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Monday Museum #6: Box Jelly

November 17, 2009

One of the functions of museums is to serve as treasure houses, showing the visitors fabulous objects that they cannot view anywhere else. Sometimes those treasures are known globally, such as the Elgin Marbles or Tyrannosaurus Sue. Other treasures are obscure, and may be well-known only to a few professionals. In the latter instances, it is gratifying when a museum takes the trouble to exhibit its best material, when perhaps less valuable and sensitive “pretty good” pieces might suffice for 99% of the visitors.

This fossil cubozoan (box jellyfish) in the Mazon Creek exhibits of the Chicago Field Museum’s splendid Evolving Planet gallery is just such a treasure. I have studied the fossil jellyfish collections at various museums, and I think that this exhibited specimen is not only the best Mazon Creek cubozoan, but that it may actually be the best fossil cubozoan “in captivity.”

Mazon Creek fossils occur in iron carbonate concretions in the Carboniferous Francis Creek Shale Formation of Illinois; for many years they have been collected in large numbers from the spoil heaps of open-pit coal mines. Many were studied scientifically and published in the 1960s to 1990s and several hundred species are known. The Field Museum has one of the finest collections of these fossils. In a windowless storeroom, secreted in the space formerly occupied by a fresh air shaft, row upon row of cabinets hold many thousands of Mazon Creek fossils that have been assigned to hundreds of species: worms, plants, jellyfish, shrimps, centipedes, scorpions, and many other groups including the wonderfully named and enigmatic Tully monsters (Tullimonstrum).

The cubozoan Anthracomedusa turnbulli is relatively rare in comparison with some of the other Mazon Creek jellyfish, but to my eye it is the most beautiful and interesting species. Mazon Creek jellies are preserved as external impressions in the siderite concretions: you can see the shape of the animal, the tentacles and the bell (“umbrella”), but there is usually almost no evidence of internal features. What makes this particular specimen unusual is that it not only has superb external preservation (or superb for a dead, squashed jellyfish, anyway), but it also shows a pyritized (“fool’s gold”) structure in each corner of the body. I have never seen pyritic structures in other Mazon Creek jellies, and I have looked at hundreds of them. I am not an expert on cubozoans, so I am not absolutely certain what these structures are, but they could possibly be evidence of the fleshy pads called pedalia.

In modern seas and oceans, cubozoans are a widespread, fascinating, and somewhat bizarre group of animals. They include some of the most poisonous creatures on the planet, such as the Australian box jellyfish Chironex fleckeri, and show some unusual adaptations such as highly evolved eyes. For many years, Anthracomedusa was the only known fossil cubozoan, but in recent years much younger ones have turned up in the Jurassic of France, and possibly in the Cambrian of Utah. Still, this Field Museum fossil stands out as a remarkable piece: a specimen worthy of detailed study, and a wonderful exhibit to share with the museum-visiting public.

Addendum (Nov. 18): After I posted this last night, the link to this new paper on cubozoan evolution showed up in my inbox this morning.  If you are interested in this subject, there is a lot of good research going on.

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