Is it an invertebrate? Is it a vertebrate? It's the Tully monster!

Further reading:
3D Tully monster probably not related to vertebrates
Has the "Tully monster" mystery finally been solved after 65 years?

Possibly what the Tully monster looked like while alive:



Show transcript:
Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.
This week we’re going to learn about an ancient creature surrounded by mystery. When I was working on last week’s updates episode, I found some new information about it and intended to include it as an update. Then I realized I was referencing a Patreon episode, which I also reworked into a chapter of the Beyond Bigfoot & Nessie book. So instead, I included the new information in this episode all about the Tully monster.
In 1955, an amateur fossil collector named Francis Tully discovered a really weird fossil. This was in one particular area of Illinois in the United States, roughly in the middle of North America. The fossil was about six inches long, or 15 cm, and Tully thought it resembled a tiny torpedo.
He took the fossil to the Field Museum of Natural History in Chicago in hopes that somebody could tell him what his fossil was. The paleontologists he showed it to had no idea what it was or even what it might be related to. It was described in 1966 and given the name Tullimonstrum, which means Mr. Tully’s monster, which is pretty much what everyone was calling it already.
300 million years ago, in what is now the state of Illinois, a strange animal lived in the shallow sea that covered part of the area. The land that bordered this sea was swampy, with many rivers emptying into the ocean. These river waters carried dead plant materials and mud, which settled to the bottom of the ocean. When an animal died, assuming it wasn’t eaten by something else, its body sank into this soft muddy mess. The bacteria in the mud produced carbon dioxide that combined with iron that was also present in the mud, which formed a mineral called siderite that encased the dead animal. This slowed decay long enough that an impression of the body formed in the mud, and as the centuries passed and the mud became stone, the fossilized body impression was surrounded by a protective ironstone nodule. That’s why we know about the soft-bodied animals from this area, even though soft-bodied animals rarely leave fossil evidence.
So what did this weird animal look like?
The Tully monster was shaped sort of like a slug or a leech, and it had a segmented body. Its eyes were on stalks that jutted out sideways, although the stalks were more of a horizontal bar that grew across the top of the head. The tail end had two vertical fins, which argues that the Tully monster was probably a good swimmer. But at the front of its body it had a long, thin, jointed proboscis that ended in claws or pincers lined with eight tiny tooth-like structures.
It’s easy to assume that the pincers acted as jaws and therefore the proboscis was a mouth on a jointed stalk, but we really don’t know. The Tully monster may have used its proboscis to probe for food in the mud at the bottom of the sea, but because the proboscis had a joint, it probably couldn’t act as a sort of straw. The pincers may have grabbed tiny prey and conveyed it to a mouth that hasn’t been preserved on the specimens we have.
The Tully monster resembles nothing else known, and is so bizarre that researchers aren’t sure where to place it taxonomically. And it wasn’t rare. Paleontologists have since found lots of Tully monster fossils in the Illinois fossil beds, known as the Mazon Creek formation. The Mazon Creek formation is also the source of highly detailed fossils of hundreds of other plant and animal species, including some that have never been found anywhere else.
Scientists have suggested any number of animal groups that the Tully monster might belong to. It might be a type of arthropod, a mollusk, a segmented worm…or it might be a vertebrate.