ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 326 



Potentially, however, bryozoans should have great value in stratigraphic studies 

 because of their common occurrence and because they can be readily identified 

 from fragmentary specimens. 



If, however, bryozoans are particularly susceptible to facies changes, as 

 has been stated by Troedsson (1928, p. 183) and mildly insinuated by Horowitz 

 and Perry (1956, p. 1707), this circumstance would detract from their effective- 

 ness in regional correlation studies. Bassler's investigation (l 9 1 1) of the early 

 Paleozoic Bryozoa of the Baltic region indicated the cosmopolitan distribution of 

 many bryozoan species. For example, Bassler (1911, p. 32) stated that 23 of the 

 58 species of the Rhinidictya bed in the Decorah Shale of the Minnesota section 

 are found in Ordovician strata of the Baltic region; this and other faunal compari- 

 sons given by Bassler do not suggest rigorous facies control of Paleozoic bryozoan 

 faunas. As will be shown in the following discussion, five of the twelve species 

 recognized in the Spechts Ferry have a considerable geographic range, which again 

 suggests that facies changes do not exercise greater control on the distribution 

 of bryozoans than on many other kinds of invertebrate fossils. 



Five of the 12 species in the Spechts Ferry have been described by Ulrich 

 (1895, p. 96-332) from rocks of Middle Ordovician age in Minnesota where they 

 are found mainly in the "Lower Third" (containing basal Stictoporella bed) and 

 "Middle Third" {Rhinidictya and Ctenodonta beds) Trenton Shales of Ulrich's strati- 

 graphic classification. These strata are now assigned to the Decorah Shale (Kay, 

 1929, p. 642, table 1). Species common to the Spechts Ferry and the Decorah of 

 Minnesota are: Batostoma winchelli (Ulrich), B. wine helli spinulo sum Ulrich, 

 Dekayella praenuntia echinata Ulrich, Homotrypa exilis Ulrich, and Stictoporella 

 frondifera Ulrich. The Spechts Ferry fauna also includes Prasopora simulatrix 

 orientalis Ulrich, a form recognized (Ulrich, 1895, p. 246-248) from rocks of 

 Trenton age near Ottawa and Peterborough in southern Ontario and from strata of 

 the same age at Trenton Falls, New York. Stictoporella frondifera , a very abundant 

 species in the Spechts Ferry, allegedly is restricted to the Stictoporella beds of 

 the Minnesota section (Ulrich, 1895, p. 184; Bassler, 1911, p. 28) in which it is 

 found abundantly. The genus Hallopora Bassler presumably is a conspicuous faunal 

 element in the Middle Ordovician bryozoan fauna of Minnesota, as Ulrich (1895, 

 p. 277-285) recognized nine species and one variety of this genus. Hallopora, a 

 very common Ordovician genus, was not found in the Spechts Ferry in the area 

 under study. The genus Stictopora Hall, apparently a commonly encountered form 

 in the Decorah strata of Minnesota (Ulrich, 1895, p. 124-138), is represented 

 sparingly in the Spechts Ferry by fragmentary unidentifiable specimens. 



Fritz (1957) described the bryozoan fauna of the Ottawa Formation, which 

 includes strata of Blackriveran and Trentonian ages. Four of the twelve species in 

 the Spechts Ferry are known also from the Ottawa Formation, including Batostoma 

 winchelli, B. winchelli spinulosum, Dekayella praenuntia echinata , and Prasopora 

 simulatrix orientalis. In addition, Fritz (1957, p. 22, 23) recorded a single speci- 

 men that is compared to Homotrypa minnesotensis Ulrich; her illustrations and 

 description indicate that the specimen is allied very closely to Homotrypa exilis , 

 a species found abundantly in the Spechts Ferry. The stratigraphic distribution 

 of these forms as given by Fritz (19 57, table 2) was not highly suggestive of 

 either Blackriveran or Trentonian age, as two species, B. winchelli and P. Simula - 

 trix orientalis , are found in Black River and Trenton equivalents in the Ottawa 

 Formation, although the latter is more abundant in rocks of Trentonian age. 



