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Journal of Paleontology

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UPPER ORDOVICIAN CRINOIDS FROM THE PLATTEVILLE LIMESTONE OF NORTHEASTERN IOWA

¹ Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070

The Platteville crinoid fauna of northeastern Iowa consists of six species: Abludoglyptocrinus charltoni (Kolata, 1975), Carabocrinus sp. Kolata, 1975, Cupulocrinus plattevillensis Kolata, 1975, Ectenocrinus simplex (Hall, 1847), Merocrinus britonensis Kolata, 1975, and Porocrinus pentagonius Meek and Worthen, 1865. Other echinoderms include an unidentified cyclocystoid and several rhombiferan and crinoid plates. The echinoderms are mostly associated with a variety of passive suspension and filter feeders, principally brachiopods and bryozoans. Complete crowns and columns are available for the porocrinid and cupulocrinid, which lived at elevations of 28 and 50–60 mm above the seafloor, respectively. Although entire stems are not preserved, specimens from other stratigraphic units suggest that Abludoglyptocrinus charltoni, Ectenocrinus simplex, and Merocrinus britonensis occupied higher areas above the seafloor. The elevation of the carabocrinid is not known. The cyclocystoid was located on the seafloor. The crinoids are ecologically separated in two ways: different elevations above the substrate and food groove width, which is correlated with the size of the food items taken. The high-level crinoids mainly ate a narrow range of small items. However, the crinoids at lower levels caught many larger food particles and utilized a much wider range of food sizes. Cup growth of Cupulocrinus plattevillensis is isometric except for the heights of the infrabasals and radials, which exhibit significant positive and negative allometry relative to overall size. The proximal brachs are strongly allometric in which the widths increase much more rapidly than the heights.