Trace fossils produced by coral corolla from the Tyrone Formation, Upper Ordovician
(Mohawkian) of central Kentucky, U.S.A. FRANK R.ETTENSOHN* and ADAM C.
LOWE, Department of Earth & Environmental Sciences, University of Kentucky,
Lexington, KY 40506.
A benched surface below the Pencil Cave bentonite in an exposure of the Upper
Ordovician (Mohawkian) Tyrone Formation at Boonesboro, Kentucky, revealed 41
circular to elliptical traces, ranging from 14 cm to 1 m in diameter, with linear to
branching grooves radiating from the center of each. The traces are generally evenly
distributed across the surface, although some occur in loose groupings. In up to five of
the traces, individuals are apparently conjoined, producing multilobed traces. Three
crinoid holdfasts were found encrusting the surface on top of individual traces and three
internal gastropod molds were found in similar situations.
Close examination of the surface revealed many shredded fragments of the tabulate
coral Phytopsis cellulosum, and similar fragments of the corallum occur locally within
the grooves of the trace, suggesting that each trace represents the impression of a colony
base in soft mud, and that the radiating grooves reflect axes of colony growth. Actual, insitu domed colonies of Phytopsis sp., preserved on bedding planes in the overlying
Curdsville Member of the Lexington Limestone at other localities, serve as models for
the way the surface must have once appeared. Apparently, this Tyrone surface reflects a
brief marine inundation of Tyrone carbonate tidal flats and the colonization of the surface
by Phytopsis colonies. The colonies were apparently abruptly removed and fragmented,
perhaps during a major storm, and the surface, by now either a firmground or
hardground, was later recolonized by a sparse marine fauna. That fauna was subsequently
snuffed out and buried by a major volcanic ash fall, which preserved the traces and the
sparse fauna that had encrusted or lived on them.
Although the surface reflects an interesting taphonomic sequence of events, the traces
may represent the first reported occurrence a trace fossils produced by skeletal coral
colonies. The ethological or behavioral classification of the traces is also seemingly
unusual. It is tempting to call them resting traces or cubichnia, but inasmuch as cubichnia
are produced by vagile organisms and the coral colonies were never vagile, their
ethological situation remains uncertain.