Article: Early Mississippian Hyolitha from northern Iowa
The exceptionally fine preservation and large number of specimens from a single late Palaeozoic locality makes the hyoliths in the Humboldt Oolite (Osagean, lower Mississippian) unique. All specimens are assigned to Gerkella humboldti n. gen., n. sp., family Hyolithidae, order Hyolithida. There is considerable variation in certain morphological features, such as transverse shape, nature of ornament and apical curvature; however, these differences are judged to be gradational. This indicates that certain features may be variable within one species and shows that establishment of hyolith species should be based upon a variety of features. Other North American late Palaeozoic hyoliths include Hyolithes carbonaria Walcott, H. milleri Sinclair, H. parvulus Girty and H. waverliensis Hyde. Their types lack important morphological features, which makes their generic identifications uncertain. Their names should not be used for any further material until better preserved topotypes become available for study.