Anthracomedusa turnbulli Johnson & Richardson, 1968 fossil jellyfish from the Pennsylvanian of Illinois, USA (public display, FMNH PE 38977, Field Museum of Natural History, Chicago, Illinois, USA).

One of the most remarkable soft-bodied fossil deposits (lagerstätten) on Earth is the Pennsylvanian-aged Mazon Creek Lagerstätte near Chicago, Illinois. In the Mazon Creek area, the Francis Creek Shale consists of concretionary gray shales. The Francis Creek concretions are composed of argillaceous ironstone, and can be fossiliferous or nonfossiliferous. The fossiliferous concretions contain land plants and terrestrial & marine animals, including nonmineralizing organisms.

This is a rare example of a fossil cubozoan jellyfish. Cubozoans are also called “box jellyfish", in reference to their perceived subquadrate bell shape. Some of the most venomous marine animals in modern oceans include species of box jellyfish.

This specimen has been compressed perpendicular to the oral-aboral axis of the jellyfish body. The central structure is the bell. Anthracomedusa had four tufts of tentacles near the periphery of the body - those are the irregularly linear structures near the margin of the concretion.

Classification: Animalia, Cnidaria, Cubozoa, Carybdeida, Carybdeidae

Stratigraphy: Mazon Creek Lagerstätte, Francis Creek Shale Member, Carbondale Formation, Desmoinesian Stage (= Westphalian D), upper Middle Pennsylvanian