Growth-Form-Analysis and Paleoecology of the Corals of the Lower Mississippian Lodgepole Formation, Bear River Range, North-Central Utah

The Mississippian (Kinderhookian-Osagean) Lodgepole Formation contains a diverse fossil assemblage. Taxa present include brachiopods, crinoids, gastropods, cephalopods, trilobites and corals. Corals and associated fauna were collected from four localities within the Bear River Range. These are, from north to south, Beirdneau Hollow, Spring Hollow, Leatham Hollow and Porcupine Dam. The well-preserved tabulate and rugose (compound and solitary) corals exhibit a high degree of morphologic variability. The colonial corals of the Lodgepole Formation (particularly Lithostrotionella, Syringopora) exhibit a morphologic gradient from platy to hemispherical forms. The six morphologic categories of colonial corals discussed in this study are identified by mean corallus diameter/corallum height ratios, by the corallite growth direction, and by the shape of the base of the colony. Type I corals have an average mean diameter/height ratio of 3.4; corallites are directed laterally away from the flat base. Type I corals are interpreted to have been adapted to offshore, quiet-water conditions. Type II corals are flattened hemispheres; they have an average mean diameter/height ratio of 4.1. Corallites are directed radially (i.e., with vertical as well as a lateral component) away from the flat colony base. Type II corals are interpreted in this study to have been adapted to shallow, moderately-turbulent environments in which vertical growth was inhibited. Type III corals have an average mean diameter/height ratio of 3.9 and are similar to Type II corals in all respects but one, namely that there is an absence of corallites on the crown of the corallum. This feature is called balding and is interpreted in this study to have been the result of desiccation and subsequent death of coral polyps. Type III corals are thus interpreted to have inhabited very shallow water wherein subaerial exposure of the crown of the corallum occurred during periods of exceptionally low tides. Type IV corals are dome-shaped or slightly-flattened hemispheres; they have an average mean diameter/height ratio of 2.3. Corallites are directed radially away from the flat base. Type IV corals are interpreted to have inhabited a depth zone intermediate between that of Type II corals (within or barely below tidal range) and Type I corals (near or below wave base). The average mean diameter/height ratio of Type V corals is 1.7. Corallites are directed almost entirely vertically away from the rounded-to-conical colony base. Type V corals are interpreted to have inhabited areas where sedimentation rates were sufficiently high to encourage vertical growth to the virtual exclusion of lateral growth. Type VI corals are composite corals, consisting of combinations of hemispherical forms and platy forms. This morphologic type is characterized by a change in the direction of growth during the astogenetic development of colony. The combinations of varying growth forms presumably reflect fluctuations in sedimentation rate