Bear Gulch Aenigmacaris Paleozoic Fossil Shrimp

Aenigmacaris cornigerum

Subphylum Crustacea, Class Malacostraca, Order Cardoida, Family Aenigmacaridae

Geological Time: Mississippian (~320 m.y.a.)

Size: Shrimp fossil is 102 mm long

Fossil Site: Heath Shale Formation, Bear Gulch Limestone, Fergus County, Montana

Aenigmacaris cornigerumThe Bear Gulch Limestone is a deposit of some 70 square km in extent and 30 m in depth that has been a source of one of the most diverse assemblages of fossil fish with some 110 species having been described over the past 30 years. Most were new to science, and provided a unique view of the marine environment of Mississippian times. Fine preservation of both fish and invertebrates is a hallmark of these deposits, presumably due to an anoxic depositional environment. This specimen is a fine part/counterpart example of the shrimp known as Aenigmacaris. The genus derives its name from the fact that the describers were unsure what subclass or superorder of Malacostacans it should be assigned to (enigmatic), while the species name refers to a hornlike process on the carapace. Its nearest relative appears to be JoanellIa from the older British Visean deposits. The shine is due to a carbonized remnant of the chitinous exoskeleton of the specimen.

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