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A new species of middle Rhuddanian Halysites (Tabulata) from Meitan, northern Guizhou, Southwest China.

INTRODUCTION

It is known that tabulate corals underwent a significant faunal change across the Ordovician-Silurian transition (Scrutton 1988; Kaljo 1996). However, lack of sufficient material from some key regions (e.g., South China) hinders further investigation of the process and mechanism of this major faunal replacement. Until very recently, Wang et al. (2014) first reviewed the tabulate occurrences in South China through this critical interval and briefly summarized its general macroevolutionary pattern across the Ordovician-Silurian boundary. Nevertheless, systematic descriptions of those tabulates are urgently needed for some further relevant investigations.

The present paper, which is part of such an effort, is to describe a new halysitid species from the middle Rhuddanian Wulipo Bed at Gaojiang, Meitan County, northern Guizhou Province, Southwest China (Fig. 1). The age determination for this bed is based on several lines of evidence (see discussion in Wang et al. 2014). It is worthy of mention that this new species, together with other associated tabulates (e.g., Propora and Paleofavosites), represents a tabulate fauna of the survival interval after the second pulse of the end-Ordovician mass extinction. Hence, for the first time, it provides a key link to bridge the gap between the coral faunas of the latest Ordovician and early Silurian in South China and enables us to further investigate the survival of corals after the major biotic event.

Biometric methods used in this paper largely follow Young & Elias (1995). The specimens illustrated and discussed here are deposited at Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences.

HALYSITES ACROSS THE ORDOVICIAN-SILURIAN TRANSITION

On the basis of new data from Southwest China and other published data from other regions, Halysites across the Ordovician-Silurian transition at species level is summarized and briefly discussed here.

The earliest known definite member of Halysites is H. praecedens Webby & Semeniuk, 1969 from the lower Katian of New South Wales, Australia (Webby & Semeniuk 1969), which is followed by Halysites sp. recorded from the stratigraphically much higher Uralba Beds of New South Wales (Hall 1975). Since then, however, there are very few fossil records of this genus in the latest Ordovician and the earliest Silurian rocks globally until the middle Llandovery. Apart from the middle Rhuddanian H. gaojiangensis sp. nov. described here, other reliable records also include (1) another indeterminate species Halysites sp. from the upper Hirnantian unnamed limestone in Shiqian County, northeastern Guizhou, Southwest China, which shows great similarity with H. gaojiangensis sp. nov. (Wang et al. 2014 and our unpublished data); (2) H. alexandricus reported from the late Hirnantian Edgewood coral assemblage of North America (Young & Elias 1995; note that such an age assignment is based on biostratigraphy of many fossil groups, and see Wang (2014) for more detailed discussion); (3) H. priscus Klaamann, 1966 from the Juuru Stage (lower Llandovery) of Estonia.

It should be noted that the age of all but two Katian Halysites records from Australia is late Hirnantian or much younger. Thus, the reappearance of Halysites in the late Hirnantian is probably related to the improvement of the marine environment after the early-middle Hirnantian glaciation.

SYSTEMATIC PALAEONTOLOGY

Subclass TABULATA Milne-Edwards & Haime, 1850

Order HALYSITIDA Sokolov, 1947

Family HALYSITIDAE Milne-Edwards & Haime, 1849

Genus Halysites Fischer von Waldheim, 1828

Type species. Tubipora catenularia Linnaeus, 1767; Silurian of Gotland (by monotypy).

Remarks. The present concept of Halysites follows Laub (1979), who considered Schedohalysites as a junior synonym of Halysites.

Halysites gaojiangensis sp. nov. Figure 2A-R, Tables 1, 2

Derivation of name. After Gaojiang village, Meitan County, northern Guizhou Province where the type locality is situated.

Holotype. NIGP160711, Wulipo Bed (middle Rhuddanian), Gaojiang, Meitan County, Guizhou.

Paratypes. NIGP160712-NIGP160714 (three specimens), all of which were collected from the same locality and horizon as the holotype.

Diagnosis. Halysites with extremely long ranks without any junctions or lacunae observed. Corallites commonly 1.29-2.16 mm long and 1.02-1.85 mm wide. Tubules do not occur between every pair of adjacent corallites. Septal spines rare to absent. Tabulae complete, widely spaced, averaging about 6 tabulae in 5 mm.

Description. (See measurements in Table 1 and Table 2). Coralla 56.22-98.19 mm wide and 30.78-47.78 mm high, growth form of coralla tabular or high domical. Corallites arranged in ranks. Ranks extremely long, somewhat parallel, highly curved to gather together, but never to form any junctions or lacunae.

In transverse sections, corallites commonly subovate or subelliptical, generally 1.29-2.16 mm long and 1.02-1.85 mm wide. Corallite walls fibrous, consisting of one layer, thick, ranging from 0.09 to 0.25 mm, average at 0.19 mm. Septal spines horn-like, fibrous, embedded in the corallite wall, rare or absent. Septal length variable, with maximum length 0.17 mm. Tubules do not occur between every pair of adjacent corallites, commonly subrectangular, rarely subquadrate; commonly 0.31-0.65 mm long and 0.08-0.29 mm wide.

In longitudinal sections, tabulae dark in colour, commonly thin and complete, flat or rarely convex, widely spaced, averaging about 5.6 in 5 mm. Tabulae structure indistinct. Tubule tabulae commonly seen, complete, flat or slightly convex, moderately spaced, averaging about 8.8-11.6 in 5 mm. The micro structure of tubule tabulae not distinguishable.

Corallites increase interstitial (between adult corallites in a rank) or lateral (at the end of a rank). Interstitial increase occurs through the expansion of an existing tubule, and new tubules are inserted on either side of each new corallite at a higher level. Lateral increase occurs along the bottom of the new rank, and new tubule is inserted above the base.

Discussion. The new species belongs to the Halysites species group whose tubules do not occur between every pair of adjacent corallites, all of which are transitional between Catenipora and Halysites. It remarkably differs from all other species of this group in having extremely long ranks without any junctions and lacunae observed.

doi: 10.3176/earth.2015.19

Acknowledgements. Deng Zhanqiu and Yu Changmin gave us some constructive suggestions on coral systematics. Wang Yi, Huang Bing, Wu Rongchang and Liang Yan helped us a lot in the field. We are grateful to the reviewers Dimitri Kaljo and Graham Young for their useful comments and suggestions. Financial support for this study came from the National Natural Science Foundation of China (projects 41221001, 41290260 and J1210006) and the State Key Laboratory of Palaeobiology and Stratigraphy. This paper is a contribution to IGCP Project 591 'The Early to Middle Palaeozoic Revolution'.

REFERENCES

Fischer von Waldheim, G. 1828. Notice sur les polypiers tubipores fossiles. In Programme pour la seance publique de la Societe Imperiale des Naturalistes, pp. 9-23.

Hall, R. 1975. Late Ordovician coral faunas from northeastern New South Wales. Journal and Proceedings of the Royal Society of New South Wales, 108, 75-93.

Kaljo, D. 1996. Diachronous recovery patterns in early Silurian corals, graptolites and acritarchs. In Biotic Recovery from Mass Extinction Events (Hart, M. B., ed.), Geological Society, London, Special Publications, 102, 127-133.

Klaamann, E. 1966. Inkommunikatnye tabulyaty Estonii [The Incommunicate Tabulata of Estonia] Eesti NSV Teaduste Akadeemia Geoloogia Instituut, 96 pp. [in Russian].

Laub, R. S. 1979. The corals of the Brassfield Formation (mid-Llandovery; Lower Silurian) in the Cincinnati arch region. Bulletins of American Paleontology, 75, 1-457.

Linnaeus, C. 1767. Systema naturae, Edition 12, Part 2. Laurentius Salvii, Holmiae, 533-1327.

Milne-Edwards, H. & Haime, J. 1849. Memoire sur les polypiers appartenant aux groupes naturels des Zoanthaires perfores et des Zoanthaires tabules. Academie des Sciences de Paris, Comptes Rendus, 29, 257-263.

Milne-Edwards, H. & Haime, J. 1850. A monograph of the British Fossil Corals: introduction. Palaeontographical Society, 3, 1-71.

Scrutton, C. T. 1988. Patterns of extinction and survival in Palaeozoic corals. In Extinction and Survival in the Fossil Record (Larwood, G. P., ed.), pp. 65-88. Clarendon, Oxford.

Sokolov, S. 1947. Novye syringoporidy Taymyra [New syringoporids of Tajmyr]. Moskovskoe Obshchestvo Ispytatelej Prirody, Byulletin (Geologiya), 22, 19-28 [in Russian].

Wang, G.-X. 2014. Coral Faunas Across the Ordovician-Silurian Transition of South China: Implications on Paleobiogeography and Macroevolution. University of Chinese Academy of Sciences, PhD thesis, 179 pp. [in Chinese, with English summary].

Wang, G.-X., Zhan, R.-B., Deng, Z.-Q. & Yu, C.-M. 2014. Latest Ordovician and earliest Silurian tabulate corals of South China. GFF, 136, 290-293.

Webby, B. D. & Semeniuk, V. 1969. Ordovician halysitid corals from New South Wales. Lethaia, 2, 345-360.

Young, G. A. & Elias, R. J. 1995. Latest Ordovician to earliest Silurian colonial corals of the east-central United States. Bulletins of American Paleontology, 108, 1-153.

Received 2 July 2014, accepted 17 November 2014

Guangxu Wang and Renbin Zhan

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (CAS), 39 East Beijing Road, Nanjing 210008, China; gxwang@nigpas.ac.cn, rbzhan@nigpas.ac.cn

Table 1. Corallum size and shape of Halysites gaojiangensis
sp. nov. W = corallum width (mm), H = corallum height (mm),
M = height to the colony's widest point (mm)

Specimen       W       H       M     Growth form

NIGP160711   70.08   43.73   32.15   High domical
NIGP160712   56.22   47.78   47.78   High domical
NIGP160713   98.19   30.78    --     Tabular

Table 2. Numerical data on Halysites gaojiangensis sp. nov. TaL =
tabularium length (mm), TaW = tabularium width (mm), TaA =
tabularium area ([mm.sup.2]), TuL = tubule length (mm), TuW =
tubule width (mm), WT = wall thickness (mm), LS = length of septa
(mm), Ta5 = number of tabulae in 5 mm

                    TaL         TaW         TaA         TuL

Minimum value      1.29        1.02         1.2        0.31
Maximum value      2.16        1.85        2.26        0.65
Range of means   1.65-1.74   1.37-1.33   1.72-1.74   0.46-0.53
Mean value         1.68        1.31        1.73        0.50
Measurements        54          54          54          28
Specimens            4           4           4           4

                    TuW         WT        LS      Ta5

Minimum value      0.08        0.09       0        5
Maximum value      0.29        0.25      0.17      7
Range of means   0.16-0.22   0.16-0.17          5.2-6.8
Mean value         0.19        0.166              5.8
Measurements        28          56                19
Specimens            4           4        4        4
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Author:Wang, Guangxu; Zhan, Renbin
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Geographic Code:9CHIN
Date:Mar 1, 2015
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