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.
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