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