Unusual Bryozoan Association
Prismopora sp.
Middle Pennsylvanian Period - Desmoinesian Series
Marmaton Group - Wewoka Formation
These bryozoan encrustations on crinoid stems were
found near Holdenville, Hughes Co., OK.
The layer produces mainly crinoid stems. A very small portion have attached
epifauna - bryozoans,
worm tubes and Crania (a brachiopod). This single specimen of Prismopora was
also found with
the crinoid stems.
1 
2
Images 1 & 2 from another web site
The bryozoan Prismopora has very unusual in
shape in that the branches are triangular
instead of round. The edges are wavy and the faces have widely spaced zooecia.
3 
4
Photo 3: Prismopora (found separately) held next to the triangular
attachment
Photo 4: Triangular end view of the Prismopora
Click on pictures (below) to Magnify
5
Close-up view of the Prismopora (found separately) held next to the
triangular attachment
6
7
The triangular shape and wavy edges of Prismopora
are identical to the
above attachment points on the bryozoan.
8
9
Crinoid column with multiple attachments
Would appreciate any input from the list about these
fossils. I think that they
could be considered a "holdfast" for the Prismopora bryozoans.
Let me know what you think of this arrangement.
Bill Rushlau
Discussion
The specimen is lovely and the triangular section
of the bryozoan colony is certainly
an erect outgrowth of a basal encrusting sheet on the crinoid stem. This is
very
informative about the variability that might be encountered in bryozoan colonies.
Without a chance to see the joined portions, one would never expect that the
sheet
and the triangular stems are part of the same colony. The triangular stems
are
certainly the most recognizable growth form, but not the only growth form.
This
is a beautiful illustration of a change in growth habit that is rarely seen.
I'd be hesitant to use the term holdfast to describe
the encrusting portion of the
colony. A holdfast is primarily a compact mass of undifferentiated tissue
that holds
an organism or colony in place. With this Prismopora specimen, the
encrusting
sheet is much larger than needed for attachment and has many zooids that indicate
the sheet is a fully functioning part of the colony. The encrusting sheet
may be simply
the early growth stages of the colony, rather than a holdfast structure.
It is always interesting to see the attachment base
of a stalked bryozoan colony, and this
is what you have illustrated. I found a similar situation in the Ordovician
genus Peronopora,
with the attachment base encrusting around a crinoid stem, and the beginnings
of a small
frond arising from the side of the column. The bryozoans might have been commensals
with the crinoid, but are more like parasites because the crinoid doesn't
benefit from the
extra weight coming from its epizoans. Of course, the crinoid might have been
dead, and
the bryozoan was just using the dead columnal as a substrate.
Prismopora links (related
to topic of discussion)
Prismopora
digitata Crockford, 1957
illustrates the "encrusting sheet" - see fig. 2
Prismopora
triquetra Hall 1883
Middle Pennsylvanian Series
Desmoinesian - Marmaton Group - Missouri
photo of 4 specimens
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