Two New Orthosphaeridium (Acritarcha) from the Middle and Upper Ordovician

Description of the new species Orthosphaeridium vibrissiferum from the Middle Ordovician Bromide Formation of Oklahoma, and O. chondrododora from the Upper Ordovician Eden Formation of Indiana extends both the geologic and geographic range of this acritarch genus. Surface features of the vesicle wall of these representatives of the early Paleozoic microplankton are shown by both light and
scanning electron microscopy. Species of this genus known to date show evolutionary trends towards decreased vesicle ornamentation, increased number of major processes, and decrease in vesicle inflation. All species have the flattened to pillow-shaped vesicle, elongate hollow processes that are plugged proximally, and the characteristic median splitting form of excystment.