Shark Egg Case (Palaeoxyris prendeli) - Mazon Creek

Palaeoxyris sp. and other shark egg cases were discovered roughly 175 years ago and because of their association with plants material in Mazon Creek concretions they were initially classified as plants. Not all researchers agreed, and only years later was the relationship with modern forms of spiral shark egg capsules established. These long, narrow, spindle-shaped fossils have surfaces marked by weak spiral lines in a helix. At least 9 species of sharks used the estuaries in the Mazon Creek area, which makes it the most diverse fossil shark nursery known. Egg cases other than those of sharks from the Paleozoic are extremely rare in the fossil record. However, many unique eggs have now been discovered in Mazon Creek fossils. The exact types of animals that laid these eggs are unknown, but there must have been at least several.