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SP-06 The Cryptostome Bryozoa from the Middle Ordovician Decorah Shale, Minnesota
Karklins, Olgerts L. (Minnesota Geological Survey, 1969)

SP-06 The Cryptostome Bryozoa from the Middle Ordovician Decorah Shale, Minnesota

Issue Date

Minnesota Geological Survey


The Middle Ordovician Decorah Shale in Minnesota is a distinctive fonnation containing an abundant diversified fauna; this study is concerned with the cryptostome Bryozoa, a group that constitutes a large part of the fauna. I have used a new approach in the interpretation of cryptoston1e zoarial structures emphasizing the dark boundary zones and their spatial arrangement in zoaria as distinct morphological features in genera of the families Rhinidictyidae, Stictoporellidae, and Ptilodictyidae. The dark boundary zones are formed by abutting or adjoining laminae and, when present, they outline zooecia or other structural segments of zoaria. In thin sections the boundaries appear as dark lines representing the edge views of planar to curved zones between zooecia and extend for different lengths throughout zoaria. Sixteen species distributed among six genera are described and illustrated. One species, Stictopora lita, is new. On the basis of the boundary zones in association with other morphological features two new genera, Astreptodictya, type species Pachydictya acuta (Hall), and Athrophragma, type species Pachydictya foliota Ulrich, are proposed. Emended generic defmitions are presented for Stictopora, Escharopora and Graptodictya. The wall structure of each genus is described in detail and, where the material permits, previously described Ordovician species are reassigned to new genera. The infonnal stictoporid and escharoporid groups of Ross (Phillips, 1960) are redefmed and three additional infonnal groups are proposed: "stictoporellid," athrophragmid, and astreptodictyid. The genera are reassigned to these groups as appropriate. The stictoporid group is defined as having zoaria with approximately linear zooecial ranges in which adjacent zooecia are separated by range boundaries laterally and zooecial boundaries longitudinally. In the "stictoporellids" the zooecial boundaries are polygonal in tangential views and the range boundaries are lacking. The escharoporid group is characterized by having well-defined wall laminae that are continuous between adjacent zooecia. There are no boundary zones in the exozone of zoaria. In the athrophragrnids the zooecial boundaries descnbe a cylindrical form in the exozone and appear approximately oval in tangential views. The walls between adjacent zooecia in the exozone may contain numerous intermittent dark zones, but there are no range boundaries. The astreptodictyid group is distinguished in having range partitions between adjacent zooecial ranges in exozone that extend throughout zoaria and are at about right angles to the zoarial swface. The zooecial boundaries are like those in the athrophragmids and the range boundaries, similar to those in the stictoporids, are located along the median of the range partitions. Geographic and stratigraphic distribution of species permits the division of the Decorah Shale into three zones from bottom to top: 1. Stictoporella angularis zone, with three restricted species, 2. Stictopora mutabilis zone, with one restricted species, and 3. Stictopora minima zone, with two restricted species. One of the remaining species, Stictopora lita, occurs primarily in the Stictoporella angularis zone, but ranges into lower part of Stictopora mutabilis zone; seven species occur primarily in the S. mutabilis zone, but range into S. minima zone, and two were found throughout the section at all seven localities. Cryptostomata are abundant throughout the Decorah Shale except in the lower part of the Stictopora mutabilis zone. The three cryptostome zones approximately coincide with ostracode zones suggested by Swain and others in 1961. On the basis of present knowledge, the Stictoporella angularis zone is the approximate biostratigraphic equivalent of the Spechts Ferry Shale Member of the Decorah Formation in Illinois, Wisconsin, and Iowa. Evaluation at the species level suggests that the cryptostomes of the Decorah Shale are more closely related to those of the Trenton Group of New York than to those of the Black River Group of New York.

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Previously Published Citation
Karklins, O.L., The Cryptostome Bryozoa from the Middle Ordovician Decorah Shale, Minnesota. Special Publication Series 6. 121 p.

Series/Report Number

121 p.

Suggested Citation
Karklins, Olgerts L.. (1969). SP-06 The Cryptostome Bryozoa from the Middle Ordovician Decorah Shale, Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy,

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