SPECHTS FERRY (MIDDLE ORDOVICIAN) 



BRYOZOAN FAUNA FROM ILLINOIS, 



WISCONSIN. AND IOWA 



T. G. Perry 



ABSTRACT 



The Spechts Ferry bryozoan fauna from ten localities in 

 northwestern Illinois, southwestern Wisconsin, and southeastern 

 Iowa includes the trepostomatous generaHomotrypa\Jlrich,Pras- 

 opora Nicholson & Etheridge, Heterotrypa Nicholson, Dekayella 

 Ulrich, Monotrypella Ulrich, Batostoma Ulrich, and Diplotrypa 

 Nicholson and the cryptostomatous genus Stictoporella Ulrich. 

 Cyclostomatous bryozoans were not found. Twelve species are 

 described and illustrated, five of which have been recognized 

 previously in other areas. The genera Homotry pa and Stictopo- 

 rella dominate the fauna . The genus Rhinidictya Ulrich (a junior 

 synonym of Stictopora Hall), commonly found in strata of Tren- 

 tonianage in Minnesota, is represented sparingly in the Spechts 

 Ferry fauna by fragmentary, poorly preserved specimens that 

 cannot be assigned to previously described species. The genus 

 Hallopora Bassler is not recognized in the Spechts Ferry fauna, 

 although it is encountered commonly in Middle Ordovician rocks 

 in Minnesota and elsewhere. The Spechts Ferry Shale cannot 

 be assigned to either Blackriveran or Trentonian on the basis of 

 its bryozoan fauna because its widely distributed species span 

 the boundary . 



INTRODUCTION 



The Spechts Ferry Shale is a distinctive unit in the Champlainian (Middle 

 Ordovician) strata of the upper Mississippi Valley. It was originally the undif- 

 ferentiated basal part of the Decorah Shale, and was correlated with Trentonian 

 strata, but later it was for many years included in the underlying Platteville Lime- 

 stone of Blackriveran age. More recently it has been returned to the Decorah 

 Shale and assigned to the Trentonian. As bryozoans are commonly abundant and 

 well preserved in the Spechts Ferry Shale, it was hoped that their study would 

 contribute toward determining the age of the shale. 



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