Archive for October 13th, 2017

Wooster’s Fossils of the Week: “Ghosts” in the Upper Ordovician of Kentucky

October 13th, 2017

This year Caroline Buttler (Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales) and I had a great project describing a cave-dwelling fauna in the Upper Ordovician of northern Kentucky. We hope that work will appear soon in the Journal of Paleontology. During our lab studies of thin-sections and acetate peels of massive trepostome bryozoans, we found several examples of clear calcite bodies in the middle of sediment-filled borings. These structures were described from the Ordovician of Estonia as “ghosts” of soft-bodied organisms by Wyse Jackson and Key (2007). They appear to be mineralized casts of organisms that were buried when sediment filled the borings that they occupied.

Meanwhile, Luke Kosowatz (’17) has a senior Independent Study project assessing bioerosion in the Upper Ordovician of the Cincinnati area. He and I have also found numerous examples of these ghosts in borings, so many that they have become a phenomenon in themselves for study. Above is an acetate peel made tangentially to the bryozoan surface showing the numerous tubular zooecia punctured by a few larger borings. Most of these borings are filled with sediment, but the two indicated by the arrows have these calcitic ghosts. This specimen is from the Corryville Formation near Washington, Mason County, Kentucky (38.609352°N latitude, 83.810973°W longitude; College of Wooster location C/W-10).

Above is one of our many heavily-bored trepostome bryozoans. This one comes from the Bellevue Formation (Katian) exposed on Bullitsville Road near the infamous Creation Museum (C/W-152). The irregular holes are the cylindrical boring Trypanites. The ghosts are not visible without sectioning.

Here is a close view of one of the ghostly calcitic casts in an acetate peel. The boundaries are sharp between the ghosts and the surrounding sediment.

The arrows above show ghosts in longitudinal cross-sections. Note their extended oval shapes. These are clearly organic shapes under these circumstances. (This is a thin-section.)

So what do the ghosts represent? They could be remains of the boring organisms themselves. If they are, they can be used to address a problem we have with bioerosion: What is the temporal relationship between the borings? How many were active in a given substrate at a given time? The percentage of borings with ghosts may give us a minimum amount of contemporary bioerosion. If, again, these are remnants of the borers themselves.

Maybe the ghosts are of later organisms that occupied the borings after the borers died? This happens often, with the secondary inhabitants called nestlers.

I know of no way to sort possible borers from nestlers with this kind of evidence.

The above image shows it’s possible that some of the ghosts are of organisms that had shells. The arrow is pointing to a dark line that may represent the remains of some type of shell. I’ve seen little tiny lingulid brachiopods in some borings before.

A fun mystery!

For technical interest, here is our photomicroscope we use to produce images like those in this post.

References:

Cuffey, R.J. 1998. The Maysville bryozoan reef mounds in the Grant Lake Limestone (Upper Ordovician) of north-central Kentucky, in Davis, A., and Cuffey, R. J., eds., Sampling the layer cake that isn’t: the stratigraphy and paleontology of the type-Cincinnatian. Ohio Department of Natural Resources Guidebook 13: 38-44.

Wyse Jackson, P.N. and Key, M.M. Jr. 2007. Borings in trepostome bryozoans from the Ordovician of Estonia: two ichnogenera produced by a single maker, a case of host morphology control. Lethaia 40: 237-252.