“Tully Monster” - Tullimonstrum gregarium

This specimen was collected in northeastern Illinois in the Mazon Creek. The fossils formed approximately 307 million years ago and are preserved in ironstone concretions. These concretions often preserved hard tissues as well as rarer soft-bodied animals, like this “Tully Monster.”

Tullimonstrum gregarium, with its strange body plan, has eluded classification for some time. It has been called a worm, an arthropod, and a mollusk, but most recent studies place it as a basal vertebrate.