Unusual Bryozoan Association
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.
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.
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
Close-up view of the Prismopora (found separately) held next to the triangular attachment
The triangular shape and wavy edges of Prismopora are identical to the
above attachment points on the bryozoan.
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.
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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