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Graptoloid evolutionary rates track Ordovician–Silurian global climate change

Published online by Cambridge University Press:  07 June 2013

ROGER A. COOPER ,
PETER M. SADLER ,
AXEL MUNNECKE  and
JAMES S. CRAMPTON
ROGER A. COOPER*
Affiliation:
GNS Science, PO Box 30368 Lower Hutt, New Zealand
PETER M. SADLER
Affiliation:
Department of Earth sciences, University of California, Riverside, CA 92521, USA
AXEL MUNNECKE
Affiliation:
GeoZentrum Nordbayern, Fachgruppe Paläoumwelt, University Erlangen-Nuremberg, Loewenichstraβe 28, Erlangen D-91054, Germany
JAMES S. CRAMPTON
Affiliation:
GNS Science, PO Box 30368 Lower Hutt, New Zealand
*
†Author for correspondence: r.cooper@gns.cri.nz
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Abstract

Graptoloid evolutionary dynamics show a marked contrast from the Ordovician to the Silurian. Subdued extinction and origination rates during the Ordovician give way, during the late Katian, to rates that were highly volatile and of higher mean value through the Silurian, reflecting the significantly shorter lifespan of Silurian species. These patterns are revealed in high-resolution rate curves derived from the CONOP (constrained optimization) scaled and calibrated global composite sequence of 2094 graptoloid species. The end-Ordovician mass depletion was driven primarily by an elevated extinction rate which lasted for c. 1.2 Ma with two main spikes during the Hirnantian. The early Silurian recovery, although initiated by a peak in origination rate, was maintained by a complex interplay of origination and extinction rates, with both rates rising and falling sharply. The global δ13C curve echoes the graptoloid evolutionary rates pattern; the prominent and well-known positive isotope excursions during the Late Ordovician and Silurian lie on or close to times of sharp decline in graptoloid species richness, commonly associated with extinction rate spikes. The graptoloid and isotope data point to a relatively steady marine environment in the Ordovician with mainly background extinction rates, changing during the Katian to a more volatile climatic regime that prevailed through the Silurian, with several sharp extinction episodes triggered by environmental crises. The correlation of graptoloid species diversity with isotopic ratios was positive in the Ordovician and negative in the Silurian, suggesting different causal linkages. Throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate.

Keywords

climatic forcingdiversityextinctionplankton
Type
Original Articles
Information
Geological Magazine , Volume 151 , Issue 2 , March 2014 , pp. 349 - 364
Copyright
Copyright © Cambridge University Press 2013 

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