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This picture shows the carapace of a decapod crustacean, the group that includes crabs, crayfish, lobsters, and shrimp. More specifically, it is Imocaris tuberuculata, a crab generally considered to be in the group Dromiacea, within Brachyura. The name means crab from the Imo Formation, which is where it was found by Frederick Schram and Royal Mapes in a roadcut along I-65 near Leslie, AR. The rocks around Leslie are a great place to hunt for invertebrate fossils, numerous specimens have come from there. Imocaris is very rare, but quite distinctive, with a carapace that looks like a frog-headed bodybuilder wearing enormous sequined parachute pants.
Imocaris is an intriguing fossil in that the Imo Formation is thought to be Carboniferous in age, in the Upper Mississippian Period roughly 320-330 million years old. Even though the fossil record of decapods goes back to the Devonian Period, few exist in the Paleozoic, not really hitting their stride until the Mesozoic Era. The fossil record of crustaceans as a whole go all the way back to the Middle Cambrian over 500 million years ago, with specimens found in the Burgess Shale. Thus, the true origin of the crustaceans must be even earlier than that, probably some time in the early Cambrian or the Ediacaran, the latest stage of the preCambrian Era.
The Arkansas Geological Survey calls the Imo Formation as a member of the Pitkin Limestone Formation. The Imo is a shale layer interspersed with thin sandstone and limestone layers found nearthe top of the Pitkin Formation. The Imo, and the Ptikin in general, demonstrate a shallow marine environment indicated by the limestone and an abundance of marine fossils. Most of the fossils are invertebrates showing off a thriving coral reef system, but you can also find conodonts and shark teeth as well. The Pitkin Limestone sits on top of the Fayetteville Shale, itself well known for fossils, particularly cephalopods. The boundary between the two can be seen along I-65 closer to Marshall.
Frederick Schram & Royal Mapes (1984). “Imocaris tuberculata, n. gen., n. sp. (Crustacea: Decapoda) from the upper Mississippian Imo Formation, Arkansas”. Transactions of the San Diego Society of Natural History 20 (11): 165–168.