Mark Wilson March 23rd, 2014
Yes, yes, I’ve heard ALL the jokes about being bored, and even intensely bored. I learn to deal with it. This week we continue to highlight fossils collected during our productive expedition to the Upper Ordovician (Cincinnatian) of Indiana (with Coleman Fitch ’15) and Kentucky (with William Harrison ’15). Last week was Coleman’s turn; this week it is William’s.
The beautiful fan-like bifoliate (two-sided) trepostome bryozoan above was collected from the lower part of the Grant Lake Formation (“Bellevue Limestone”) at our locality C/W-152 along the Idlewild Bypass (KY-8) in Boone County, Kentucky (N 39.081120°, W 84.792434°). It is in the Maysvillian Stage and so below the Richmondian where Coleman is getting most of his specimens. I’ve labeled it to show: A, additional bryozoans encrusting this bryozoan; B, a very bored section; C, a less bored surface showing the original tiny zooecia, monticules, and a few larger borings.
The other side of this bryozoan is more uniform. It has an even distribution of small borings and no encrusters. This likely means that at some point after the death of the bryozoan and subsequent bioerosion this side was placed down in the mud while the exposed opposite side was encrusted.
A closer view of the upwards-facing side (with the encrusting bryozoan at the top) shows just how intense the boring was prior to encrustation. Some of the borings are close to overlapping. The encrusting bryozoan has its own borings, but far fewer and significantly larger.
In this close view of the downwards-facing side we see lots of the small borings. Some are star-shaped if they punched through the junction of multiple zooecia. Note that these borings are rather evenly spread and seem to have about the same external morphology and and erosion. Likely they were all produced about the same time. It must have been a crowded neighborhood when all those boring creatures were home.
The questions that are provoked by this specimen are: (1) Were there any borings produced while the host bryozoan was still alive? (We may find elements of bioclaustration with some holes); (2) Why are zones B and C in the top image so different in the amount of bioerosion? Could zone C have still been alive at the time and resisted most bioeroders? Maybe zone C was covered by sediment? (But the margin is very irregular); (3) Why are the later encrusting bryozoans (zone A) so much less bioeroded?; (4) How do we classify such tiny pits that are between microborings and macroborings in size? (Trypanites is becoming a very large category) (5) What kind of organism made so many small pits? Were they filter-feeders as we always say, or was something else going on? (Sectioning specimens like this may reveal some internal connections between the pits.)
William has plenty of fun work ahead of him!
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Erickson, J.M. and Waugh, D.A. 2002. Colony morphologies and missed opportunities during the Cincinnatian (Late Ordovician) bryozoan radiation: examples from Heterotrypa frondosa and Monticulipora mammulata. Proceedings of the 12th International Conference of the International Bryozoology Association. Swets and Zeitlinger, Lisse; pp. 101-107..
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Vogel, K. 1993. Bioeroders in fossil reefs. Facies 28: 109-113.
Wilson, M.A. and Palmer, T.J. 2006. Patterns and processes in the Ordovician Bioerosion Revolution. Ichnos 13: 109–112.