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

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

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 SETTINGIgneous 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-­

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

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

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 (jyrong@nigpas.ac.cn), 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 (markes.e.johnson@williams.edu) 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.

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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

AFR23) is interpreted as a rocky shore under a windward,

(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.

SEDIMENTARY PETROLOGYAdditional information regarding the composition of the basal few centimetres of sedimentary rocks on the protected, leeward side of the palaeo-­island (locality AFR19) is based

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

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

and are very hard as a result of cementation.In thin section the rocks are seen to be composed of

grains of various sizes in matrix. The detritus comprises particles of about 1-­4 mm in diameter, commonly chloritised to varying degrees, but with boundaries often unclear due

Figure 1. A-­Carea, 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.

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to crystallisation of the matrix. The mineral composition of the rocks includes augite, quartz and plagioclase, with some

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

with no particular orientation. No muscovite is present. The matrix comprises about 40% of the total, and consists

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 FAUNAFossils 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

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 IslandTabulate 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 IslandFossils 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

Okopites 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 IslandIt 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

Clathrodictyon gotlandense Mori, 1968 and Actinostromella slitensis Mori, 1968.

Table 1. Distribution of tabulate corals and stromatoporoids at localities AFR19, AFR21a and AFR23 on the shores of the late Silurian Bater Island.

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SYSTEMATIC PALAEONTOLOGYThe 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, 1850Order FAVOSITIDA Wedekind, 1937Family 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 alShihniulan 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. dividusof 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

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.

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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.

Age and distribution. Silurian;; South and North China.

Mesoculipora cf. divida (Deng & Yang, 1985) (Fig. 2A-­B)

cf. 1985 Submesofavosites dividus;; Deng & Yang, p. 117,

Figured material. A single specimen with transverse and longitudinal sections, NIGP 133622a-­b.

Description. Corallum massive. Corallites of two sizes;; larger ones rounded-­octagonal to sub-­circular in outline and 1.1-­1.2 mm in diameter;; smaller ones tetra-­pentagonal in outline and 0.3-­0.8 mm in diameter, distributed between larger ones;; adjacent corallites obviously different in size. Walls thin, 0.1-­0.15 mm thick;; microstructure of walls unknown. Mural pores developed, very small, about 0.1 mm in diameter. Septal spines rare, very small, not visible in the walls, short and not penetrating the lumen, straight, not curved and not upwardly inclined. Tabulae complete, horizontal to slightly wavy, usually spaced 0.25-­0.4 mm apart, a few spaced 0.5 mm apart.

Comparsion. The specimen differs from those of Mesoculipora divida from the same formation and region in having smaller 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 0.3-­0.8 mm in M. cf. divida, 0.7-­1.1 mm in M. divida), and in having fewer angular and mural pores.

Locality. AFR19

Family PACHYPORIDAE Gerth, 1921

Thamnopora Steininger, 1831

Type species. By original designation;; Thamnopora madreporacea Steininger, 1831, p. 10.

Age and distribution. ThamnoporaDevonian rocks worldwide (Hill 1981, p. 584) but occurs in the basal part of the Xibiehe Formation (upper Ludlow, upper Silurian) of Inner Mongolia (also see Chi 1976;; Deng & Yang 1985).

Thamnopora cf. neimongolensis Chi, 1976 (Fig. 2C-­H)

cf. 1976 Thamnopora neimongolensis;; Chi, p. 112, pl. 47,

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.

Description. Corallum ramose, composed of cylindrical branches of similar diameter (4-­5 mm). Corallites diverging

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 and seen only in peripheral region. Mural pores rare, about 0.1 mm in diameter. Tabulae complete, horizontal or weakly concave, spaced 0.3-­0.6 mm apart.

Comparsion. Though it is abundant in Devonian strata, the only species of Thamnopora known in the Silurian of China are (Dubatolov, 1959) and T. neimongolensis Chi, 1976 from the Xibiehe Formation at Sujidongwu and Bateaobao respectively, Inner Mongolia. The latter species is differentiated from the former in having much smaller corallites (0.4-­1.0 mm in diameter in T. neimongolensis compared to 1.3-­1.8 mm in diameter in ) and more slender branches (6-­11 mm in diameter in T. neimongolensis compared to 12-­17 mm in diameter in ). The present specimens are similar to T. neimongolensis in many features but have a smaller corallum (4-­5 mm) and corallites (0.2-­0.3 mm in diameter axially and 0.25-­0.5 mm peripherally). In addition, T. neimongolensis has rare septal spines but these have only been seen in the peripheral regions of our specimens;; and there are 1-­2 rows of mural pores in T. neimongolensis but they are very rare and small in our specimens.

Localities. AFR19 (common) and AFR21a (extremely rare).

Striatopora Hall, 1851

Type species. By monotypy;; Hall, 1851, p. 400, from the Rochester Shale (Wenlock), Lockport, New York, USA.

Age and distribution. Silurian to Permian;; cosmopolitan.

Striatopora macrosepala Chi, 1976 (Fig. 3A-­C)

1976 Striatopora macrosepala

Figured material. Two specimens with longitudinal sections, NIGP 133618, 133619;; one specimen with a tangential section, NIGP 133612.

Description. Fragmentary corallum composed of branches approximately 4.5 mm in diameter. Corallites generally rounded tetra-­pentagonal in outline, diverging from axial region, opening obliquely to the surface of branches in tangential section, 0.8-­1.0 mm in diameter in axial region, and increasing to 1.5-­2.0 mm diameter peripherally. Walls slightly thickened, 0.2-­0.3 mm in width. Mural pores rare, 0.1mm in diameter, one row on wall of corallites. Both septal spines and tabulae absent.

Comparison. Striatopora macrosepalafrom 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.

Locality. AFR19.

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Striatopora sp. (Fig. 3D)

Figured material. A single incomplete specimen with an oblique section, NIGP 133620.

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.

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).

Locality. AFR19.

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.

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Cladopora Hall, 1851

Type species. By subsequent designation of Miller (1889, p. 178);; Cladopora seriata Hall, 1851, from the Lockport Dolomite (upper Silurian), Lockport, New York, USA.

Age and distribution. Silurian and Devonian;; cosmopolitan.

Cladopora obesa Deng, sp. nov. (Fig. 3E-­H)

Etymology. Latin obesa, fat or stout, referring to the walls of the corallites.

Type material. Holotype, tangential, longitudinal and transverse sections, NIGP 133616a-­c;; paratype, transverse and longitudinal sections, NIGP 133617a-­b).

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.

Description. Corallum ramose. Corallites diverging from axial region, polygonal or subcircular in outline in cross section, with walls 0.12-­0.15 mm thick axially and increasing to 0.3-­0.35 mm thick peripherally. Mural pores 0.05-­0.1 mm in diameter, arranged in a single row on walls.

Comparison. Cladopora obesa is characterised particularly by the absence of septal spines and tabulae.Eighteen species of Cladopora have been recorded from

the Silurian and Devonian of China, three of them from the Silurian (Lin et al. 1988). Of these Silurian species, C. cf. seriatathe Xibiehe Formation at Gashaomiao, about 20 km west of Bater Obo, differs from D. obesa in the smaller size of the corallum (2-­2.5 mm) and corallites (0.2-­0.3 mm), the

thinner wall (about 0.02 mm), and the smaller and much rarer mural pores (0.03 mm). Also similar to C. obesa is C. shiqianensis Zhou in Yang et al.5), the earliest known species of the genus, from the upper Xiangshuyuan Formation (mid Aeronian), Leijiatun, Shiqian, northeastern Guizhou. The similarities include the size of 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 arrangement in a single row, and the absence of septal spines. Cladopora obesa is distinguished by the less variable size of the corallum, and the absence of tabulae (they are relatively rare in C. shiqianensis).

Locality. AFR19.

Taxopora Sokolov in Dzyubo & Mironova, 1961

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,

Mironova 1961, p. 68).

Remarks. Hill (1981, p. 576) regarded Taxopora as a junior subjective synonym of Cladopora, in which the corallites are initially parallel to the axis of the branches but gradually diverge to intersect the surface obliquely in lozenge-­shaped calices. However, in Taxopora the corallites are not initially parallel to the axis but diverge from it, and hence the present author (DZQ) regards Taxopora as a valid genus.

Age and distribution. Silurian, Wenlock to Ludlow;; Altay, Salair, Inner Mongolia, North China (Sokolov in Dzyubo & Mironova 1961;; Dubatolov & Ivanovskii 1977;; Chi 1976;; Deng & Yang 1985;; Lin et al. 1988).

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.

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Taxopora sp. (Fig. 4A)

Figured material. A single specimen with a transverse section, NIGP 133625.

Description. Corallum ramose, composed of small cylindrical branches about 2 mm in diameter. A few corallites diverging from axial region of branches, bending gradually to meet branch surface at an acute angle, rounded-­polygonal in outline, 0.1-­0.15 mm in diameter axially and about 0.2 mm peripherally. Walls thin, 0.05 mm in width axially and increasing to 0.1 mm peripherally. Mural pores small, 0.05 mm in diameter. Septal spines and tabulae not seen.

Comparison. This species is characterised by the small corallum and rare corallites. Two species of Taxopora have previously been recorded from the Xibiehe Formation in Inner Mongolia -­ T. sailairica and T. gashaomiaoensis, from Bateaobao and Gashaomiao respectively (Deng & Yang

species differs from T. salairica Mironova in Dzyubo &

corallum (2.5 mm in T. sailairica) and corallites (0.2 mm axially and 0.3 mm peripherally in T. sailairica), and thicker walls (0.1 mm axially and 0.2 mm peripherally in T. sailairica).

Locality. AFR19.

Family COENITIDAE Sardeson, 1896

Planocoenites Sokolov, 1952

Type species. By original designation;; Coenites orientalis

Devonian (Eifelian) of Gorny Altay.

Age and distribution. Late Llandovery, Silurian to Givetian, Middle Devonian (Hill, 1981);; Estonia, Urals, Altay, Siberian Platform, Inner Mongolia, New South Wales and Ontario.

Planocoenites sp. (Fig. 4B-­C)

Figured material. Two specimens, NIGP 133623 in oblique section and NIGP 133624 in longitudinal section.

Description. Corallum very small, thinly laminated, only 0.5-­1.5 mm thick. Corallites suboval in transverse section, 0.2-­0.3 mm in diameter in lower region and 0.3-­0.4 mm in upper, initially thin-­walled and recumbent on substrate, then turning with sharply thickened walls to open in narrow, cresentic calices on surface of corallum. Walls 0.05 mm thick in lower region and 0.15-­0.3 mm in upper region. Mural pores small and rare, seen in lower region. Septal spines and tabulae lacking.

Comparison. This indeterminate species is rather similar to Plancoenites qaganhebuensis Deng & Yang (l985, p.

Formation of the Bateaobao area, Inner Mongolia, in the appearance of the corallum in longitudinal sections, the similar wall thickness, the presence of rare mural pores, and the absence of septal spines and tabulae. The former species can be differentiated mainly by the much smaller corallum (0.5-­1.5 mm compared to 2-­3 mm in P. qaganhebuensis).

Locality. AFR19.

Order HELIOLITIDA Frech, 1897Family HELIOLITIDAE Lindström, 1876

Okopites Bondarenko, 1978

Type species. By original designation;; Okopites okopiensis Bondarenko, 1978, p. 26, from the Rashkovsk beds (upper

Age and distribution. Late Silurian;; Podolia, New South Wales and North China. Early Devonian (Lochkovian);; Kazakhstan.

Okopites subtiles Deng & Yang, l985 (Fig. 4D-­E)

1985 Okopites subtiles Deng &

Figured material. A single specimen with longitudinal and transverse sections, NIGP 133621a-­b.

Description. Corallum massive, composed of cylindrical corallites and small, prismatic or rounded-­polygonal coenenchymal tubules. Corallites round in outline, 1.4-­1.5 mm in diameter in transverse section. Coenenchymal tubules 0.2-­0.3 mm in diameter, with walls 0.04-­0.06 mm thick;; surrounding the corallites in an aureole of usually 17-­19 tubules, with one to four rows of tubules between corallites. Wall smooth or occasionally weakly plicate. Septal spines

complete, horizontal and weakly wavy, spaced from 0.5 mm to 0.6 mm apart;; diaphragms complete, horizontal, spaced 0.3-­0.4 mm apart.

Comparison. The specimen has slightly larger corallites and more aureolar tubules than the type material of O. subtiles but is otherwise similar.

Locality. AFR21a.

Phylum PORIFERA Grant, 1836Class STROMATOPOROIDEA Nicholson & Murie, 1878Order CLATHRODICTYIDA Bogoyavlenskaya, 1969Family CLATHRODICTYIDAE Kühn, 1939

Clathrodictyon Nicholson & Murie, 1878

Type species. Clathrodictyon vesiculosum Nicholson & Murie, 1878.

Clathrodictyon gotlandense Mori, 1968 (Fig. 5A-­B)

1968 Clathrodictyon gotlandense

Figured material. One specimen with longitudinal and tangential sections, NIGP 133626a-­b.

Description. Coenosteum generally small;; laminae continuous, slightly crumpled in places, 0.04-­0.09 mm thick, numbering 5-­7 in 1 mm. Interlaminar spaces usually wider than thickness of laminae. Pillars vertical and straight, rarely oblique, generally circular in tangential section, numbering 6-­8 in 1 mm;; Y-­shaped pillars very rare. Width of interlaminar spaces varying maximally from 0.10 to 0.26

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mm, more generally 0.18-­0.21 mm. Microstructure compact

Comparison. C. gotlandense was originally described from the upper Silurian of Gotland, and is herein recorded from

characterised by very dense skeletal structures similar to the type specimens from Sweden but is distinguished from them by the absence of astrorhizae, a difference here considered

Localities. AFR19 and AFR23.

Actinostromella Boehnke, 1915

Type species. Actinostromella tubulata Boehnke, 1915.

Actinostromella slitensis Mori, 1968 (Fig. 5C-­D)

1968 Actinostromella slitensis

1985 Actinostromella slitensis

Figured material. One specimen with transverse and longitudinal sections, NIGP 133627a-­b).

Description. Coenosteum dome-­like, conical, massive or irregular, composed of very thin laminae and vertical pillars.

Laminae of radical processes with a regular or irregular arrangement. Number of laminae 20-­22 or up to 25 in 1 mm. Pillars long and continuous, passing through some laminae. Number of pillars up to 23 in 1mm. Pseudozooidal tubes 3-­5 in 1 mm, well developed, running parallel to pillars, commonly circular in tangential section, 0.04-­0.05 mm in diameter. Dissepiments present in pseudozooidal tubes. Astrorhizae not seen.

Comparison. Actinostromella slitensis was established on the basis of specimens from the upper Silurian of Sweden, and is abundant in the Xibiehe Formation of Damao Joint Banner, Inner Mongolia. The present specimens differ from the types only in having somewhat less distinct pseudozooidal tubes.

Localities. AFR19 and AFR23.

CONCLUDING REMARKSPetrographic and palaeontological data add important new details regarding the late Ludlow (late Silurian) biota from the Bater palaeo-­island, especially with respect to the palaeoenvironments of the more sheltered, leeward, southeastern side (localities AFR19 and AFR21a) and the windward, rough-­water, northwestern side (locality AFR23). As originally recorded by Rong et al. (2001), no basal conglomerate occurs along the present southeastern margin of the palaeo-­island. Indeed, the lithoclastic greywacke deposited in the basal few centimetres above the unconformity

Figure 5. A-­B, Clathrodictyon gotlandense Mori, 1969, longitudinal and tangential sections, NIGP 133626a, b. C-­D, Actinostromella slitensis Mori, 1969, longitudinal and tangential sections, NIGP 133627a, b. All from Locality AFR23. Sale bars represent 1 mm.

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on Ordovician diorite points to rapid deposition of poorly sorted material weathered directly from the igneous source. This is entirely consistent with a quiet-­water setting with little wave agitation. The fauna surrounding the island includes tabulate corals (tabulatoids and heliotoids), stromatoporoid sponges, and a few brachiopods and crinoids, all being benthic in habit. Most tabulates and stromatoporoids may have lived in situ, although transportation of some tabulates cannot be excluded. The brachiopods and crinoids were probably transported from an offshore area nearby. There are no rugose corals, bryozoans or evidence of boring organisms within the basal part of the upper Silurian rocks at these sites, although many such elements were common in shallow marine environments almost everywhere during the late Silurian.Tabulate corals occur commonly at sites AFR19 and

AFR21a, but not at AFR23 where only stromatoporoids were found (Fig. 1). The biota at AFR19 is the most diverse at both genus and species levels. In this setting, the large dome-­shaped stromatoporoid Actinostromella slitensis directly colonised the unconformity surface and continued throughout many metres of the succeeding ‘quiet-­water’ silty limestones. The stromatoporoids from AFR19 and

shore environment. The tabulate corals from the sheltered, rocky-­shore setting of AFR19 are much more diverse than

et al. (2001), amounting to a total of eight taxa including one new pachyporid species and one heliolitid species. The diversity of tabulate corals at this locality is in sharp contrast to their absence at AFR23, considered to be a windward, rough-­water setting. At site AFR21a only a single species of stromatoporoids and two of tabulate corals were discovered.According to Scrutton (1997, l998), the association of

tabulate corals with a rocky-­shore environment is uncommon in the fossil record. Only two other examples are known;; one from the Upper Ordovician of Manitoba, Canada, on Hudson Bay (Johnson & Baarli 1987) and the other from the Lower Pennsylvanian of Arkansas, USA (Webb 1993). However, these North American localities involve only a very few taxa. Thus, the sheltered rocky shore of Silurian Bater Island in Inner Mongolia provides evidence of many more tabulate corals than previously recognised, living in a comparable environment.

ACKNOWLEDGEMENTSWe are grateful to Li Wenguo (Inner Mongolian Geological Survey, Hohhot) and Su Wenbo (China University of

in l999. Huang Bing (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences) helped with technical aspects of the project. Travel to Nanjing by Johnson was supported by the Whiting Foundation

the NSFC (41296260, 41221001), BGSC (1B0110123, 1212011120115) and NGS (6326-­98). This is a contribution to IGCP Project 591.

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