Every one came out of the same quarry. Every specimen of this animal that anyone has ever found, anywhere on Earth, came out of one coal pit in northern Illinois. And nobody can tell you what it is.
It arrives as a gray lump the size of a fist. Hit it with a hammer on a cold morning, and it splits into two halves that fit back together. Pressed flat inside is something that was swimming here 308 million years ago.
Why does one pit have all of them? The lump is the answer. An animal dies and settles into the mud. It begins to rot. The rotting changes the water around the body, and that sets iron and carbon hard, like a shell, around it. The stone closes before the animal is gone. Most fossil sites keep the hard parts. This one kept the soft parts.
A pipefitter named Francis Tully found the first one in the 1950s, collecting on his weekends. He carried his lumps into the Field Museum in Chicago, expecting to be told what he had. Nobody there could tell him. So the staff called it Tully's monster, and the name stuck.
So what did he have? A soft body, banded like a shelled peanut, with a fin at the tail. Off the front runs a long snout, thin as a shoelace, ending in a claw with small teeth inside it. And the eyes are not on the head. They sit on the two ends of a stiff bar that juts straight out sideways, the way a hammerhead's do. No living thing has that combination. It is also smaller than you are picturing. Of 116 specimens that were measured, exactly one was whole enough to give a full length. Nine centimeters. It would sit in your palm.
In 2016, a team said they had solved it. A pale stripe runs the length of the body, and they read it as the rod that every backbone on Earth is built around. Inside the eyes they found grains of color stacked in layers, the way a backboned animal's eye stacks them. Their answer was a fish with no jaws, a cousin of the lamprey.
The next year another team took that apart. The stripe was in the wrong place, they said, and the wrong shape. Then the eyes went. Somebody checked squid, and squid stack those same grains in layers too. A squid has no backbone. And in 2023, a team scanned specimens in three dimensions, and the head came back in segments, in a way no backboned animal's head is.
So why can nobody settle this? The answer is in the rock. The snout is the part you would sort the animal by. Of 511 specimens examined, 14 keep a complete one. There is no bone here and no shell. What survives is a stain and an outline, flattened. And the stone set all the way through the body, so the fine detail sits inside a mineral that grew through it.
Illinois made it the state fossil in 1989. An official state animal that officially nobody can identify.
