Buttler and Wilson—Ordovician cave-dwelling bryozoans from Kentucky
Bryozoan colonies growing upwards bored by Trypanites
569
3.0 m Seawater Hardground 2.0 m
Limestone with shale interbeds
1.0 m
Figure 3. Reconstruction of living positions of bryozoan colonies of Stigmatella personata.
0 Road level Figure 1. Location of the hardground within the Corryville Formation.
the seafloor and currents washed away large patches of soft clay underneath it, forming cavities 5–20cm high (based on field measurements) with the hardground as a solid roof. Bryozoans then occupied the ceilings of the cavity below the hardgrounds as well as the hardground upper surfaces, producing the cryptic and exposed communities we see today on the tops and bottoms of the Corryville hardground slabs (Fig. 3). The same phe- nomenon has been documented with hardgrounds exposed in the Middle Jurassic of Utah (Wilson, 1998). We do not know how long these small caves remained open and connected to normal marine circulation, but we suspect a substantial interval because the cryptic bryozoans hanging pendantly from the ceilings grew to considerable sizes—up to 93mm in diameter and 50mm high.
Figure 2. Large colony of Stigmatella personata (NMW2017.9G.1.1) growing downwards from the hardground (black triangle), both the bryozoan and substrate extensively bored (white arrows). Black arrow indicates the way up of sediments.
(Patzkowsky and Holland, 1996). In the North American stage terminology it is Maysvillian. The Corryville consists of very fossiliferous limestones
(packstones and wackestones) and shales deposited on a carbonate-siliciclastic ramp under considerable storm influence (Holland, 1993). The sediments accumulated in a deep subtidal environment (between fair-weather wavebase and storm wave- base) on the paleocontinent Laurentia ~20°S of the paleoequator (Holland and Patzkowsky, 2007; Vogel and Brett, 2009, fig. 2).
Hardground and cave development
Hardgrounds are synsedimentarily cemented, in situ, rocky seafloors (Wilson and Palmer, 1992; Taylor and Wilson, 2003).
Materials and methods
The specimens (NMW 2017.9G.1-7) forming the basis of this work were collected from exposures in 1999 (38.609352°N latitude, 83.810973°W). Longitudinally oriented thin sections were prepared to examine internal structures and photographed using a Canon 70D on a Leica Z6 microscope. Colony measurements are given in Table 1.
Repository and institutional abbreviation.—NMW - Amgueddfa Cymru – National Museum Wales, Cardiff, UK.
Systematic paleontology
Phylum Bryozoa Ehrenberg, 1831 Class Stenolaemata Borg, 1926
Superorder Palaeostomata Ma, Buttler, and Taylor, 2014 Order Trepostomata Ulrich, 1882
They are common throughout the Cincinnatian Series, especially in limestones formed in subtidal paleoenvironments (Palmer, 1982). Typical hardgrounds, such as the Corryville example in this study, are only a few centimeters thick and have less- indurated sediments above and below. They are thus commonly found as shelf-like projections from outcrops or loose slabs, allowing easy access to their bored and encrusted surfaces. This Corryville hardground was at some point exposed on
Bryozoan colonies growing downwards bored by Trypanites
Clay-rich sediments
Hardground
Upper Ordovician System Corryville Formation
Katian Stage
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