A NEW SPECIES OF CLAM SHRIMP FROM A PECULIAR GREEN CHERT FACIES OF THE JURASSIC MORRISON FORMATION, WYOMING

During fieldwork on the Morrison and Cloverly Formations in Natrona County (WY), Copeland MacClintock collected several fossil clam shrimp. The clam shrimp were found in a two-foot thick bed of green chert near the top of the Morrison Formation in the south Square Top Butte section. The fossil clam shrimp stand out with a slight blue color when compared to the green matrix. This unusual, fossil-bearing lithology was interpreted as a secondarily chertified claystone. At the time of deposition, this bed likely represented a floodplain with ephemeral ponds.

In comparison to Asia, clam shrimp are very poorly known in the Mesozoic of North America. In the Jurassic of the western US, clam shrimp have only been reported from the Moenave Fm., Morrison Fm., and Navajo Sandstone. The clam shrimp from the south Square Top Butte section seem to be distinct from the forms reported from the Morrison Formation in Colorado and Utah. The clam shrimp studied herein are tentatively assigned to the family Loxomegaglyptidae. Future study will clarify the relationship of the Morrison Formation clam shrimp to those elsewhere in the world.