Electron Microscope Investigation of Orthograptus quadrimucronatus from the Maquoketa Formation (Late Ordovician) in Iowa

Both the transmitting and scanning electron microscopes were used to study specimens of an orthograptid of the O. quadrimucronatus group from the Elgin Member of the Maquoketa Formation in Iowa. One technique employed to prepare the specimens for study with the transmitting electron microscope is relatively simple in that the specimens were studied directly after being freed from the limestone matrix and cleaned with hydrofluoric acid, without going through the several preparation steps commonly taken by electron microscopists. Specimens were also sectioned and examined with the transmitting electron microscope following the commonly-used preparation procedures. This study indicates that the microstructure of the cortical layer of the metasicula and thecal walls is built up of two types of sublayers that alternate. One type of sublayer is a meshwork and the other is two sets of fibers oriented normal to each other to give the aspect of a fabric. The fusellar layer of the periderm of the metasicula and thecal walls is formed of meshwork that appears similar to the meshwork sublayer of the cortical layer.