In 1905, in a lab at New York’s American Museum of Natural History, a fossil sat partially entombed in a rock.

Even in its half-prepared state, paleontologists working on the bones emerging from the American West could tell it was something extraordinary: big jaws and flesh-shearing teeth a giant predator unlike anything they'd ever seen.

But there was a problem.

It was a whole lot like something that someone else had seen.

Rumor had reached New York that a rival team had also just discovered a big meat-eater, one that sounded remarkably similar.

So even though the fossil wasn't yet fully free from the rock, the team rushed to publish and name the animal.

They called it Tyrannosaurus rex, the tyrant lizard king.

While the scientific name is iconic today, here's the thing: it wasn't the first name this dinosaur received.

It wasn't even the second.

And of all the names given to this dinosaur over the years, maybe T. rex shouldn't have been the one that stuck at all The first recorded discovery of a fossil that would later become known as T. rex was a single tooth found in 1874, by a student hiking up a Colorado mountain.

And despite being sent to paleontologist Othniel Charles Marsh, the tooth wasn’t recognized as anything important at the time.

Not long after, more fossils like it started to emerge, right around when the Bone Wars of the late nineteenth century moved into full swing.

During the bone wars, paleontologists Marsh and Edward Drinker Cope were locked in an intense rivalry, racing to discover and name as many new dinosaurs as possible.

In roughly fifteen years, they described over 130 species.

But that breakneck pace led to mistakes.

A lot of them.

In 1890, Marsh received a large foot bone, followed by a few leg bones, from a fossil collector in Wyoming.

He suspected that the bones came from a big meat-eating dinosaur and named it Ornithomimus grandis, which means grand bird-mimic, that same year.

He later described the species as a gigantic carnivore that was the most destructive enemy of the horned dinosaurs.

But little did he know, Cope was also busy naming fossils from what would turn out to be the same mysterious meat-eater.

In 1892, Cope received two vertebrae from South Dakota that were in pretty rough shape, but he thought they were parts of a new type of horned dinosaur.

He named it Manospondylus gigas, meaning giant porous vertebrae.

But what he didn’t know was that those bones belonged to the same animal that Marsh had named just two years earlier.

And believe it or not, Marsh and Cope weren’t the only ones naming this dinosaur.

In 1900, Barnum Brown, a fossil hunter working for the American Museum of Natural History was sent west to look for Triceratops fossils.

Instead, he discovered fossils of a large meat-eating dinosaur...and I’ll give you one guess what animal it turned out to eventually be.

But the quarry where he found that dinosaur also contained fossils from other animals including bony armor from a herbivorous ankylosaur.

And while studying the fossils back at the museum, paleontologist Henry Fairfield Osborn accidentally merged the two, mistakenly thinking the armor belonged to the carnivore.

He eventually named this giant plated dinosaur Dynamosaurus imperiosus, meaning the powerful imperial lizard a name almost as cool as Tyrannosaurus rex.

Now, it’s worth mentioning that Osborn was a problematic scientist with straight-up terrible views on race, immigration, and eugenics.

So while we don’t want to give him much attention, unfortunately he was at the helm of a major museum, with lots of resources and excellent fossil collectors like Brown.

And in 1902, Brown, guided by a local rancher, made another discovery for the museum: a partial skeleton from Montana, in rocks dating to the Cretaceous Period.

This specimen included parts of the skull, vertebrae, hips, and more painting a much more complete picture of the creature.

But the fossils were completely encased in hard sandstone, and fully excavating them from the ground took two summer field seasons.

And even then, the work of further removing the rock from the bones back at the museum, known as preparation, was slow and challenging.

That’s when the rumor arrived that a rival crew from the Carnegie Museum in Pittsburgh had just discovered something strikingly similar in Wyoming.

So, even though the skull back in New York was still being prepared, there was no time to wait.

Scientific priority meant that it was critical to publish a description of the fossil before someone else did.

On October 4, 1905, Osborn published a description and gave the fossil its official name: Tyrannosaurus rex.

Except at this point, it technically had four official names.

And when it comes to naming new species, there can be only one scientific name.

Which raises the question, are we calling the world’s most famous dinosaur by the wrong name?

Do we even know T. rex?

See, scientists have a set of formal rules for scientific naming.

The rules have been developing ever since the 18th century, when the tradition of assigning a two-part scientific name became popular.

For animals, living or extinct, these are called the International Code of Zoological Nomenclature.

And according to the code, naming a new species requires three things.

First, describe the species, including what makes it unique from others we already know about.

Second, choose a type specimen to represent the species, one that shows off its unique characteristics.

And finally, give it a scientific name, which must include a genus and a specific or species name, traditionally derived from Latin or Greek.

Simple, right?

Except what happens when competing scientists unknowingly give the same species two different names?

Or, in the case of T. rex, four?

That’s where The Principle of Priority comes in.

This rule says that whatever name a species was given first is its official name.

And any other names should be retired.

So, does this mean that Tyrannosaurus rex should actually be called the first name given to it, Ornithomimus grandis?

Well, not so fast the principle of priority says that you use the oldest name unless that name has been invalidated.

Meaning only a valid older name could overthrow Tyrannosaurus rex.

So, is Ornithomimus grandis valid?

Well actually, there’s a problem.

When Marsh named those leg bones in 1890, he was adding that new species to an existing genus one with its own defining characteristics.

Animals that belong to Ornithomimus have ostrich-like legs, a long slender neck, and a toothless, beaked skull.

And T. rex, of course, doesn’t have any of those things.

So, Marsh had added the dinosaur to the wrong genus, and as a result, we can’t resurrect that name because it isn’t where that dinosaur belongs.

And if that hadn’t already disqualified it, the original fossils were later reported lost.

Without the specimens, there was no way to confirm what Marsh had actually found so Ornithomimus grandis was out of the running.

But what about the second name, given by Cope, Manospondylus gigas?

Well, the vertebrae Cope used were pretty worn down, making them poor type specimens for a new species.

Because a type specimen needs to clearly show what makes the species unique something these weathered bones couldn’t do.

So even when paleontologists realized that Manospondylus gigas and Tyrannosaurus rex might be the same, it became obvious the vertebrae were in such bad shape, they couldn’t properly define the species.

And if that wasn’t enough, it turns out that one of those specimens had also been lost!

So Manospondylus gigas was invalid, too.

And that leaves just one final potential usurper to dethrone T. rex: Dynamosaurus imperiosus.

By 1906, when the skull of the Tyrannosaurus rex type specimen was finally freed from the rock, it became clear there’d been a mistake.

T. rex and Dynamosaurus imperiosus were the same dinosaur.

The plated carnivore had, of course, never existed its armor belonged to a different animal entirely.

Which, in theory, means Dynamosaurus would overthrow the Tyrant Lizard King, right?

After all, it was found first.

Turns out, no...by a single page.

Both names appeared in the same 1905 article, but Tyrannosaurus rex was described first, and Dynamosaurus imperiosus in the section right after.

This made the name T. rex technically older.

Which means that the one true name for this dinosaur is Tyrannosaurus rex.

And this name went unchallenged for almost 100 years, until an old name suddenly threatened to make a resurgence.

In 2000, scientists from the Black Hills Institute reported finding more bones from the original Manospondylus quarry where those two worn vertebrae had come from If they could prove the new fossils belonged to Manospondylus gigas, they could resurrect the 1892 name.

This would have been bad news for Tyrannosaurus rex, of course, given that the principle of priority would recognize an 1892 name before a 1905 one.

Suddenly, it seemed possible again that T. rex could be overthrown.

Luckily, people working on the International Code of Zoological Nomenclature had recently decided that they had enough of obscure, old scientific names toppling iconic ones.

On January 1, 2000, a new rule took effect that could reverse the rule of priority under special circumstances, by making a younger scientific name a protected name for a species.

To qualify, the younger name must have been in use for more than 50 years and the older name must have been published before 1899 In these cases, the younger name can become the official one and the older scientific name becomes forgotten.

So, because the name Tyrannosaurus rex has been in common use for well over 50 years, it easily meets the first step to be a protected name.

And because Manospondylus gigas was published in 1892, it means we can forget it.

So, because of a few misidentifications, a couple of lost fossils, and some important technicalities, the name Tyrannosaurus rex won out.

But here’s a funny little wrinkle: that same year the new reversal rule took effect, a paleontologist rediscovered that tooth found in Colorado back in 1874, among the fossils at Yale’s Peabody Museum.

And he recognized that it belonged to T. rex.

Which means that, among all these arguments over names, Marsh actually had the first T. rex fossil to ever fall into a paleontologist’s hands right under his nose, sitting in a drawer unidentified and unnamed.

While it may seem like naming a new species is filled with boring details and tiny technicalities, these names help paleontologists understand evolutionary relationships and track how life has evolved over time.

And it’s worth remembering that our understanding of these extinct icons is shaped by the people who found and described them as well as what they decided to call them.