northward. Depositional environments could have varied from place to place in the Yangtze epeiric sea.
The Jiebei section is located at the boundary between Chongqing and Guizhou provinces (Fig. 1). The stratigraphic details were given by Li et al. (2018) who described the lower Hanchiatien Formation as brownish to greenish shales with minor silstones, which cap the Shihniulan Formation disconformably. In this section, there are two bioherms 85-120 m above the base of the Hanchiatien Formation. At the bottom of the second bioherm, there is a layer (∼1.7 m thick) of cross-bedded, oolitic grainstone with ripples ( Fig. 2). Lacking any photic-related fossils, the bryozoan bioherms demonstrate an unusual heterozoan marine community, mainly produced by internal waves (Li et al., 2018). Our interpretation is that the lower Hanchiatien Formation in Jiebei represents mid-outer ramp settings.

Material and methods
All bryozoans were collected from the bioherms (Figs. 2, 3) in June 2016 by QJL. Thin sections were made in the laboratory of the GeoZentrum Nordbayern, Friedrich-Alexander University Erlangen-Nuremberg, Germany. Bryozoans were studied in thin sections using a binocular microscope under transmitted light. In total, 42 thin sections were prepared from rock samples. The spacing of structures is measured as the distance between their centers. Statistics were summarized using arithmetic means, sample standard deviations, coefficients of variation, and minimum and maximum values. Based on the genus-level occurrence databases of Silurian bryozoans (Hu, 1982(Hu, , 1990Xia and Qi, 1989;Anstey et al., 2003), pair-group cluster analysis (Euclidean similarity index) and detrended correspondence analysis were implemented with the PAST statistical package (version 2.16; Hammer et al., 2001) to assess the paleobiogeographical relationship of the Telychian bryozoan fauna from the South China Block. Both cluster analysis and detrended correspondence analysis make no assumptions about the data structure, so they were suitable for our large-scale paleobiogeographical studies (Shi, 1993). To ensure the robustness of our results, taxa that were potentially poorly known were removed from the published presence-absence dataset (see Appendix).
Repositories and institutional abbreviations.-The newly studied material is deposited at the Naturmuseum Senckenberg (SMF), Frankfurt am Main, Germany. Other cited repositories are: NIGP = Nanjing Institute of Geology and Paleontology, China.
Remarks.-Asperopora sinensis n. sp. differs from Asperopora bellum (Pushkin, 1976) from the middle Silurian of Belarus and Norway in its slightly smaller autozooecial apertures (mean aperture width 0.11 mm vs. 0.14 mm [Pushkin, 1976], respectively) and in its more abundant mesozooecia (mean 8.12 per aperture vs. 7.4 [Pushkin, 1976], respectively). Asperopora sinensis n. sp. differs from Asperopora aspera (Hall, 1852) from the middle Silurian of North America and Europe in its less abundant and larger mesozooecia (mean 8.12 per aperture vs. 10.5 [Ernst et al., 2015], respectively), and in having one or two acanthostyles around each autozooecial aperture instead of 1−5 (Ernst et al., 2015) in the latter species. Etymology.-The species is named after the type locality, the village of Jiebei in China.
Etymology.-The species is named because of its thin branches (Latin tenuis = thin, narrow).
Description.-Reticulate colonies with straight, frequently bifurcating branches, joined by short, wide dissepiments. Autozooecia arranged in two alternating rows on branches, having circular apertures with moderately high peristomes, two or three spaced per length of a fenestrule. Peristomes containing 10−12 nodes. Peristomal nodes 0.015−0.020 mm in diameter. Fenestrules oval. Keels wide, low. Keel nodes high, with moderate diameter and spacing, rounded to oval in their cross sections. Microacanthostyles on the reverse colony surface abundant, regularly spaced in longitudinal rows, 0.010−0.015 mm in diameter. Interior description.-Autozooecia long, rectangular in the mid-tangential section, with well developed vestibule; axial wall straight; aperture positioned at distal end of chamber. Hemisepta absent. Diaphragms present. Internal granular skeleton thin,  Journal of Paleontology 95(2):252-267 262 continuous, with obverse keel, nodes, peristome, and across dissepiments. Outer lamellar skeleton thin to moderately thick. Vesicular skeleton present.
Etymology.-The species name refers to the small size of this species (Latin parvulus = very small).

Discussion
In the Jiebei section, bryozoans occur exclusively in the carbonate interbeds in the lower part of the Hanchiatien Formation (Figs. 2, 3). Characterized by reticular colonies, Moorephylloporina can be found in all types of facies in these meter-scale reefs, indicating its eurytopic nature. However, such distribution could be also explained by taphonomic processes and postmortem transportation of Moorephylloporina fragments from reef areas inhabited by this species within the whole reef. Although the abundance of Moorephylloporina is relatively low in the framestone (Fig. 3.3) of the reefs, these fenestellid bryozoans provide hard substrata for Fistulipora and Asperopora. In contrast, branched Trematopora and Leioclema tend to occur out of the reef core (framework) (Fig. 3.1). Like Champlainopora (Atactotoechus) chazyensis Ross, 1963 in some Ordovician reefs (e.g., Cuffey et al., 2002), Trematopora and Leioclema might have formed reef-flank thickets developed under more agitated conditions. Moorephylloporina, Trematopora, and Leioclema represent pioneering bryozoans that were able to grow on weakly lithified substrata. In contrast, encrusting Fistulipora, Hennigopora, and Asperopora relied more on hardgrounds and occupied a large proportion in the reefs, layered on top of one another and so forming densely compact framestones (Li et al., 2018) that represent a typical crust-mound stage from an evolutionary ecological perspective (Cuffey, 2006).
The Telychian of the Upper Yangtze Platform is characterized by terrigenous sediments (e.g., Rong et al., 2012), and bryozoans are mostly reported from interbedded argillaceous limestones (or marls) (Hu, 1982(Hu, , 1990Xia and Qi, 1989), probably related to internal-wave deposits (Li et al., 2018) in some cases. Systematic studies of Telychian bryozoans are limited Figure 8. Paleobiogeographical affinities of Jiebei bryozoan association and selected Telychian bryozoan faunas (data from Anstey et al., 2003Anstey et al., , updated 2020: (1) the hierarchical relationship based on pair-group cluster analysis (Euclidean similarity index); (2) detrended correspondence analysis. All analyses performed using PAST (Hammer et al., 2001). Table 9. Distribution of bryozoan species in the Telychian localities of South China. compared to other macrofossil groups, e.g., brachiopods, trilobites, and corals. Here, we provide the first detailed comparison of the composition of the bryozoan fauna (Table 9). In the South China block, Asperopora and Moorephylloporina are only reported from the Hanchiatien Formation, whereas Fistulipora and Trematopora are widely distributed across the platform during the early Telychian, recorded in all three studied sections in the literature. Apart from the Jiebei section, Hennigopora and Leioclema have been documented from one other Telychian section, indicating a moderately wide paleogeographic distribution. It is worth noting that bryozoans in some sections are not diverse, or else have been inadequately sampled. Further studies should be carried out to confirm their distributional patterns on the platform.
Bryozoans from the Jiebei section belong to genera with predominantly cosmopolitan distributions during the Telychian. Outside of South China, representatives of the genus Fistulipora are known from North America and Siberia. Species of Hennigopora are known from the USA (New York, Indiana) and Siberia. The genus Leioclema is known from North America, Europe (England, Ukraine), and Tuva (Russia). Records of Asperopora are known from North America (New York, Canada), Sweden (Gotland), and Siberia, whereas Trematopora is known from North America and China. The fenestrate genus Moorephylloporina is largely restricted to the Ordovician, except for two species from the lower Silurian (Llandovery) of Siberia, and the new species described here. It disappeared during the lower Silurian.
The early Silurian was a period of exceptional cosmopolitanism for benthic species with dispersive larvae (e.g., Cocks, 2001). Bryozoans, as in other groups of benthic organisms, showed low provinciality (e.g., Tuckey, 1990;Anstey et al., 2003;McCoy and Anstey, 2010;Buttler et al., 2013). The cluster analysis and detrended correspondence analysis (Fig. 8) reveal distinct clustering of South China with Siberia and Indiana (USA). Bryozoan faunas of Estonia seem to be close to those of New York (USA), whereas Gotland (Sweden) clusters with Anticosti (Canada) and Michigan (USA). These results agree with existing paleogeographical reconstructions for the early Silurian (e.g., Cocks and Torsvik, 2002). However, most existing references for bryozoans in the lower Silurian need critical reassessment, because the descriptions are often far from present-day standards.