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

Published online by Cambridge University Press:  20 May 2016

Chen Xu ,
Michael J. Melchin ,
H. David Sheets ,
Charles E. Mitchell  and
Fan Jun-Xuan
Chen Xu
Affiliation:
Nanjing Institute of Geology and Paleontology, Academia Sinica, Nanjing, China,
Michael J. Melchin
Affiliation:
Department of Earth Sciences, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada,
H. David Sheets
Affiliation:
Department of Physics, Canisius College, 2001 Main Street, Buffalo, New York 14208
Charles E. Mitchell
Affiliation:
The State University of New York at Buffalo 14260-3050.
Fan Jun-Xuan
Affiliation:
Nanjing Institute of Geology and Paleontology, Academia Sinica, Nanjing, China,
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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.

Type
Research Article
Information
Journal of Paleontology , Volume 79 , Issue 5 , September 2005 , pp. 842 - 861
Copyright
Copyright © The Paleontological Society 

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