Abstract
Approximately 10,000 determinable specimens of Pentremites pulchellus Ulrich (Blastoidea, Echinodermata) from a single quarry in the Pella Formation near Oskaloosa, Mahaska County, Iowa, have been examined for abnormal morphologic characters. The total frequency (2.7%) and the frequency among different types of abnormalities (ambulacral 29%; basal 46%; symmetrical 12%) are significantly different in the Pella material than in previously published studies (Wanner, 1932; Macurda, 1980). The greater frequency of abnormalities in the Pella Formation than in previous studies may reflect higher environmental stress because of variable temperatures or salinities or both. The kinds of abnormalities are the same among published studies, which suggest that the underlying biological causes of abnormalities were similar throughout the geologic history of the blastoids. Ambulacral abnormalities are symmetrically distributed, but the division and inversion of basal plates show an asymmetrical distribution as previously recorded by Macurda (1980). The Pella Formation is of Genevievian age (Upper Mississippian) based on the occurrence of Pugnoides ottumwa (White) and convex ambulacra in Pentremites pulchellus. Other unequivocal guide fossils are unknown in the Pella Formation.
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