Wooster’s Fossil of the Week: A bored and formerly encrusting trepostome bryozoan from the Upper Ordovician of Indiana
Mark Wilson March 20th, 2015
The lump above looks like your average trepostome bryozoan from the Upper Ordovician. I collected it from the Whitewater Formation of the Cincinnatian Group at one of my favorite collecting sites near Richmond, Indiana. In this view you can just barely make out the tiny, regular holes that are the zooecia (calcitic tubes that held the bryozoan individuals — the zooids). There are bits of other fossils stuck to the outside, so it’s not particularly attractive as fossils go. (Except that all fossils are fascinating messengers in time.)
With this closer view you can see my initial interest in this particular bryozoan. Again, the regular, tiny holes are the zooecia. The larger pits are borings by worm-like, filter-feeding organisms. These borings are either in the ichnogenus Trypanites (if they are cylindrical) or Palaeosabella (if they are clavate, meaning clubbed at their distal ends). Such borings are common in all types of skeletal fossils in the Upper Ordovician — so common that they are part of the evidence for the Ordovician Bioerosion Revolution. So, let’s flip this ordinary, bored bryozoan over and see what’s underneath:
Here’s the main scientific beauty! We’re looking at the underside of the bryozoan. Ordinarily we’d expect to see a shell here that the bryozoan was encrusting, but the shell is gone. We’re gazing directly at the attachment surface of the bryozoan. It’s as if the colony had encrusted a sheet of glass and we’re looking right through it. The shell it was originally attached to has been removed either through dissolution (it might have been an aragonitic bivalve) or physical removal (it may have been a calcitic brachiopod). The borings are now much more prominent. They penetrated through the bryozoan into the mysterious missing shelly substrate. Some are small pits that just intersected the shell, others are horizontal as the boring organism turned at a right angle when it reached the shell and drilled along the bryozoan-shell interface. Removing the shell exposed the distal parts of these borings — parts that ordinarily would have been hidden by the encrusted shell.
Here is a closer, labeled view of this bryozoan basal surface. A is the earliest encruster recorded in this scenario; it is a small encrusting bryozoan that was first on the shelly substrate and then completely overgrown (or bioimmured) by the large trepostome. B shows that the trepostome was growing on a shell that already had borings from a previous encruster-borings combination that must have fallen off; these are grooves in the substrate that the trepostome filled in as it covered the shell. C is one of the many later borings that cut perpendicularly through the bryozoan and worked along the shell-bryozoan interface; as described above, only when that shelly substrate was removed would these be visible. In this surprisingly complex story, B represents an earlier version of C. We thus know that the shell was encrusted by one bryozoan, bored, and then that bryozoan was freed at its attachment (and not found in our collection). The same shell was then encrusted by this second bryozoan, which recorded the groove (or “half-borings”) made during the first encrustation.
These half-borings were first described in 2006 when my students Cordy Dennison-Budak and Jeff Bowen worked with me on them and we had a GSA abstract. Coleman Fitch is presently completing his Senior Independent Study enlarging the database for these features and developing detailed interpretations. The main implication from this work is that thick trepostome bryozoan encrusters often “popped off” shells, leaving no signs of their presence unless there were these half-borings in the shell surfaces and bryozoan undersides. Paleoecology and taphonomy on a very small scale!
Taylor, P.D. 1990. Preservation of soft-bodied and other organisms by bioimmuration—a review. Palaeontology 33: 1-17.
Wilson, M.A., Dennison-Budak, W.C., and Bowen, J.C. 2006. Half-borings and missing encrusters on brachiopods in the Upper Ordovician: Implications for the paleoecological analysis of sclerobionts. Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 514.
Wilson, M.A. and Palmer, T.J. 2006. Patterns and processes in the Ordovician Bioerosion Revolution. Ichnos 13: 109-112.
Wilson, M.A., Palmer, T.J. and Taylor, P.D. 1994. Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.