What is the truth about the "mutant" of Tully, the mysterious organism that has puzzled scientists for half a century and lived about 300 million years ago?

The fossil of Tullimonstrum gregarium, a mysterious animal that lived about 300 million years ago, has surprised paleontologists for more than half a century, as it is difficult to classify because of its strange body formations.

Recently, a group of researchers proposed a hypothesis that the Tully monstrosity belongs to vertebrates, and that it resembles unjawed fish such as tubefish and mudfish. If so, Tully may fill a gap in the history of early vertebrate evolution, but there is considerable discrepancy in recent studies on its classification.

More recently, a research study by Japanese researchers, published in the journal Palaeontology on April 16, used three-dimensional imaging technology and revealed detailed characteristics of a Tully teratomy that strongly suggest it is an invertebrate. However, his exact classification and the type of invertebrate to which he belongs has not been determined.

A strange monster in a well-preserved location

In the fifties, Francis Tully was enjoying his hobby of searching for fossils at a site known as Mazon Creek Lagerstätte in Illinois, United States, when he discovered what would later be known as the "Tully Mutant."

But the 15-centimeter marine monstrosity, dating back nearly 300 million years, has become a mystery to scientists since its discovery, as they have not been able to place it in a particular taxonomic position among the chain of organisms.

In contrast to dinosaur bones and creatures with solid armor that are often found as fossils, Tully's mutant was soft-bodied.

Maison Creek Lagerstadt is a very popular place among paleontologists and geologists, with a wide variety of rocks and mud containing fossils of marine animals, flora and wildlife dating back to the Late Carbon Period (about 300 million years ago).

The site is unique, since the fossils it contains are often soft to the touch and carry subtle details of the organisms that lived in that period, which makes it possible to deduce valuable information about the development of life at that time.

In 2016, a group of scientists in the United States proposed a hypothesis suggesting that Tully's mutant was a vertebrate animal. If true, this may decipher the mystery of how vertebrates evolved.

Maison Creek retained the details of Tully's monster body very accurately (University of Tokyo)

Tully's Monster controversy settled

Despite considerable efforts, studies have been published in recent years, some supporting and some rejecting this hypothesis. But a recent study by a team from the University of Tokyo and Nagoya University of Japan put an end to this debate.

The team studied more than 150 fossilized mutants and more than 70 other fossils of various animals at the Maison Creek site, and the team was able to create colorful three-dimensional maps of those fossils, which showed the exact variations found on the surface of these fossils.

These three-dimensional data showed that features previously used to classify Tully's teratoma as a vertebrate animal are not actually consistent with those found in vertebrates.

According to the report published by the University of Tokyo, Tomoyuki Mikami, a researcher at the National Museum of Nature and Science and leader of the study, said, "Based on a number of evidence, the hypothesis that a mutant is a vertebrate cannot survive, as a mutant has a division in the area of its head extending from its body, and this trait is unknown in any lineage of vertebrate, suggesting that it is related to non-vertebrates."

Although the researchers are confident in this study that Tully is not a vertebrate, in the next step they must still classify the group of organisms to which Tully terogen belongs, as it may be invertebrate chordates (such as fish-like animals known as seminators) or a species of protozoa (a variety of animals including, for example, insects, roundworms, earthworms and snails).