Directional orientations of solitary rugose corals

Abstract

Unattached solitary rugose corals lying parallel to bedding are common in many Ordovician and younger Paleozoic units, but their directional orientations have seldom been examined. Interpretations based on occurrences in North American Upper Ordovician and Lower Silurian strata provide a foundation for such studies. Distinct directional patterns resulted from preferred orientation with respect to water motion. They indicate transportation rather than biologic orientation, if it can be shown that the corals were not preserved in life position or were abraded to a significant degree before burial. Transported, slightly curved specimens having trochoid to ceratoid form were aligned parallel to currents, with the apex pointing upstream, and (or) were rolled nearly perpendicular to currents or almost parallel to crests of advancing waves, with the apex facing either way but directed slightly upstream. Therefore, unimodal orientation patterns, bimodal patterns with equal peaks that are opposite one another but slightly skewed, and trimodal patterns that are a combination of these can be used to determine flow directions. An apparently bimodal pattern with nearly equal and directly opposite peaks has been observed in a case involving mostly subcalceoloid corals. These individuals may have been aligned parallel to currents or to the direction of wave progression, with the apex facing either way. Random directional distributions do not necessarily indicate low-energy environments. They could have resulted from changes in flow direction during the time in which the sampled stratigraphic interval was deposited or from the effects of bioturbation on corals that were initially preferentially oriented.Directional patterns of solitary rugose corals are of value in paleoecology (recognition of transported assemblages) and basin analysis (determination of paleocurrent directions). They may also prove to be useful in making paleoenvironmental reconstructions (type of water motion, nature of substrate) if the reasons for different types of orientation patterns can be established with experimental work and additional data from the geologic record.