Tullimonstrum gregarium, commonly known as the Tully Monster, is an extinct, soft-bodied marine animal from the Carboniferous period. Its unique features include a long, flexible proboscis with claw-like structures and a pair of stalked eyes. It inhabited shallow coastal waters, likely feeding on small prey.

Tullimonstrum gregarium, commonly known as the Tully Monster, is a prehistoric species known only from the Mazon Creek fossil beds in Illinois, USA. These fossils date back to the Pennsylvanian subperiod of the Carboniferous period, approximately 307 to 309 million years ago.

During the time Tullimonstrum gregarium existed, the area that is now Illinois was a coastal delta near the equator, characterized by warm and humid climatic conditions. The environment likely consisted of shallow marine waters, estuaries, and coastal swamps.

Tullimonstrum is believed to have been a soft-bodied, free-swimming creature inhabiting the murky waters of the coastal delta environment. Its morphology suggests it may have been a predator or scavenger, using its elongated proboscis with a claw-like structure to grasp prey or detritus. The exact ecological niche of the Tully Monster remains a subject of scientific inquiry, due to its unique anatomy and lack of clear modern analogs.