Mark Wilson June 15th, 2014
We have a very simple trace and body fossil combination this week that provides a stratigraphic and structural geologic tool. Above is a bit of scleractinian coral from the Matmor Formation (Middle Jurassic, Callovian) of Makhtesh Gadol in southern Israel. The coral skeleton was originally made of aragonite. It has been since recrystallized into a coarse sparry calcite, so we can no longer see the internal skeletal details of the coral. In the middle of this polished cross-section is an elliptical hole. This is a boring made by a bivalve (the trace fossil Gastrochaenolites). Inside the boring you see a separate elliptical object: a cross-section of a bivalve shell. This could be the bivalve that made the boring or, more likely, a bivalve that later occupied the boring for a living refuge. This, then, is the trace fossil (Gastrochaenolites) and body fossil (the bivalve shell) juxtaposition.
That stratigraphic and structural interest is that the boring and the bivalve shell are partially filled with a yellow sediment. This sediment has gravitationally settled to the bottom of these cavities (at slightly different levels). These holes have thus acted as natural builders’ levels showing is which way was down and which was up at the time of deposition. We can tell without any clues from the recrystallized coral the “way up” before any later structural deformation (or in this case rolling around on the outcrop) changed the orientation of the coral. Pretty cool and simple, eh? The name for this feature is a geopetal structure. There are some faulted and folded sedimentary rock exposures in the world where we search diligently for these little clues to original orientation (see, for example, Klompmaker et al., 2013). Not all geopetal structures have fossil origins (i.e., Mozhen et al., 2010), but most do. A little gift from paleontology to its sister disciplines.