Shanchi Peng

Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian System

Important steps have been made toward global subdivision of the Cambrian System. The consensus of opinion is that the system should be subdivided into four series representing subequal spans of time. The lower two series will correspond approximately to the traditional Lower Cambrian, the third series will correspond approximately to the traditional Middle Cambrian, and the fourth series corresponds approximately to the traditional Upper Cambrian. The fourth series, called the Furongian Series, has been ratified by the International Union of Geological Sciences (IUGS). Selection of horizons representing evolutionary events that can be correlated on a global scale, and which can be used as stage or series boundaries, is much more difficult for the lower two series intervals of the Cambrian than it is for the upper two series intervals of the Cambrian. The most practical concept for subdividing the system into stages involves the establishment of two stages each for the first and second series, and establishment of three stages each for the third and fourth series. The lowermost stage of the Furongian, called the Paibian Stage, has been ratified by the IUGS, and the International Subcommission on Cambrian Stratigraphy (ISCS) is nearing decisions on the levels that will used to mark the bases of the upper two stages of the Furongian. Concerning stage boundaries within the proposed third series of the Cambrian, the ISCS is close to decisions on two levels that will be used to mark boundaries.

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).

Two Cambrian agnostoid trilobites,Agnostotes orientalis (Kobayashi, 1935) andLotagnostus americanus (Billings, 1860): Key species for defining global stages of the Cambrian System

The systematics, stratigraphic distribution and geographic distribution of two widely distributed agnostoid species from the Furongian Series of the Cambrian are reviewed. The speciesAgnostotes orientalis (Kobayashi, 1935) andLotagnostus americanus (Billings, 1860) have been described from around the world under a variety of species and subspecies names, and have been assigned to a variety of genera. Nomenclatural problems have hindered to some extent a full under standing of the value of these two forms for intercontinental correlation. Both species have relatively narrow stratigraphic ranges and intercontinental distributions in open-shelf lithofacies. These conditions make these species excellent candidates for defining the bases of stage boundaries within the Furongian Series.

Proposal and prospects for the global Lower-Middle Cambrian boundary

Abstract In recent years paleontologists have been working on the global stratotype section and standard point (GSSP) for the Lower-Middle Cambrian boundary in North America, China, Morocco, Spain and Russia. Continually, they suggest the potential first appearance datum (FAD) of the Middle Cambrian, such as Oryctocephalus indicus, Ovatoryctocara granulata, Acadoparadoxides mureroensis, Hupeolenus and Arthricocephalus chauveaui as being relevant to the potential stratotype section for the Lower-Middle Cambrian boundary. There are only a few trilobites distributed widely enough in the Early to Middle Cambrian connection, so virtually no trilobite(s) can act as the FAD for the global Middle Cambrian. The easily identified Oryctocephalus indicus is widely distributed in three realms of the global Cambrian. Its first appearance position is linked to the extinction of old species coevolution of new ones. It appears to be a comparatively good FAD of Middle Cambrian. The continuous and unbroken Wuliu section (Ba...

CONODONTS FROM THE WA’ERGANG SECTION, CHINA, A POTENTIAL GSSP FOR THE UPPERMOST STAGE OF THE CAMBRIAN

Furongian (upper Cambrian) conodonts from the Shenjiawan Formation, Wa’ergang section, Taoyuan County, Hunan Province, South China are described. The Wa’ergang section has been proposed as a potential GSSP for the base of the uppermost stage of the Cambrian System, at the first appearance of the agnostoid trilobite Lotagnostus americanus . The Shenjiawan Formation consists of limestone with intercalations of marlstone and shale. The conodont yielding is low and the preservation is moderate. The conodont fauna includes proto-, para- and euconodonts. Conodont taxa characteristic of North China, South China and Korea occur together with cosmopolitan taxa. The biostratigraphic distribution of conodonts is directly correlated with the agnostoid trilobite biozones, with the base of the Proconodontus posterocostatus Zone very close to the base of the Lotagnostus americanus trilobite Zone.

Revision of the oryctocephalid trilobite genera Arthricocephalus Bergeron and Oryctocarella Tomashpolskaya and Karpinski (Cambrian) from South China and Siberia

Abstract. Restudy of the limestone slab containing the type suite of Arthricocephalus chauveaui Bergeron, 1899 (Trilobita), the type species of Arthricocephalus Bergeron, 1899, shows that specimens of three species, each representing a separate genus, are present on the slab and were likely included in the original concept of the species. Besides A. chauveaui, the slab contains specimens of Oryctocarella duyunensis (Qian, 1961) and Duyunaspis duyunensis Zhang and Qian in Zhou et al., 1977. Bergeron (1899) illustrated one exoskeleton from the type suite of A. chauveaui only and apparently based much of his written description on that specimen. This specimen was clearly intended to be the type specimen of A. chauveaui. Subsequently, Lane et al. (1988) designated another specimen, which was not originally illustrated, as the lectotype, and this specimen is referable to Oryctocarella duyunensis. As revised here, Bergerons illustrated specimen should be regarded as the lectotype of A. chauveaui. Such designation stabilizes the concepts of both Arthricocephalus and A. chauveaui. Arthricocephalites Qian in Lu et al., 1974 and Haliplanktos Blaker and Peel, 1997 are regarded as junior synonyms of Arthricocephalus. Oryctocarella Tomashpolskaya and Karpinski, 1961, which was previously regarded as a junior synonym of Arthricocephalus, is revived as a valid genus. Oryctocarella includes a number of species previously assigned to Arthricocephalus. The type species of both Arthricocephalus and Oryctocarella, together with related species, are described on the basis of new material.

Carbon-Isotope Excursions Recorded in the Cambrian System, South China: Implications for Mass Extinctions and Sea-Level Fluctuations

Cambrian carbonates with abundant fossils of agnostoid trilobites deposited on the southern slope (Jiangnan slope belt) of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangcun Section of western Hunan, South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbon-isotope excursions to depositional environment changes, mass extinctions and eustatic events, we collected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Furongian Series. Three distinctive negative excursions CNEwc-1, 2, 3 were separately tested in the Lower Terreneuvian Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic productivity, mass extinctions and eustatic events.