Rare, Fossil Horseshoe Crab (Euproops) Pos/Neg - Mazon Creek, Illinois

What we have for you is an extremely rare specimen of extinct horseshoe crab from Grundy County, Illinois. Adding to the rarity of this fossil is the inclusion of both the positive and negative concretions of rock. This astonishing horseshoe crab is of the species, Euproops danae, which scuttled the intertidal zones of Pennsylvanian (320-300 mya) eastern United States until its demise in what is thought to be a massive flood. Euproops danae is commonly collected from terrestrial sites at Mazon Creek, suggesting that this horseshoe crab may have been a freshwater species. This species of Xiphosurid (horseshoe crabs) is an early relative of the few extant species of horseshoe crab that live today.

This particular specimen is nicely preserved with many shell ridges still visible. This species is notable for its rather large head-shield with trailing spines, which can be observed on the positive side of the rock. Euproops fossils are often found completely missing the tail because of the soft-tissue joint between the body and tail. This specimen has the distinct impression of the tail in the negative and a thick ridge on the positive where the tail should be which makes this piece exceedingly more collectable.

Grundy County is home to a portion of the Francis Creek Shale. This formation dates back to the Carboniferous-age, roughly 355 to 300 million years ago. During the carboniferous epoch, much of the midwestern United States was submerged in tropical waters or shorefront property. This is why many of the fossils extracted from the Francis Creek Shale are marine fauna.