Enigmatic Mazon Creek Tully Monster

Tulimostrum gregarium

Geological Time: Pennsylvanian (~300 million years ago)


Size (25.4 mm = 1 inch): Fossil is 80 mm and 70 mm long on a 95 mm by 43 mm and 73 mm by 40 mm nodule pair

Fossil Site: Francis Creek shale, Braidwood, Illinois (Mazon Creek)

Tulimostrum gregariumDescription: The Mazon Creek deposits of the region near Braidwood, Illinois rival the other famous Lagerstatten of the Burgess Shale, Solnhofen, and Liaoning for the variety and quality of preserved fossils. Many exquisitely preserved specimens are found in the ironstone nodules that make up the deposits. This specimen is the posterior half of the famous Tully Monster, named for Francis Tully who first collected it in 1958. It is endemic to Illinois, and is the state fossil. It shows the feature posterior to the “eye bar”. The true affinities of this mystery animal are not known, some scientists think it may be related to the Gastopoda (snails), but this is far from decided at this point. Tullys are not often seen in a complete state, and this is a fine partial specimen.

click to enlarge

Tully Monster

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