THE RAFT OF THE MEDUSA: EXCEPTIONAL PRESERVATION OF A BOX JELLYFISH FROM A LATE CARBONIFEROUS LAGERSTÄTTE IN THE MIDCONTINENT OF NORTH AMERICA

Well-documented Late Paleozoic Lagerstätten are rare, but other potentially significant deposits have been virtually ignored in the modern literature. Notable examples occur in Carboniferous black shales of the midwestern United States, in strata deposited in either restricted nearshore environments or brackish lagoons. These deposits contain sharks, bony fishes, brachiopods, bivalves, oligochaetes, cnidarian medusae (jellyfish), and algae. Fossils are preserved in a variety of ways, most notably as carbon films.

Here we describe an overlooked but spectacular medusa from one of these deposits, the Late Carboniferous (Desmoinesian) Mecca Shale Member of the Linton Formation. This specimen – long ago consigned to the collections of the Field Museum of Natural History – was collected from a quarry in western Indiana in 1973. It represents a cubozoan (box jellyfish) preserved in part and counterpart along a shale parting.

This fossil resulted from a complex taphonomic sequence. The bell is filled with well-sorted fine to medium quartz sand, while the extremities are carbon films with a sand and silt coating. A body of pure quartz sand is very unusual in the middle of a black shale bed; this resulted from sediment rafting by the jellyfish, a process analogous to ice rafting. The medusa apparently stranded on a beach, ingested sand as it attempted to free itself, and then was washed or rafted into a lagoon where it was buried in anoxic mud.

The specimen is tentatively assigned to Anthracomedusa, a genus that occurs with very different preservation in the contemporaneous Mazon Creek Lagerstätte (Essex fauna). Its features are entirely consistent with those of extant chirodropid cubozoans, including fine preservation of tentacles, pedalia, pedalial canals, and possibly velarium. Together, these features permit morphologic comparison far beyond what can generally be done with fossil medusae.

The preservation of this fossil and of different jellyfish at other black shale sites in the Midwest, along with the associated biotas, indicates that such sites may contribute significantly to the Late Paleozoic Lagerstätten record.