A HIGH-DENSITY EDRIOASTEROID-ENCRUSTED HARDGROUND FROM THE BRANNON MEMBER OF THE LEXINGTON LIMESTONE (UPPER ORDOVICIAN, KATIAN, MOHAWKIAN)

Edrioasteroids are important components of Paleozoic marine communities that, as obligate encrusters, cement themselves to hardgrounds, lithified clasts, and biotic hard substrates. A hardground encrusted by edrioasteroids, when subjected to rapid burial, presents a time-poor, in situ “snapshot” of an ancient assemblage that allows ecological study on a fine scale. A new species of Isorophid edrioasteroid from the Upper Ordovician of central Kentucky occupies remarkable densities unknown from Lower-Middle Paleozoic occurrences, although to date, just in a single bed. Previously described Ordovician edrioasteroid occurrences can achieve densities of up to twenty-five individuals per square meter, but specimens of Curvitriordo n. sp. reach densities three orders of magnitude greater than any other known Ordovician edrioasteroid occurrence.

The monospecific assemblage of Curvitriordo n. sp. is described from the Brannon Member of the Lexington Limestone (Upper Ordovician, Katian, Mohawkian). This new species is unique in having a well-developed pedunculate zone between the peripheral rim and the edge of the ambulacra. Specimens from two slabs with 17 and 53 individuals in near-contact have diameters ranging from 6.83–12.06 mm (average 9.01 mm) and 3.81–10.63 mm (average 7.14 mm) respectively. While small for the genus, specimens show a variety of morphological features, such as long curved ambulacra, fully developed cover plate cycles, and a well-developed pedunculate zone, suggesting maturity. The individuals show a dispersed distribution and show no preferred orientation upon the encrusted surface.

The Brannon Member consists of ~2 –5 m of intercalated, sparsely fossiliferous calcisiltite and shale rhythmites, and in some locations a one-meter zone of seismically-deformed cherty calcisiltite. Fifteen centimeters below this zone of deformation is a laterally discontinuous phosphatic grainstone lens with a cryptic hardground on its upper surface which yields the edrioasteroids as well as occasional Sphenothallus holdfasts. Curvitriordo n. sp. inhabited a deep-water hardground that experienced little disturbance until burial by a thin layer of silty mud. These were likely opportunistic organisms that settled gregariously where conditions on an otherwise inhospitable seafloor permitted.