Diversification of Ordovician Chitinozoans from South China, and Its Relationships with Environment Changes

INTRODUCTION

A more systematic research on the Ordovician chitinozoans in China started in the early 1980s. Hou and Wang (1982) reported several Ordovician chitinozoan fossils found in the Huanghuachang Section, Yichang, Hubei Province. Some articles on chitinozoans were subsequently published from the Yangtze platform and its margin and from the Tarim block by Chinese geologists in the last 20 years. Wang and Chen (2004) preliminarily discussed the diversification of Ordovician chitinozoans from China based on those data. However, the above-mentioned studies appear to be relatively limited and sporadic in comparison with comparatively widespread Ordovician strata and rich chitinozoan succession in China. Most, if not all, investigations are not concerned with the causes of chitinozoan diversification events vs. the paleogeographic and paleoenvironmental evolutions. To this end, the authors from 2004 onwards have systematically processed large amounts of the Ordovician chitinozoan samples from different facies across the Yangtze platform and its south margin for delineating Ordovician chronostratigraphic unit boundary and for correlating precisely the chitinozoan succession with the relevant conodont and graptolite zones from the stratotype or typical sections (Chen et al., 2009). This article is a continuation of the research for the purposes of revealing the Ordovician chitinozoan diversity and its relationship with the paleogeographic and paleoenvironmental evolution.

MATERIALS AND METHODS

For past five-odd years have been studied 19 typical Ordovician sections representing different facies of the Ordovician in South China, the results from these sections may more fully reflect the changes of Ordovician chitinozoan distribution and development in different stages, and in different facies of the same stage.

The ranges of chitinozoan genera and species, and the assemblage characteristics are not fully alike from section to section at the same level due to sedimentary facies etc.. In order to obtain a comprehensive, objective and integrated chitinozoan ranges in the Ordovician in the area under study, this article uses the graphic correlation method by Shaw (1964) and Stouge et al. (2005) to constitute a composite standard section. The graphic correlation between the standard reference Huanghuachang Section and the composite standard section indicates that most of the important chitinozoan data fit with a straight line, the LOC (line of correlation) (Fig. 1). Consequently, the ranges of important chitinozoans in the Huanghuachang Section are credible. And they could be served as the precise correlation for different facies. The Huanghuachang Ordovician section, owing to an intensive study in bio- and chronostratigraphy, is thought to be the typical section for calibrating the Ordovician in South China. For this reason, the chitinozoan ranges in the section may be applied to define precisely the time slice (TS) of the generic or specific ranges of chitinozoans in the composite standard section.

Figure  1.  The graphic correlation between the Huanghuachang Section, Yichang, Hubei (standard reference section) and the composite standard section. 1. Lagenochitina daobaowanensis; 2. Eremochitina baculata; 3. Euconochitina symmetrica; 4. Euconochitina fengxiangensis; 5. Lagenochitina chongqingensis; 6. Clavachitina langei; 7. Conochitina exilis; 8. Conochitina pseudocarinata; 9. Lagenochitina combazi; 10. Lagenochitina esthonica; 11. Lagenochitina lata; 12. Sagenachitina dapingensis.

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Following the 19 time slices of Webby et al. (2004), and the chitinozoan diversity parameters of Paris et al. (2004), the authors calculate the chitinozoan balanced total diversity (BTD), the turnover ratio (TR), the origin ratio (Od), the extinction ratio (Ed), the species originate rate per Ma (Odi) and the species extinction rate per Ma (Edi) in the 19 time slices (TS1a–TS6c) in the Ordovician of South China. The results are plotted in Fig. 2.

Figure  2.  Ordovician chitinozoan species diversity of South China.

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RESULTS

The Ordovician chitinozoan BTD curve in Fig. 2 shows that the BTD of chitinozoan species upincreased gradually during the Lower–Middle Ordovician after chitinozoans made their first appearance in the middle Tremadocian, and a gentle fluctuation took subsequently place within the middle Floian (TS. 2b) and the early Dapingian (TS. 3a). The chitinozoan BTD reached the maximum value by the early Darriwilian (TS. 4a). The BTD value began to decrease by degree after the late Middle Ordovician. There existed a mild fluctuation in the middle Katian (TS.6a). It reached, thereafter, the minimum value by the Hirnantian (TS. 6c). A change from rise to drop in chitinozoan species BTD is concomitant with the evolution of Ordovician South China geography—from shallow water shelf to passive continental margin in the Lower–Middle Ordovician, then transforming to foreland basin in the Late Ordovician (Xu et al., 2001). This phenomenon seems to prove that the shallow water shelf is favorable for chitinozoan diversification and the subsequent foreland basin might be contrarily unfavorable for chitinozoan diversification by virtue of the deepening water column and high instability of sea environments.

The BTD maximum value in the period TS.4a poorly matches the peaks of the remainder, but in the middle–late Tremadocian (TS. 1c–1d), the late Floian (TS. 2c), the late Dapingian (TS.3b) and the middle Katian (TS.6a), the positive TR value undoubtedly denotes the tendency of chitinozoan diversification, with the positive TR value in the middle–late Tremadoc being obviously related to the early origins and adaptive radiation of chitinozoan species, for instance. On the one hand, the greatest negative anomaly in the TR value in the Middle Ordovician, may be related to the entire migration of chitinozoans, that was probably imputed to the stratigraphic gaps between the upper TS.4b and lower TS.4c, and on the other hand it is more likely to bear a relation to the meteorite-impact event in the mid Middle Ordovician (TS. 4b), that may deteriorate the forming and ovulating of metazoan eggs, a direct consequence of chitinozoan sudden fall in number (Schimitz et al., 2007).

Patterns of the BTD and TR curves are the same as those of the Od and Ed curves whereas patterns of the Od and Odi curves are analogous of the TR curve, but in opposition to the BTD curve during the Early–Middle Ordovician. In the Late Ordovician, the Od and Odi curves are similar in pattern to the BTD and TR ones. The Ed and Edi curves are contrast to the remained curves. By comparison, it is worthy of noting that the evolutionary radiation duration chitinozoan species appeared mainly during the Early–Middle Ordovician. With the onset of the Late Ordovician in South China, chitinozoans passed into waning-dominated times in diversification.

CONCLUSIONS

(1) The chitinozoan species radiation in South China happened predominantly in the Early–Middle Ordovician and subsequent extinction, or gradual disappearance occurred at the beginning of the Late Ordovician, and lasted for most of the Late Ordovician. In the period of Early–Middle Ordovician chitinozoan radiation, there were two small-scale extinction events, being in the middle Floian and early Dapingian, respectively. In the chitinozoan extinction process of the Late Ordovician, a small-scale radiation event in the middle Katian occurred.

(2) An Early Ordovician chitinozoan radiation took place mainly on the Yangtze platform and its southern passive margin, while stepwise disappearance of Late Ordovician chitinozoans is considered to have relations with the deepening sea waters, as a result of the formation of the Yangtze foreland basin. It may be deduced that the deeper waters and turbulent paleogeographic environments are harmful to the differentiation and development of chitinozoans.

(3) The first massive extinction of chitinozoans occurring in the middle Darriwilian was contemporary with the large-scale meteorite impact event recorded on the Yangtze shallow platform. Therefore, if the theory of eggs or capsules is accepted, then the largescale asteroid impact may be disastrous to the decline of the ovulating amount of metazoans and thus to their ultimate doom in the latest Ordovician.