Chen Xu (陈旭)

Patterns and processes of latest Ordovician graptolite extinction and recovery based on data from south China

Abstract We have studied the pattern of graptolite species turnover during the latest Ordovician mass extinction based on four continuous Ashgillian to earliest Llandovery sections together with data from more than 30 other published sections. The studied sections represent relatively shallow-water and deeper-water belts in the Yangtze Platform region. Using temporally scaled range data, species diversities and extinction and origination probabilities have been calculated using several analytical methods, including a capture-mark-recapture method. We test the statistical significance of these results and the apparent taxonomic selectivity of extinction and origination via Monte Carlo simulations and contingency analysis. Graptolite species diversity within the Yangtze Platform rose steadily during the late Ashgill, until in the mid-late Paraorthograptus pacificus Chron, when rising extinction risk overtook origination. Diversity dropped to very low levels during the early Hirnantian when extinction probabilities attained significantly elevated rates for a period of 600–900 Ky. The period of high extinction risk was followed immediately by a short period of very high origination probability. A second, short period of high extinction risk occurred at the end of Hirnantian time. The Hirnantian extinction events marked a change from relatively low, steady origination and extinction probabilities to a prolonged period of elevated extinction risk and highly variable origination probability that extended well into the Rhuddanian. Extinction and origination was highly selective during the Hirnantian and favored both the survival and diversification of the Normalograptidae relative to the Dicranograptidae, Diplograptidae, and Orthograptidae. The main phase of extinction in the latest Rawtheyan and early Hirnantian was coincident with continental glaciation in the Southern Hemisphere. The resulting changes in ocean circulation and oxygenation appear to have almost completely eliminated the preferred habitat for most graptolite species. The Yangtze Platform region, however, may have served as a refugium for many taxa that disappeared earlier in other regions as well as a host site for the initiation of graptolite rediversification. Following the end of the glaciation, conditions favorable for graptolite proliferation were restored but graptolite communities remained unstable for much of the late Hirnantian and early Rhuddanian. Accordingly, the Hirnantian mass extinction appears to have fundamentally altered graptolite species dynamics as well as clade dominance patterns. A full understanding of the history of life requires an expanded, hierarchical theory of evolution that gives to mass extinctions (and other levels of selection) an appropriate role in determining clade and diversity histories.

GRAPTOLITES FROM THE QILANG AND YINGAN FORMATIONS (CARADOC, ORDOVICIAN) OF KALPIN, WESTERN TARIM, XINJIANG, CHINA

Abstract Late Ordovician rocks of the Qilang and Yingan formations from the Kalpin area in the Tarim region of western Xinjiang, China (Tarim palaeoplate) contain a moderately diverse graptolite fauna. The fauna from the Qilang Formation contains Corynoides calicularis Nicholson, 1867; Dicranograptus clingani resicis Williams and Bruton, 1983; Lasiograptus costatus Lapworth, 1873; Pseudoclimacograptus scharenbergi (Lapworth, 1876); and Glossograptus sp. among other species. This assemblage most likely corresponds to the upper Climacograptus (Climacograptus) bicornis Zone? to lower C. (Diplacanthograptus) lanceolatus Zone of Australia. The Qilang Formation also yields the new taxon, Amplexograptus maxwelli spinousus new subspecies. The overlying Yingan Formation yields a more diverse assemblage that includes Climacograptus (Diplacanthograptus) spiniferus Ruedemann,1912; C. (D.) lanceolatus VandenBerg, 1990; Orthograptus quadrimucronatus (Hall, 1865); Amplexograptus praetypicalis Riva,1987; Dicellograptus pumilus Lapworth,1876; and D. morrisi Hopkinson, 1871. This assemblage most likely corresponds to the Corynoides americanus Zone through the C. (D.) spiniferus Zone of eastern Laurentia, or to the D. clingani Zone of Scotland and central Newfoundland. The presence of a C. (D.) lanceolatus and C. (D.) spiniferus succession in the Yingan Formation also suggests correlation with the Eastonian 1 and 2 of Australasia. The Yingan Formation faunas represent an offshore Pacific Province assemblage dominated by cosmopolitan epipelagic species. The fauna is most similar, both in terms of species composition and relative abundance patterns, to those of the Appalachian Basin. The Yingan faunas differ from the latter in the absence of Laurentian endemic species (except for Amplexograptus praetypicalis), and in the common occurrence of dicellograptids.