This charming little creature  is Tullimonstrum gregarium,   but its friends call it the Tully Monster.

It’s an extinct species that lived along an  ancient coastline in what is now Illinois,   more than 300 million years ago. At first glance, you might think it’s  some sort of strange fish with stalk eyes,   or maybe a slug with a claw on its face,   or maybe even some kind of odd ocean-dwelling  worm, but what it really is … we have no idea.

No, really. Over the past 70 years, paleontologists have found  literally thousands of Tully Monster fossils,   and they’ve analyzed them over and over using  the most sophisticated techniques available,   and the more we learn, the  more confusing this animal is. So, let’s follow in these scientists’ footsteps   and explore exactly what it takes to stump  multiple generations of paleontologists. [♪ INTRO] Ever since the mid-1800s, folks  in Illinois have reported finding   well-preserved fossils in  and around local coal beds.

Starting in the early 1900s, these beds became  the focus of extensive coal mining operations,   which uncovered even more of  these fossils, and over time,   an incredible ancient ecosystem came into view. Nowadays, these rock layers are known as  the Mazon Creek fossil beds, and they date   to a little over 300 million years ago,  during the Late Carboniferous Period. At that time, Illinois was  just south of the equator,   and it was a tropical paradise  for all sorts of life.

The fossils of Mazon Creek tell the  story of an ancient river delta system. From swamp and shoreline deposits, paleontologists  have identified the remains of horsetails,   early trees, insects, amphibians, and  some of the earliest known reptiles. And in the adjacent ocean deposits,  there are fossils of jellyfish,   crustaceans, and lots of fish, including  sharks, lampreys, hagfish, and more.

Mazon Creek is an incredibly rich fossil locality. Hundreds of thousands of fossils have been  found, representing some 700 ancient species. Most of these fossils are found  hidden away in iron-rich nodules,   which split open to reveal incredibly  well-preserved fossils, often including   the remains of soft tissues like  internal organs and eyeballs.

In 1958, a fossil collector named  Francis J. Tully brought a very unusual   Mazon Creek fossil animal to the attention of  paleontologists at the Field Museum in Chicago. These animals were small, ranging in size from  around 6 centimeters to over 30 centimeters long,   with a squishy torpedo-shaped body composed  of many segments like a worm or a centipede.

On its tail are broad, triangular  flaps that look like tail fins. Just behind its head is a straight  structure that cuts across the body   and seems to support a pair of round  organs on either end – this is often   called the “transverse bar.” And on  its face is a long, spiky proboscis. Now you might be thinking, that sounds  like a really strange creature to me,   but surely Illinois paleontologists had  some idea what sort of critter this was.

And you’d be wrong. Baffled by this bizarre species,  local paleontologists and fossil   hunters took to calling it “Tully’s  monster,” or “The Tully Monster.” Eventually, in 1989, the Tully Monster would  be named the official state fossil of Illinois. In 1966, the Tully Monster appeared  in a scientific publication written   by E.

S. Richardson Jr, who described  the fossils in all their unusual glory,   and who gave the species an official  scientific name: Tullimonstrum gregarium. But Richardson mostly side-steps the  question of the Tully Monster’s identity,   simply calling it “wormlike,” and leaving  the details to be argued by other scientists.

Like a coward. And argue they did! Paleontologists have been debating the true  identity of the Tully Monster ever since.

We’ll get to why it’s so  hard after this short break! This SciShow video is supported by DeleteMe,   the subscription service that provides  set it and forget it peace of mind. When you sign up, their privacy  experts reduce your risk of phishing,   doxing, swatting, and harassment.

They’ll continuously remove your data  using their own tech and no third parties. Or, if you set it and don’t forget  it, DeleteMe makes real human   privacy advisors available for custom  requests and help when you need it. This service has been used by judges,  journalists, and our executive producer!

They reviewed over 1,000 records for her and saved  her 34 hours of reviewing and removing listings. You can get 20% off DeleteMe consumer plans  when you go to joindeleteme.com/SCISHOW and   use promo code SCISHOW at checkout,  or use the QR code on screen. Believe it or not, identifying fossil  animals is often pretty straightforward.

You just have to find the distinguishing   characteristics of whatever group  of life that animal belongs to. When paleontologists can’t identify an  ancient animal, it’s usually because the   fossils aren’t preserved very well, and those  distinguishing features are missing or obscured. There are many, many such cases, but  the Tully Monster isn’t one of them.

We know a ton about this species’ anatomy. Because Thousands of Tully  Monster fossils have been found,   and lots of them are really well-preserved. Scientists have been able to deduce that the  transverse bar is a pair of stalked eyes,   and that its long proboscis was flexible  and ended in a toothy, mouth-like structure.

They’ve even observed its internal organs! With all this information to work with,   you’d think figuring out the Tully Monster’s  evolutionary relationships would be easy. But the problem is, the features we can  identify just don’t make sense together.

There’s no one group they all fit with. It’s clearly an animal, and it has bilateral  symmetry, meaning its left and right half   are a mirror image – just like us and  insects and worms and lots of others. This narrows things down a bit, but not much.

Some scientists have suggested the Tully Monster  might be a relative of a group called ribbon   worms, also known as proboscis worms, which –  as you might have guessed – have a proboscis! But these worms don’t have the  same shape of proboscis that the   Tully Monster does … also, they don’t have eyes. Others have compared it with  polychaetes, or bristle worms,   which also have a proboscis and  they have eyes, but the Tully   Monster doesn’t have the bristly appendages  or the distinctly-shaped head of these worms.

Some have pointed out that the Tully  Monster’s proboscis looks similar to   the toothy, tongue-like radula of  molluscs like octopus and snails,   but the Tully Monster is missing a lot of  the internal organs we see in molluscs. Still others have compared the Tully Monster  to the bizarre animals of the Cambrian Period,   like the stalk-eyed, claw-faced Opabinia,  which are thought to be cousins of arthropods. But these animals are separated in time from  the Tully Monster by around 200 million years.

Somewhere within that squishy body are the   features that link the Tully  Monster to its true cousins,   but paleontologists have had a really hard time  figuring out exactly which features those are. The reality is that the Tully Monster doesn’t  look very much like any other known animals. Over the course of its evolution, it’s either  lost some key features of its ancestors,   or it’s evolved new features that are similar  to those of its distant relatives … or both!

The end result is an animal that has become  very different from its closest cousins. For example, one author in 2025  suggested a possible relationship   with a group of mollusks called sea elephants. These are a type of sea slug with long,  round bodies, a flexible proboscis,   and a pair of well-developed eyes – all  pretty similar to the Tully Monster.

But the Tully Monster is missing some  of the iconic features of sea elephants,   such as the swimming fin and stabilizing organs. So if it is a sea slug, it must have  evolved away those features at some point. And if it isn’t a sea slug, it’s evolved  a bunch of sea-slug-like features which   were probably used for sea-slug-like behaviors.

That’s what makes this so tricky. You have to figure out which features  come from its ancestors in its group,   and which happened along later. As the decades have passed, paleontologists  have taken a closer and closer look at the Tully   Monster – down to the molecular scale – trying  to find the clues to finally solve this puzzle.

And in 2016, it seemed for a moment  that the answer had finally been found. That year, two separate studies found evidence  to suggest that the Tully Monster … was a fish! The first of those studies took a scanning  electron microscope to the Tully Monster’s   eyes and found that they were made up of  layers of tiny structures called melanosomes.

This structure, they pointed out, was  only known from one group: vertebrates. The other study examined over a  thousand Tully Monster specimens,   paying close attention to the  structure of its various features. Among their findings was that one of its internal  structures, a long, narrow feature which had been   previously identified as part of the guts,  was actually the remains of a notochord.

A notochord is a rod of tissue that  runs down the back of chordate animals,   a group that includes vertebrates  and their closest relatives. That study also identified a handful of  other vertebrate features, including angled   blocks of muscle tissue, gill slits, and  vertebrate-like teeth on that proboscis. With this collection of anatomical  features, they suggested that the   Tully Monster belongs to an early branch  of aquatic vertebrates – that is, fish!

More specifically, they proposed it was a  jawless fish closely related to lampreys. That team even ran a statistical comparison  between the features of a bunch of different   fossil critters, and the numbers grouped  the Tully Monster with lamprey-like fish. Now, you might be wondering how all  those distinguishing features could   have gone unnoticed for so long, and  the researchers wondered the same thing.

They pointed out that fossilization  might have been getting in the way. Some of these features had been degraded so  much that they had become unrecognizable. And the Tully Monster’s tail – which  is flattened top-to-bottom instead   of side-to-side like in fish – could have  become twisted during the burial process,   giving it a falsely un-fish-like appearance.

And so, 2016 was the year that the Tully  Monster was officially identified as a fish. It was a very neat and satisfying solution   to this long-standing mystery …  perhaps too neat and satisfying. In the years that followed, numerous other  studies came out which cast doubt on … well,   just about all of those findings.

Some paleontologists pointed out that those   vertebrate features couldn’t  have been vertebrate features. The supposed notochord extends in  front of the Tully Monster’s eyes,   which doesn’t happen in vertebrates. The supposed gill slits are too widely spaced and   some are present in the wrong parts  of the body, like near the tail.

And those alleged muscle segments don’t run   the full length of the body  like they do in vertebrates. And on top of all that, there are lots of fish  preserved in Mazon Creek, including jawless   fish like lampreys and hagfish – and none of  them ever preserve those anatomical features. They had those features, but we can’t see them.

So, either the Tully Monster had  these iconic vertebrate features,   but they were all unprecedented  shapes and fossilized uniquely   well for some reason … or those are not  iconic vertebrate features after all. That same study also pointed out  some problems with the statistical   comparison that linked the Tully Monster to fish. See, a statistical analysis is  only as good as the data it uses.

And some of that data had apparently been based   on outdated interpretations of  some of the Mazon Creek fossils. That analysis also barely included any  invertebrate animals, so the only things   the Tully Monster could be compared to were  vertebrates and their closest relatives. So, jawless fish might have been the best  available match, but the options were limited.

A few years later, a 2019 study revisited  the topic of the Tully Monster’s eyes. As I said, the 2016 study found that the eye   structure of the Tully Monster  was a lot like vertebrate eyes. But it turns out not much research had been done   on the eye structures of other  animals, like squid or octopus.

The 2019 study found that these animals  also have eyes composed of melanosome   layers, meaning this is not a sure-fire  identifier of a vertebrate animal. They also did a spectroscopic analysis  on the Tully Monster’s eyes to determine   their chemical composition, and its  chemistry didn’t match fish or octopus! And so, just a few years after it seemed  the mystery had been solved, a whole pile   of scientific evidence had accumulated to say  that the Tully Monster wasn’t in fact, a fish.

But then in 2020, another team of researchers  did their own chemical analysis on the various   fossilized tissues of the Tully Monster and a  whole bunch of other Mazon Creek fossil animals. They found that these chemical signals could  reliably distinguish between invertebrate and   vertebrate animals, and the Tully Monster  had the chemistry of a vertebrate animal! And just like that, vertebrates rejoined the long   list of potential suspects for  the Tully Monster’s identity.

It has some features that seem to say vertebrate,  and others that seem to say absolutely not. The Tully Monster is either a very unusual fish,   or an invertebrate that evolved  some very fish-like features. And we still don’t know which.

The story of the Tully Monster is an  incredible tale of scientific mystery. But more than that, it’s a beautiful demonstration  of how paleontologists do what they do. Sorting through the tangle of debates and  hypotheses reveals a whole bunch of the   different tools and techniques that  scientists use to identify fossils,   as well as the shortcomings and  potential flaws of those methods.

The Tully Monster forces us to look past the  question of how you can identify a fossil,   to the much more interesting question of why there  are some fossils we can’t seem to figure out. Someday, surely, paleontologists will find the   right combination of clues  to put this debate to rest. And as much as that will be a day for celebration,   it will be bittersweet to see the end of  this fascinating scientific discussion.

Fortunately, there are plenty of  other mystery fossils out there. [♪ OUTRO]