Check the listing for details. VERY RARE EUPROOPS HORSESHOE CRAB WITH EXTREMELY LONG TAILSPINE - MAZON CREEK!. Listed at 359.99 USD. This is a very rare invertebrate fossil. The ancient arthropod is very inflated and 3-D and has been fossilized in a large, round siderite concretion. (Horseshoe Crab). Formation: Carbondale Formation, Francis Creek Shale.