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Graptolites from near the Ordovician-Silurian boundary in Illinois and Iowa

Published online by Cambridge University Press:  14 July 2015

David K. Loydell
Affiliation:
1School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK, ;
Andrew Mallett
Affiliation:
1School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK, ;
Donald G. Mikulic
Affiliation:
2Illinois State Geological Survey, Champaign, Illinois 61820,
Joanne Kluessendorf
Affiliation:
3Department of Geology, University of Illinois, Urbana, Illinois 61801,
Rodney D. Norby
Affiliation:
2Illinois State Geological Survey, Champaign, Illinois 61820,

Abstract

The Wilhelmi Formation of Illinois and the Mosalem Formation of Iowa contain monospecific assemblages of the stratigraphically important species Normalograptus parvulus Lapworth. This species is confined elsewhere to the uppermost Ordovician Normalograptus persculptus Biozone and the lower part of the lowermost Silurian Parakidograptus acuminatus Biozone. The presence of this graptolite raises the possibility that the lowermost parts of the Wilhelmi and Mosalem formations are of late Ordovician age rather than of early Silurian age as previously thought.

Type
Research Article
Copyright
Copyright © The Paleontological Society

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