There is only one known species of Tullimonstrum gregarium and its fossils have only been found at the Mazon Creek fossil site, in the state of Illinois (USA).

Fossils of this species appear a lot here. Hundreds of its fossils were also displayed at the Field Museum in Chicago after initial discoveries in the 1950s.

Based on the traces left on fossils, scientists have painted them as an animal with squid-like fins, long eye stalks, crab-like claws, a sticky upper jaw and a rather complex appearance.

The Tully Monster’s intricate body structure makes it difficult for scientists to classify it. They compared it to groups of species ranging from molluscs, arthropods, to later newly listed species such as lampreys.

“This animal is really not easy to classify, its body is so strange. It has eyes located on a long stem growing from the middle of its body, and a crab-like claw at the end of a long trunk growing from its body. There are even many other structural features of this creature that are difficult to understand. But after all, maybe the Tully Monster is just a fish,” said Lauren Sallan, who teaches at the University of Pennsylvania and is a member of the research team.

Tullimonstrum gregarium has a very strange structure that has caused scientists to debate for decades. Photo: Nobu Tamura/Wikimedia.

The first discovery of the Tully Monster was announced in 1955 by amateur fossil collector Francis Tully, leading to decades of debate as its strange properties prevent it from being classified as a monster. any group of species.

For a while, researchers classified it as a mollusc like a sea cucumber, or an arthropod like a lobster, until a new study published in March 2016 classified it. it belongs to another group of animals.

A group of researchers working at Yale University, led by paleontologist Victoria McCoy, confirmed that the straight line running along the body of that ancient creature is not the intestinal tract as previously researched. This is a bone bar, which is the basis of later spine formation.

This bony bar is an important feature that evolved into the backbone in later vertebrates, including fish, amphibians, reptiles, birds and mammals.

The team also found evidence of gill slits in some of the more than 1,200 specimens they analyzed, making it more fish-like than previous studies.

This feature may have been overlooked by previous researchers, they said, because many individuals of this species tend to die in a position that maintains the front or back rather than the sides of the body. This obscures their gill slits when petrified.

The second study by researchers at the University of Leicester (UK) also concluded that the Tully Monster is a vertebrate animal, after scanning images of this species’ eyes through an electric microscope. molecule, showing that there is a melanosome structure inside it.

This paleont had a very complex eye structure, meaning it must have been a vertebrate. The Tully Monster’s eyes may have evolved dozens of times to become more complex in structure.

However, two newly published studies only increased the number of questions about this strange creature. A new debate has broken out, scientists always want to classify it to gain an understanding of the evolution of backbones and vertebrates.