572


Journal of Paleontology 92(4):568–576


Long autozooecia with diaphragms along length. Styles present in patches, mesopores absent.


Occurrence.—Ordovician, Katian, midwestern USA (Ohio, Indiana, Kentucky).


Description.—The zooaria are massive in form and multi- laminar with the new parts of the colony overgrowing older ones. They range in size and are up to 93mm in diameter and 50mm in height. Monticules of clusters of larger autozooecial chambers form regular conical structures on colony surface. Endozones restricted to short recumbent autozooecia budded from exterior basal colony walls. In exozones, autozooecial boundaries polygonal in cross-section. Autozooecia in dis- ordered pattern on colony surface. Zooecial walls thin, cortex thickness regular throughout exozone; microstructure poorly preserved. Zooecia polygonal in transverse section, 0.23mm maximum mean diameter and 0.38mm maximum mean dia- meter in monticules. Thin basal diaphragms abundant in some parts of zooecial chambers and lacking in others, spaced on mean average 0.19mm. Styles are observed in localized patches. Mesopores are absent.


Remarks.—Stigmatella personata was originally described by Ulrich and Bassler (1904) as a smooth branching colony char- acterized by having no mesopores and few styles. Dyer (1925) proposed S. personata lobata, which differed in colony mor- phology, forming irregular lobate masses covered by low mon- ticules. Utgaard and Perry (1964) regarded the differences between S. personata lobata and the type species as variations in growth form. Fritz (1973) however disagreed and considered therewas a significant difference in the numbers of styles between the two. The colonies from the Corryville Formation show var- iation of style number within the colony. Encrusting trepostome colonies are known to develop erect branches, so species cannot be distinguished by external morphological form.


Bryozoan colonies


Taxonomy and form.—The bryozoan colonies are recognized as having lived on an exposed hardground surface and having inhabited small caves beneath it. The Corryville Formation is estimated to have been deposited below the fair-weather wave- base and above the storm wavebase, and so well within the photic zone (Holland and Patzkowsky, 2007). We investigated as to whether there were differences between those that grew upwards in presumably well-lit waters and those that grew downwards from the ceilings in the darker, light-restricted caves. There would have been differences in the water currents in the two environments and colony growth may have been affected by gravity. Taxonomically no differences were recognized between


the upwards-growing, exposed bryozoans and those living pendantly in the caves. All the large colonies are identified as the trepostome Stigmatella personata Ulrich and Bassler, 1904 (Fig. 4.1, 4.2) regardless of their growth orientation. Only one other encrusting trepostome species is recognized; one small colony of Monticulipora is observed growing partly over a small single layer of S. personata and the celling of the cave


(Fig. 4.3). This colony is in turn overgrown by a large S. personata. There are few discernible anatomical differences between


the bryozoan colonies from the two locations. The pendant, cave-dwelling S. personata on average has longer zooecial tubes than its exposed equivalent, possibly caused by environ- mental factors in the different locations. A maximum zooecial length of 22.5mm and was measured in colonies growing downwards and 16.5mm in those growing upwards. The mean average of the downward growth is 10.6mm (range 3.7–22.5mm; SD 4.96mm) and upward 5.6mm (range 0.89–16.6mm; SD 4.14 mm). The spacing between diaphragms in the zooecial chambers


was measured to determine if there is a difference between the colonies in different orientations. The diaphragms are commonly widely spaced, but in some locations are found closer together. The intervals between the first five diaphragms from the zooecial opening at the colony surface or at the point of overgrowth were measured over 200 times in colonies growing up and growing down (~420 measurements). The results showed there was only a difference of 0.005mm in growth direction, a mean average of 0.196mmrecorded for the colonies in caves and 0.191mm for those on the hardgrounds. However, within one colony the distances are variable as can be seen in Figure 4.1.


Growth and overgrowths of colonies.—The autozooecial chambers in the colony generally grew long, up to 22.5mm, and straight, but can be seen to have localized changes in direction of colony growth (Figs. 4.1, 5.1). Thickening of the zooecial walls and the localized development of styles can also occur (Fig. 5.2), sometimes accompanying a change in growth direction. These developments may be controlled by external microenvironmental effects.


Discussion


The colonies of Stigmatella personata are multi-layered because of self-overgrowth (Fig. 5.3). The intracolonial nature of the overgrowths is recognized in thin section (Fig. 5.4) by uninterrupted zooecial chambers adjacent to those overgrowing part of the colony, which has sediment that infills zooecia. This suggests the zooids in the overgrown part of the colony died and the living zooids overgrew them. The area of overgrowth can be localized or extend over most of the colony (Fig. 5.3, 5.5, 5.6). The death of parts of the colony may be due to localized predation, although no evidence can be seen in the skeletal walls. Modern-day cyclostome bryozoans are predated by a variety of organisms, including echinoderms, nudibranch sea slugs, pycnogonids, small crustaceans, and fish (Hayward and Ryland, 1985; Lidgard, 2008). The colonies living in the cave environment may have had some protection from predation compared to those growing in exposed environments. This may be the reason that longer autozooecial tubes are recognized in the bryozoans growing down in caves. In colonies growing upwards and those growing down, the


overgrowths are marked by sediment infilling the zooecial chambers in the older part of the colony (Fig. 5.4). Lev et al. (1993) observed ‘clay drapes’ on the top of colony-wide bands


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