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.

Functional morphology of coral shape and passive hydrodynamic self-righting in recentManicina areolata

“…The fact that we demonstrated here that Manicina coralla can 'flip over' while being oriented parallel to the current, rather than just being rolled perpendicular to the current may have some important considerations for paleoenvironmental reconstruction in muddy environments using similarly shaped rugosans (ELLAS 1984, ELLAS et al 1987, ELLAS et al 1988, BOLTON & DRIESE 1991. It has been claimed that orientation of horn-shaped corals would be preferentially perpendicular to the prevailing currents, since coralla would primarily be rolled.…”

External morphology and paleobiology of Heliophyllum halli (Zoantharia, Rugosa), from the Middle Devonian Hamilton Group of New York State

“…based on the morphology of a cardinal septum, generally an ar rangement of thickened septa around the cardinal plane or devel opment of a cardinal fossula where the septum is short. Elias (1984) and Elias et al (1988) indicated that, in several Ordovician genera, the cardinal area is oriented in the downcurrent direction and occurs on the convex side of the corallum, so that the plane of corallum bilaterality is vertically oriented. No such preferred orientation of bilateral corals has been determined for H. halli; however, specimens here reported were not collected in situ.…”

“…A coral with a straight growth axis and growth lines perpen dicular to it apparently lived partially or largely buried in sub strate, with the rim of the calice situated at some viable distance above a soft substrate (Hubbard, 1970;Elias, 1984;Elias and Buttler, 1986;Elias et al, 1988;Neuman, 1988). Fedorowski (1997, p. 30) stated that it is proven that Caninia lived in a per pendicular position, based on examination of in situ specimens and on growth lines in this Carboniferous genus.…”

“…Curvature and geniculation.-Curved solitary corals have gen erally been interpreted as unattached, with their convex side un derneath and their concave side up (Elias, 1984;Elias et al, 1988;Neuman, 1988). Such corallites of Heliophyllum apparently lived free on the sediment surface, or settled somewhat into soft sub strate (Fig.…”

“…Interpretations applied to this group of rugosans, having no modern representatives, should utilize concepts that are generally accepted as applicable. Hubbard (1970), Elias (1984), Neuman (1988), Elias and Buttler (1986), Elias et al (1988) and Scrutton (1998) have presented summaries of variation in shapes and sup port structures in solitary rugosans and their meanings as reflect ing substrate conditions, rates of sedimentation and disturbance as factors in controlling shape. In evaluating their variability with substrate, Neuman (1988) illustrated hypothetical life histories for Silurian corals from Gotland (Sweden) and Elias (1984) and Elias et al (1988) did much the same for Ordovician corals.…”