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

A new halysitid species, Halysites gaojiangensis, is systematically described from the middle Rhuddanian (Llandovery, Silurian) Wulipo Bed at Gaojiang, Meitan County, northern Guizhou Province, Southwest China. It provides new data for further investigation of the tabulate faunal change across the OrdovicianSilurian transition. Halysites across the OrdovicianSilurian transition is reviewed and briefly discussed.


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) 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.Remarks.The present concept of Halysites follows Laub (1979), who considered Schedohalysites as a junior synonym of Halysites.
Figure 2A-R, Tables 1, 2 Derivation of name.After Gaojiang village, Meitan County, northern Guizhou Province where the type locality is situated.
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 1.29-2.16mm long and 1.02-1.85mm 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.In transverse sections, corallites commonly subovate or subelliptical, generally 1.29-2.16mm long and 1.02-1.85mm 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.

Description. (See measurements in
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 microstructure 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.

Fig. 2 .
Fig. 2. A-R, Halysites gaojiangensis sp.nov.from the Wulipo Bed at Gaojiang, Meitan, Southwest China.A-G, NIGP160711, holotype: A, exterior lateral view of corallum; B, E, transverse section and its local enlargement; C, D, F, G, longitudinal section and its three local enlargements; H-K, NIGP160712, paratype: H, I, exterior lateral view of corallum and its local enlargement; J, K, transverse and longitudinal sections.L-P, NIGP160713, paratype: L, exterior lateral view of corallum; M, longitudinal section; N, O, P, transverse section and its two local enlargements.Q, R, NIGP160714, paratype: transverse and longitudinal sections.
. 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.
Corallites arranged in ranks.Ranks extremely long, somewhat parallel, highly curved to gather together, but never to form any junctions or lacunae.