COMPARATIVE PALEOECOLOGY OF EARLY MISSISSIPPIAN (KINDERHOOKIAN) CRINOIDS FROM THE LODGEPOLE FORMATION, IDAHO, MONTANA, AND WYOMING

The Early Mississippian Lodgepole Formation of Montana and subadjacent states was deposited on a distal carbonate ramp. Situated within 10° N of the paleoequator, this region was bounded by the Antler Highlands to the west, the Central Montana Trough to the north, and the Transcontinental Arch to the southeast. In this setting, many fossils were exceptionally preserved, including large bedding surfaces with several hundred articulated crinoids that often bear complete stalks and holdfasts.

Lodgepole crinoids generally occur in two distinct facies-controlled associations. The lower association occurs in crinoid-bryozoan packstones. Fenestrate bryozoans are unusually abundant and well-preserved in this facies and occur alongside a diverse assemblage of brachiopods, crinoids, and rare articulated trilobites. Crinoids are mostly represented as compressed, slightly disarticulated calyxes, but rare crowns also occur. The lower crinoid association is dominated by Cusacrinus and Platycrinites, but additional actinocrinitids, periechocrinids, cyathocrinitids, advanced cladids, and taxocrinids constitute important faunal elements. Such high diversity in the lower crinoid zone has not been previously recognized. Macroinvertebrates in this fauna are significantly larger than those of the upper crinoid association and are interpreted as inhabiting a more proximal environment.

The upper crinoid association occurs mostly in thin laminated mudstones. Specimens are typically preserved as crowns with at least some of their stalks. Bedding surfaces with multiple individuals often reflect low diversity. Monospecific concentrations of Platycrinites are particularly common, but similar “stands” of Rhodocrinites, Cusacrinus, Abatocrinus, Dichocrinus, and even Goniocrinus have been recovered.

Crinoid faunas of the Lodgepole Formation are most similar to those of the Wassonville Cycle of Iowa. Most genera and many species are shared between the two regions, including Gilmocrinus and Nactocrinus, which are here reported as occurring in the Lodgepole for the first time. Other similarities include the dominance of camerate crinoids and higher abundance of advanced cladids in offshore environments. Moreover, cyathocrinitids and flexibles are most common in the shallower facies of both areas.