Coral-stromatoporoid faunas from the shores of a late Silurian island, Inner Mongolia, North China RONG JIAYU, MARKES E. JOHNSON, DENG ZHANQIU, DONG DEYUAN, XUE YAOSONG, B. GUDVEIG BAARLI & WANG GUANGXU
RONG JIAYU, JOHNSON, M.E., DENG ZHANQIU, DONG DEYUAN, XUE YAOSONG, BAARLI, B.G. & WANG GUANGXU, 2013:04:26. Coral-stromatoporoid faunas from the shores of a late Silurian island, Inner Mongolia, North China. Memoirs of the Association of Australasian Palaeontologists 44, 95-105. ISSN 0810-8889. Previous research on a small continental island called Bater Island in south-central Inner Mongolia focused on palaeogeographic relationships in the context of the North China Block, and on the prevailing pattern of atmospheric and oceanic circulation interpreted from Ludlow (upper Silurian) strata surrounding an Ordovician diorite inlier. Preliminary information initially available regarding the palaeontology of a rare rocky-shore environment preserved around this ancient island is revised and expanded based RQVWXG\RIWKHGRPLQDQWIRVVLOIDXQDRIFRUDOVDQGVWURPDWRSRURLGVSRQJHV7KHIDXQDKHUHGHVFULEHGDQGLOOXVWUDWHGIRUWKH¿UVW time, includes seven genera of tabulate corals (Mesoculipora, Thamnopora, Striatopora, Cladopora, Taxopora, Planocoenites and Okopites) and two genera of stromatoporoids (Clathrodictyon and Actinostromella). Compared to other rocky-shore faunas from North America, the diversity of the tabulate corals from the more sheltered, leeward side of Bater Island is high (AFR19), whereas no tabulates have been discovered on the windward, rough-water side of the island where only stromatoporoids were found. Bater Island is the only known locality worldwide to feature stromatoporoids in a rocky-shore setting. Rong Jiayu (
[email protected]), Deng Zhanqiu, Dong Deyuan, Xue Yaosong and Wang Guangxu, State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China;; M.E. Johnson (
[email protected]) and B.G. Baarli, Department of Geosciences, Williams College, Williamstown, MA 01267, USA. Received 12 June 2012. Keywords: Late Silurian, palaeo-island, corals, stromatoporoids, systematic palaeontology, Inner Mongolia.
BATER ISLAND is the name given by Rong et al. (2001) to a small palaeo-island composed of Ordovician diorite surrounded by Ludlow (upper Silurian) strata of the Xibiehe Formation in the Bateaobao (= Bater Obo) region, Darhan Mumingan Joint Banner, south-central Inner Mongolia (Fig. 1). The elongated and bilobate outline of the unconformity between Silurian marine strata and Ordovician igneous rocks UHSUHVHQWVDQDQFLHQWLVODQGURFN\VKRUH7KLVZDVWKH¿UVW palaeo-island to be described in the geological literature of China, and is notable among only a few examples worldwide of former islands that preserve sedimentary facies showing a clear contrast between windward and leeward rocky-shore environments (Johnson et al. 2004). Subsequent studies focusing on solving the precise age of Bater Island based on conodonts related the palaeo-island’s subsidence to global changes in Silurian sea level, and re-evaluated the location and orientation of the Sino-Korean Plate in terms of its palaeogeography (Johnson et al. 2001, 2004). The stratigraphy and faunas of the mostly Ludlow age Xibiehe Formation were previously described by Li et al. (1985), with contributions on stromatoporoids by Dong (l985) and tabulate corals by Deng & Yang (l985). Recently, Wang (2005) discovered conodonts including Ozarkodina uncrispa from the lower Xibiehe Formation, which was correlated with the O. crispa Biozone of the upper Ludlow. Only a preliminary account of the rocky-shore and near-shore faunas particular to Bater Island was made by Rong et al. (2001), based on comparisons with published materials on
the Xibiehe Formation. The purpose of the present report is to provide a revised and more complete list of the Bater Island fauna as determined from collections brought to the Nanjing Institute of Geology and Palaeontology for further study in 1999. This contribution focuses on new taxa previously unrecognised from the study area, with an emphasis on tabulate corals and stromatoporoids. Greater attention is also paid to the sedimentology of the basal strata immediately above the unconformity surface on the Ordovician diorite. GEOLOGICAL SETTING Igneous basement rocks in the Bater Obo region, about 220 km northwest of the capital city Hohhot, include granodiorite, basalt and diorite from a continental shelf setting, probably on the Sino-Korean Plate. Silurian strata that rest unconformably on these rocks are well exposed in the Baoerhantu and Bater Obo areas (Fig. 1). Bater Island is located within 3 km of the prominent topographic landmark known as Bater Obo, and approximately 50 km south of the Chinese-Mongolian border. The palaeo-island is oriented roughly SW-NE, is bilobate-elliptical in outline, measures 610 m by 200 m, and has a core of Ordovician diorite. Upper Silurian strata belonging to the Xibiehe Formation (Ludlow- 3ĜtGROt DUHZUDSSHGDURXQGWKHGLRULWHFRUHDQGGLSDZD\ from it in all directions in a more or less radial pattern. A 2-3 m thick basal conglomerate bed consisting of eroded diorite cobbles and small boulders occurs exclusively on WKHQRUWKZHVWHUQÀDQNRIWKHSDODHRLVODQG,QFRQWUDVWWKH
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Figure 1. A-CPDSVVKRZLQJWKHORFDWLRQDQGVLPSOL¿HGJHRORJLFDOVHWWLQJRIWKHODWH/XGORZ 6LOXULDQ %DWHU,VODQGLQWKH%DWHU2ER area, Darhan Mumingan Joint Banner, northeast of Bayinobo Sumu, southern Inner Mongolia, North China. D, diagrammatic map of Bater Island showing the three localities from which tabulate corals and stromatoporoids were collected from late Silurian strata surrounding the island. Revised from Rong et al. ¿J
VRXWKHDVWHUQÀDQNRIWKHSDODHRLVODQGH[KLELWVPDLQO\VLOW\ limestone and patches of quartz-rich silt in direct contact with the underlying diorite. The silty limestone reaches an exposed thickness of about 50 m. The depositional HQYLURQPHQWRQWKHQRUWKZHVWHUQÀDQNRIWKHLVODQG ORFDOLW\ AFR23) is interpreted as a rocky shore under a windward, URXJKZDWHU VHWWLQJ ZKLOH WKH FRHYDO VRXWKHDVWHUQ ÀDQN (AFR19 and AFR21a) represents a leeward, calm-water setting (Rong et al. 2001;; Johnson et al. 2001, 2004). About 30 m of topographic relief is shown by the diorite now exposed above the rim of the exhumed palaeo-island. Submergence of the island during the late Silurian required a minimum rise in sea level of approximately 30 m.
on a petrographic analysis by one of us (Xue Yaosong). In hand-specimens, rocks are dark grey with a rusted-iron colour when weathered, and greyish green on unweathered surfaces. A few impressions of brachiopods with costae (i.e. external moulds of the pentamerid Conchidium? sp.) are evident, in addition to some brachiopod fragments with calcareous shells. Columnar tabulate corals (Cladopora sp.) are common, both as cavities (3.5-4 mm in diameter) on the ZHDWKHUHGVXUIDFHDVZHOODVVROLG¿OO PPLQGLDPHWHU within the rocks. Their orientation suggests that branching corals were not buried in situ, and they were subject to more breakage than other, compact corals. The rocks show no HYLGHQFHRIVWUDWL¿FDWLRQDQGQRJUDGDWLRQLQVHGLPHQWVL]H and are very hard as a result of cementation. SEDIMENTARY PETROLOGY In thin section the rocks are seen to be composed of Additional information regarding the composition of the grains of various sizes in matrix. The detritus comprises basal few centimetres of sedimentary rocks on the protected, particles of about 1-4 mm in diameter, commonly chloritised leeward side of the palaeo-island (locality AFR19) is based to varying degrees, but with boundaries often unclear due
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Table 1. Distribution of tabulate corals and stromatoporoids at localities AFR19, AFR21a and AFR23 on the shores of the late Silurian Bater Island.
to crystallisation of the matrix. The mineral composition of the rocks includes augite, quartz and plagioclase, with some SODJLRFODVHVXEMHFWWRZHDNFKORULWLVDWLRQDQGFDOFL¿FDWLRQ,Q bulk, the detritus occupies about 20% of the total, dominated mainly by silt to coarse sand (up to 2.5 x 1.5 mm in size, angular to subangular in shape, but occasionally rounded and 0.12 mm in diameter). Quartz sand constitutes 10% of the total. Plagioclase grains generally are silty. Biotite is a UDUHFRPSRQHQWÀDN\LQIRUPDERXWPPORQJDQG with no particular orientation. No muscovite is present. The matrix comprises about 40% of the total, and consists FKLHÀ\ RI PLFURFU\VWDOOLQH TXDUW] DQG FU\SWRFU\VWDOOLQH FKORULWH ZLWK PLQRU ¿QH TXDUW] SODJLRFODVH FKORULWH DQG augite. Lithologically the rocks are greywackes in the sense that sorting is very poor. The constituent grains indicate that the sediment was derived from fractured and weathered medium-basic igneous rocks, and was deposited rapidly adjacent to the source. DIVERSITY AND DISTRIBUTION OF FAUNA Fossils were collected on the margins of the palaeo-island at three locations, AFR19 and AFR21a on the southeastern side and AFR23 on the northwestern side (Fig. 1). Each RIWKHVHORFDOLVHGIDXQDVLVEULHÀ\VXPPDULVHGEHORZDQG all of the taxa present are listed in Table 1;; taxa marked with an asterisk are not described or illustrated due to the paucity or poor preservation of the material. Among the tabulates, thamnoporids (including Cladopora, Thamnopora, Striatopora, and Taxopora) are the most common, whereas coenitids (Planocoenites) and heliolitids (Okopites) are rare. It is noteworthy that Cladopora, Thamnopora and Planocoenites have commonly been recorded from Devonian rocks but also are known from the Upper Silurian of North China. Locality AFR19, southeastern side of Bater Island Tabulate corals recorded from this locality by Rong et al.
(2001, p. 239-240) were Mesoculipora cf. divida (Deng & Yang, 1985), Thamnopora sp. and Cladopora sp. After closer examination of the collected fauna, we now recognise seven species in six genera, namely M. cf. divida, Thamnopora cf. neimongolensis Chi, 1976, Striatopora cf. microsepala Chi, 1976, Striatopora sp., Cladopora obesa Deng, sp. nov., Taxopora sp. and Planocoenites sp. There are three species of stromatoporoids from AFR19, including Clathrodictyon gotlandense Mori, 1968 (recorded by Rong et al. 2001, p. 239 as C.? microstriatellum Riabinin, 1953), Actinostromella slitensis Mori, 1968 and Hexastylostroma sp. The last is not described or illustrated here because of the poor preservation of the material. Locality AFR21a, southern side of Bater Island Fossils are much less abundant and diverse at this locality than at AFR19. Rong et al. (2001, p. 239) reported only two taxa of tabulate corals (heliolitoid gen. et sp. indet. and the tabulatoid Cladopora sp.), but after re-examination we KDYHQRZLGHQWL¿HGOkopites subtiles Deng & Yang, 1985 and Thamnopora cf. neimongolensis Chi, 1976. A single indeterminate stromatoporoid species (Plexodictyon sp.) was also listed by Rong et al. (2001) from AFR21a;; no further information on this species is available as a result of our study. Locality AFR23, northwestern side of Bater Island It is noteworthy that no tabulate corals have been discovered at this locality. Two stromatoporoid taxa (Clathrodictyon microstriatellum Riabinin, 1953 and Syringostromella sp.) were recorded from AFR23 by Rong et at. (2001, p. 240). We have restudied the original material, and on the basis of WKHEHVWSUHVHUYHGVSHFLPHQVKDYHLGHQWL¿HGClathrodictyon gotlandense Mori, 1968 and Actinostromella slitensis Mori, 1968.
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Figure 2. A-B, Mesoculipora cf. divida (Deng & Yang, 1985), transverse and longitudinal sections, NIGP 133622a, b. C-H, Thamnopora cf. neimongolensis Chi, 1976;; C, longitudinal section, NIGP 133611;; D-E, longitudinal and transverse sections, NIGP 133610a, b;; F-G, longitudinal and transverse sections, NIGP 133613a, b;; H, longitudinal section, NIGP 133614. All from Locality AFR19. Scale bars represent 1 mm.
SYSTEMATIC PALAEONTOLOGY The following accounts of the tabulate corals and stromatoporoid sponges are by Deng Zhanqiu and Dong Deyuan, respectively. All sectioned specimens described are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGP). Subclass TABULATA Milne-Edwards & Haime, 1850 Order FAVOSITIDA Wedekind, 1937 Family OCULIPORIDAE Lin & Jia in Lin et al., 1988 Mesoculipora Jin in Jin et al., 1982 1985 Submesofavosites Deng & Yang, p. 117. Type species. By original designation;; Mesoculipora sichuanensis Jin in Jin et al SO¿JIURPWKH Shihniulan Formation (mid-upper Aeronian, Llandovery) of Guanyinqiao, Qijiang County, Chongqing City, Southwest China.
Other species. Mesoculipora divida (Deng & Yang, 1985). Remarks. Mesoculipora is characterised mainly by the presence of two size groups of corallites in the same corallum. The genus resembles Oculipora Sokolov, 1952 in general structure but the latter differs in lacking angular pores, and in having thicker walls and a special morphology of the smaller corallites in transverse section. Mesoculipora was originally assigned to the Multisoleniidae Fritz, 1950 by Jin et al. (1982) because of the presence of angular and mural pores, but was later transferred to the Oculiporidae by Lin et al. (1988) based mainly on the presence of two size groups of corallites with very small individuals and the existence of both angular and mural pores. The present author (DZQ) agrees with this assignment. When erecting Submesofavosites, with type species S. dividus IURP WKH ;LELHKH )RUPDWLRQ /XGORZ3ĜtGROt of Huaaobao, Inner Mongolia, Deng & Yang (1985) were unaware of the paper by Jin et al. (1982) in which Mesoculipora was established. Lin et al. (1988, p. 139) subsequently listed Submesofavosites as a subjective
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from axial region of branches, bending gradually to meet the surface of branch at or almost at right angles. In transverse section, corallites polygonal in outline, rarely rounded- polygonal, 0.25-0.3 mm in diameter in axial region and 0.25-0.5 mm in peripheral region. Walls 0.08-0.12 mm thick axially and 0.1-0.35 mm peripherally. Septal spines very rare Age and distribution. $HURQLDQ /ODQGRYHU\ WR 3ĜtGROt and seen only in peripheral region. Mural pores rare, about 0.1 mm in diameter. Tabulae complete, horizontal or weakly Silurian;; South and North China. concave, spaced 0.3-0.6 mm apart. Mesoculipora cf. divida (Deng & Yang, 1985) (Fig. 2A-B) Comparsion. Though it is abundant in Devonian strata, the cf. 1985 Submesofavosites dividus;; Deng & Yang, p. 117, only species of Thamnopora known in the Silurian of China are 7 NKDO¿QL (Dubatolov, 1959) and T. neimongolensis SO¿J Chi, 1976 from the Xibiehe Formation at Sujidongwu and Figured material. A single specimen with transverse and Bateaobao respectively, Inner Mongolia. The latter species is differentiated from the former in having much smaller longitudinal sections, NIGP 133622a-b. corallites (0.4-1.0 mm in diameter in T. neimongolensis Description. Corallum massive. Corallites of two sizes;; compared to 1.3-1.8 mm in diameter in 7NKDO¿QL) and more larger ones rounded-octagonal to sub-circular in outline slender branches (6-11 mm in diameter in T. neimongolensis and 1.1-1.2 mm in diameter;; smaller ones tetra-pentagonal compared to 12-17 mm in diameter in 7 NKDO¿QL). The in outline and 0.3-0.8 mm in diameter, distributed between present specimens are similar to T. neimongolensis in larger ones;; adjacent corallites obviously different in size. many features but have a smaller corallum (4-5 mm) and Walls thin, 0.1-0.15 mm thick;; microstructure of walls corallites (0.2-0.3 mm in diameter axially and 0.25-0.5 unknown. Mural pores developed, very small, about 0.1 mm peripherally). In addition, T. neimongolensis has rare mm in diameter. Septal spines rare, very small, not visible septal spines but these have only been seen in the peripheral in the walls, short and not penetrating the lumen, straight, regions of our specimens;; and there are 1-2 rows of mural not curved and not upwardly inclined. Tabulae complete, pores in T. neimongolensis but they are very rare and small horizontal to slightly wavy, usually spaced 0.25-0.4 mm in our specimens. apart, a few spaced 0.5 mm apart. Localities. AFR19 (common) and AFR21a (extremely rare). Comparsion. The specimen differs from those of Mesoculipora divida from the same formation and region in having smaller Striatopora Hall, 1851 corallites of both size groups (larger corallites 1.1-1.2 mm in M. cf. divida, 1.2-1.8 mm in M. divida;; smaller corallites Type species. By monotypy;; 6WULDWRSRUDÀH[XRVD Hall, 1851, 0.3-0.8 mm in M. cf. divida, 0.7-1.1 mm in M. divida), and p. 400, from the Rochester Shale (Wenlock), Lockport, New York, USA. in having fewer angular and mural pores. synonym of Mesoculipora without any discussion of the reason. The present author (DZQ) agrees with this conclusion as Mesoculipora and Submesofavosites are generically identical in basic characters of the corallites and general structure.
Locality. AFR19
Age and distribution. Silurian to Permian;; cosmopolitan.
Family PACHYPORIDAE Gerth, 1921
Striatopora macrosepala Chi, 1976 (Fig. 3A-C)
Thamnopora Steininger, 1831
1976 Striatopora macrosepala&KLSSO¿JDE
Type species. By original designation;; Thamnopora Figured material. Two specimens with longitudinal sections, NIGP 133618, 133619;; one specimen with a tangential madreporacea Steininger, 1831, p. 10. section, NIGP 133612. Age and distribution. Thamnopora LV NQRZQ FKLHÀ\ LQ Devonian rocks worldwide (Hill 1981, p. 584) but occurs Description. Fragmentary corallum composed of branches in the basal part of the Xibiehe Formation (upper Ludlow, approximately 4.5 mm in diameter. Corallites generally upper Silurian) of Inner Mongolia (also see Chi 1976;; Deng rounded tetra-pentagonal in outline, diverging from axial region, opening obliquely to the surface of branches in & Yang 1985). tangential section, 0.8-1.0 mm in diameter in axial region, and increasing to 1.5-2.0 mm diameter peripherally. Walls Thamnopora cf. neimongolensis Chi, 1976 (Fig. 2C-H) slightly thickened, 0.2-0.3 mm in width. Mural pores rare, cf. 1976 Thamnopora neimongolensis;; Chi, p. 112, pl. 47, 0.1mm in diameter, one row on wall of corallites. Both septal spines and tabulae absent. ¿JDE Figured material. Two complete specimens with longitudinal and transverse sections, NIGP 133610a-b, 133613a-b;; and two incomplete specimens with longitudinal sections, NIGP 133611, 133614. The specimens are not well preserved but some details of internal structures are visible.
Comparison. Striatopora macrosepala ZDV ¿UVW UHFRUGHG from the Xibiehe Formation at Xipiangou and Huaaobao, Damao Banner. Our specimens are almost identical to the types, differing only in that the calices of the corallites are larger.
Description. Corallum ramose, composed of cylindrical Locality. AFR19. branches of similar diameter (4-5 mm). Corallites diverging
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Figure 3. A-C, Striatopora macrosepala Chi, 1976;; A, tangential section, NIGP 133612;; B, longitudinal section, NIGP 133618;; C, longitudinal section, NIGP 133619. D, Striatopora sp., oblique section, NIGP 133620. E-H, Cladopora obesa Deng, sp. nov.;; E-F, H, holotype, tangential, longitudinal and transverse sections, NIGP 133616a, b, c;; G, paratype, transverse and longitudinal sections, NIGP 133617a, b. All from Locality AFR19. Scale bars represent 1 mm.
Striatopora sp. (Fig. 3D) Figured material. A single incomplete specimen with an oblique section, NIGP 133620.
Comparison. Although only a single, poorly preserved specimen is available, it is assigned to Striatopora on the basis of the corallites meeting the surface at an acute angle on small branches, and having thickened walls peripherally (Oliver 1966). The specimen is most similar to S. macrosepala but differs in having a distinctly smaller corallum and corallites, and thinner walls (S. macrosepala with diameters of corallum and corallites 4.5 mm and 0.8-2.0 mm respectively, and with wall 0.2-0.3 mm thick).
Description. Cylindrical branch of corallum is 3-3.5 mm in diameter. Corallites rounded polygonal in outline in oblique section, 0.25-0.3 mm in diameter axially, branching to the surface at acute angles. Walls 0.05-0.1 mm thick. Mural pores rare, about 0.1 mm in diameter. Septal spines and tabulae not seen. Locality. AFR19.
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Figure 4. A, Taxopora sp., transverse section, NIGP 133625;; locality AFR19. B-C, Planocoenites sp.;; B, oblique section, NIGP 133623;; C, longitudinal section, NIGP 133624;; locality AFR19. D-E, Okopites subtiles Deng & Yang, 1985, transverse and longitudinal sections, NIGP 133621a, b;; locality AFR21a. Scale bars represent 1 mm.
Cladopora Hall, 1851
thinner wall (about 0.02 mm), and the smaller and much rarer mural pores (0.03 mm). Also similar to C. obesa is C. Type species. By subsequent designation of Miller (1889, shiqianensis Zhou in Yang et al. S SO¿J p. 178);; Cladopora seriata Hall, 1851, from the Lockport 5), the earliest known species of the genus, from the upper Dolomite (upper Silurian), Lockport, New York, USA. Xiangshuyuan Formation (mid Aeronian), Leijiatun, Shiqian, northeastern Guizhou. The similarities include the size of Age and distribution. Silurian and Devonian;; cosmopolitan. the corallum (2.5-3.5 mm in C. shiqianensis), the size of the mural pores (0.05-0.08 mm in C. shiqianensis) and their Cladopora obesa Deng, sp. nov. (Fig. 3E-H) arrangement in a single row, and the absence of septal spines. Cladopora obesa is distinguished by the less variable size of Etymology. Latin obesa, fat or stout, referring to the walls the corallum, and the absence of tabulae (they are relatively of the corallites. rare in C. shiqianensis). Type material. Holotype, tangential, longitudinal and Locality. AFR19. transverse sections, NIGP 133616a-c;; paratype, transverse and longitudinal sections, NIGP 133617a-b). Taxopora Sokolov in Dzyubo & Mironova, 1961 Diagnosis. Corallum with cylindrical branches about 3.5 mm in diameter. Corallites usually opening obliquely to surface of branches, 0.3-0.35 mm in diameter axially and 0.4-0.5 mm peripherally in transverse section. Mural pores small. Septal spines and tabulae not developed.
Type species. Sokolov (in Dzyubo & Mironova 1961, p. 68) designated Taxopora xenia as type species but did not describe, diagnose or illustrate it. Two other species, T. altaica from the Chagyr Suite (Wenlock) of the Altay and T. salairica from the Baskuskan Suite (Silurian) of Salair, ZHUH GLDJQRVHG DQG ¿JXUHG E\ 0LURQRYD LQ ']\XER Description. Corallum ramose. Corallites diverging from Mironova 1961, p. 68). axial region, polygonal or subcircular in outline in cross section, with walls 0.12-0.15 mm thick axially and increasing Remarks. Hill (1981, p. 576) regarded Taxopora as a junior to 0.3-0.35 mm thick peripherally. Mural pores 0.05-0.1 mm subjective synonym of Cladopora, in which the corallites in diameter, arranged in a single row on walls. are initially parallel to the axis of the branches but gradually diverge to intersect the surface obliquely in lozenge-shaped Comparison. Cladopora obesa is characterised particularly calices. However, in Taxopora the corallites are not initially by the absence of septal spines and tabulae. parallel to the axis but diverge from it, and hence the present Eighteen species of Cladopora have been recorded from author (DZQ) regards Taxopora as a valid genus. the Silurian and Devonian of China, three of them from the Silurian (Lin et al. 1988). Of these Silurian species, C. cf. Age and distribution. Silurian, Wenlock to Ludlow;; Altay, seriataRI'HQJ
