GARDEN CITY OF ECHINODERMS: A NEW EARLY ORDOVICIAN LAGERSTÄTTE FROM IDAHO AND UTAH

The Early Ordovician was a time of rapid evolution among marine invertebrates and was a formative interval for the Paleozoic Evolutionary Fauna. During this time, many echinoderm groups first appeared, including crinoids, rhombiferans, and asteroids. However, knowledge of basal echinoderm phylogeny and ecology is limited by the abrupt appearance of many echinoderm classes and few complete specimens.

A new Early Ordovician echinoderm fauna from the Garden City Formation of southeastern Idaho and northeastern Utah adds significantly to the known diversity of the earliest Ordovician echinoderms, and especially for crinoids, has the potential to reveal important information about their evolutionary and ecological history. The Garden City Formation preserves a surprisingly rich and abundant echinoderm fauna, including the world's stratigraphically oldest crinoids and North America's youngest cornute stylophorans. To date, nearly 200 echinoderms have been collected from the Garden City, most since 2003. These include (in order of decreasing diversity) at least 33 species of crinoids, eocrinoids, edrioasteroids, rhombiferans, asteroids, mitrates, and cornutes.

The Garden City Formation lies stratigraphically between the Upper Cambrian St. Charles and Middle Ordovician Swan Peak Formations. It ranges in thickness from 370-550 m (1200-1800 ft.) and is of early Ibexian to early Whiterockian age. The lower half of the section, dominated by carbonate mudstones and (intraformational) flat-pebble conglomerates, has yielded all of the articulated echinoderms. Most specimens are buried atop small (<0.5 m) thrombolitic mound layers, many still attached to mound surfaces. Articulated echinoderms associated with similar mound horizons are known from the coeval Fillmore and Wah Wah Formations of western Utah, but the composition of the Garden City Lagerstätte is notably different. For example, camerate crinoids are much more abundant and diverse in the Garden City Formation. This may be attributed to differences in depositional environments; the Fillmore and Wah Wah represent mixed carbonate-siliciclastic settings, whereas the Garden City is dominated by carbonates. Although Garden City echinoderm studies are in their infancy, it is clear that this fauna provides an important new window into the Great Ordovician Biodiversification Event.