The Tully Monster

Every now and again, scientists discover fossils that are so bizarre they defy classification, their body plans unlike any other living animals or plants. The Tully Monster, a 300 million-year-old fossil is one such creature.

Amateur collector Francis Tully found the first of these fossils in 1955 in a fossil bed known as the Mazon Creek formation. He took the strange creature to the Field Museum of Natural History, but paleontologists were stumped as to which phylum Tullimonstrum belonged.

The species Tullimonstrum gregarium, "Tully's common monster", as these fossils later were named, takes its genus name from Tully, whereas the species name, gregarium, means "common", and reflects its abundance. The term monstrum ("monster") relates to the creature's outlandish appearance and strange body plan.

The fossil remained a puzzle, and interpretations likened it to a worm, a mollusc, an arthropod, or a vertebrate. Since it appeared to lack characteristics of the well-known modern phyla, it was speculated that it could be representative of a stem group to one of the many phyla of worms that are poorly represented today.

 It was suggested that similarities to Vetustovermis planus were present. Others pointed to a general resemblance between Tullimonstrum and Opabinia regalis, although noted that they were too distant.

At first glance, Tully looks slug-like. But where you would expect its mouth to be, the creature has a long thin appendage ending in what looks like a pair of claws. Then there are its eyes, which come outward from its body on stalks.

Tully is so strange that scientists have even been unable to agree on whether it is a vertebrate (with a backbone, like mammals, birds, reptiles and fish) or an invertebrate (without a backbone, like insects, crustaceans, octopuses and all other animals). In 2016, a group of scientists claimed to have solved the mystery of Tully, providing the strongest evidence yet that it was a vertebrate. But even then, there are still arguments being made whether this creature is an invertebrate or a vertebrate.

There have been many attempts to classify the monster. The majority of these studies have focused on the appearance and some of its more prominent features.

The body plan of the Tully Monster is so unusual in it’s entirety that it will greatly expand the diversity of of whatever group it ultimately belongs to, changing the way we think about that group of animals.

Video here and here about even more information on this topic.