Preservation and paleoecology of a Middle Ordovician hardground community

Limestone beds in the Middle Ordovician (Trentonian) Bobcaygeon Formation, exposed near Kirkfield, Ontario, exhibit irregular, bored and encrusted surfaces indicative of early lithification. These hardgrounds were formed in extensively burrowed carbonate sediments and their hummocky surface topographies were inherited, in part, from a pre-existing pattern of burrow tunnels. A diverse community, including bryozoans, brachiopods, crinoids, edrioasteroids, and paracrinoids, colonized these hard substrates. In addition, most surfaces are riddled with small, cylindrical boreholes (Trypanites) which represent dwellings of softbodied organisms. Some hardgrouind surfaces were inhabited by multiple generations of organisms. Remains of the older generations of encrusters were strongly abraded and nearly obliterated. Superimposed upon these worn remnants are well-preserved remains of rather fragile organisms, e.g. complete hybocystitid crinoids and edrioasteroids. Evidently, certain hardground surfaces were rapidly buried by muds, resulting in the in situ preservation of the last generation of attached organisms. Slightly differing subcommunities of organisms inhabited various microhabitats provided by the irregular hardground surfaces. Thus, the relative abundance of bryozoans and echinoderms encrusting the roofs of small crevices differs from that on the exposed upper surfaces of the hardgrounds. This is the geologically oldest known hardground community in which microhabitat subdivision can be recognized. However, polarity between the subcommunities is not as pronounced as in geologically younger hard substrate faunas.