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.