Buttler and Wilson—Ordovician cave-dwelling bryozoans from Kentucky


in colonies of Prasopora from the Middle Ordovician Martinsburg Formation of south-central Pennsylvania. They


suggested that these represent a turbidity episode in which fine-grained particles smothered the colony. This was then followed by a regeneration of zooids. In the colonies from Kentucky, we consider that an influx of sediment may not be responsible for killing parts of the colony by smothering because often only localized sections are affected. Modern colonies are known to be able to clean the surface of sediment. Polypides in the colony generate water currents to feed and to remove sediment and waste (Lidgard, 1981), and there are examples of zooids cleaning the colony using tentacles (Dick, 1984). It is probable that the areas that had chambers infilled with sediment had no living animals inside. Sediment could have entered the cave environment during


a storm episode, but why was it retained on the colony surface and not dislodged due to gravity? Alternatively, it is possible that dead parts of the colony had become covered in biofilms. Gerdes et al. (2005) recognized that bryozoans represented a common settling ground for a wide range of epizoic biofilms, including cyanobacteria and fungi. Sediment may have adhered to the surface of the biofilm, trapping it between the layers when the colony overgrew.


Borings


All colonies in both growth orientations have been extensively bored (Fig. 6.1–6.6). The borings are located throughout the colony and can also be seen in the hardground substrate (Fig. 2). They all have a similar form: straight with a cylindrical cross section. Two different types are recognized, but both are iden- tified as Trypanites Magdefrau, 1932. This trace fossil, which is known from throughout the Phanerozoic, was extremely com- mon during the Late Ordovician when it was a significant bioeroder of hard substrates (Wilson and Palmer, 2006). The two types are found in the exposed and cave environment.


Trypanites boring Type A.—These are the smaller of the two varieties (Fig. 6.1) and have a slightly different structure, depending upon the whether they are boring into the bryozoan colonies or into the hardground substrate. The tubes range in diameter from 0.74mm to 1.62mm, with a mean diameter of 1.25mm, and the maximum length measured is 16mm. These borings are confined to a single overgrowth within


the bryozoan colonies. The borings are often covered over by subsequent layers of the colony (Fig. 6.2). This suggests that the borings were not occupied by living organisms at this time; possibly they had died in a storm-sedimentation event. Type A borings appear circular in cross section in hand


specimens with straight sides to the tube. When observed in transverse sections of the bryozoan colony (Fig. 6.1), the borings are bounded by the calcite walls, which creates a polygonal rather than circular structure (Fig. 6.3). Even though calcite colony walls were eroded in the boring process, excavating the cylindrical cavity parallel to the walls was the way of least resistance for the borer. This was also observed by Wyse Jackson and Key (2007) in Ordovician bryozoans from Estonia. They found the borings oriented roughly perpendicular to the colony surface minimized intersection with skeletal walls


573


of the bryozoan zooecia. The strength of the calcite diaphragms within the bryozoan colony also resulted in a structure with a stepped appearance when they appear to form a more resistant barrier and cause the burrow to be offset to the adjacent chamber (Fig 6.2). There are some Type A borings that cut through zooecial chambers at a 90˚ angle. Type A borings are commonly infilled with micrite. Some


contain an additional cylindrical tube of calcite (Fig. 6.4). These appear to be similar to the ‘ghosts’ of organic material described by Wyse Jackson and Key (2007). They interpreted the ghosts as the sparry cement-filled cast of the boring organism that was killed by infilling of matrix into the larger boring it had excavated. This may have occurred during a storm event that buried the host colony. This is consistent with evidence for the colonies being covered in sediment and disrupting the growth.


Trypanites boring Type B.—The second variety has a larger tube size; these tubes range in diameter from 2.4mmto 3.2mm, with a mean diameter of 2.9mm and the maximum length measured is 39mm. These cut through several layers of overgrowth (Fig. 6.5). The borings are infilled with various sediments, some containing numerous dolomite rhombs and others with larger fossil fragments, including cryptostome bryozoans, brachiopod shell, and echinoderm fragments (Fig. 6.6). There are no cylindrical calcite ‘ghosts’ present in these structures.


Bioclaustration


The primary bryozoan in this study, Stigmatella personata, hosts several bioclaustration structures (Figs. 7.1, 7.2). Bio- claustration was originally described by Palmer and Wilson (1988) as a process by which soft-bodied symbionts are entombed within the growing skeletons of other organisms. Taylor (1990) expanded the definition to include the embedment of skeletal organisms as well. There are only four bioclaustra- tion ichnotaxa formally described in Paleozoic bryozoans: Anoigmaichnus odinsholmensis Vinn et al., 2014, from the Middle Ordovician (Darriwilian) of Estonia; Catellocaula val- lata Palmer and Wilson, 1988, in Upper Ordovician (Katian) trepostomes; Caupokeras calyptos McKinney, 2009, in Middle Devonian fenestrates (see also Suárez Andrés, 2014); and Chaetosalpinx tapanilai Ernst et al., 2014, in Middle Devonian cystoporates. The bioclaustration structures in S. personata are


flat-bottomed tubular structures. They appear to represent soft-bodied fouling organisms that spread across the apertures of three to six zooecia, effectively halting their growth. Neigh- boring zooecia grew up and eventually overtopped them, forming keyhole-like cross-sections, as in Figure. 7.1. Patches of sparry calcite and sediment appear to be additional examples of “ghosts” of soft tissues (Fig. 7.1). In some bioclaustration structures, there is a hint of a recrystallized skeleton for the embedded organism (Fig. 7.2). The bioclaustration structures in S. personata do not fit the


descriptions of current ichnotaxa, but with only cross-sections, we do not have enough information to erect a new ichnotaxon. Wealso cannot identify the symbiont that left these features. We know enough, though, to classify these bioclaustrations within


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