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Southern Tyrant: 10 Facts about the New Tyrannosaurus Species

Updated: Jan 21

The internet has been buzzing with news of the latest addition to the roster of prehistoric titans, one of the largest apex predators the New World has ever seen! But what kind of discovery is this, what might have been misreported by the hype-train, and what does this new addition to the lineage of Tyrant dinosaurs tell us about the past's revolving door of apex predators?

In this GojiCenter article, paleontology educator Jack Blackburn will be walking you through the dinosaur that has taken some of the Internet by storm over the past few days right out the gate of the new year.

How was it found, why was it mistaken for a T.rex, if there is likely a new species at all, and what kind of habitat and prey was this apex predator living with? 

Some of these topics might be covered in a longer format with a future video, but this will be a good summary for anyone curious about this bygone beast and wanting it without the jargon or misreporting so common in news articles. 

painting by Sergey Krasovskiy

Tyrannosaurus mcraeensis 

Name Meaning: “Tyrant Reptile from the McRae” 

Location: Southwestern USA and probably Mexico, ~73-70 million years ago 

Height: 11.5-12.5 feet (3.5-3.75 meters)

Length:  37-39 feet (11-12 meters) 

Weight: 5-8 tons (4,500-7,250 kilograms)

Diet:  Large ceratopsians and hadrosaurs, possible sauropods, small and swift prey as a teen

 #1 What Did It Look Like? How Big Was It?

In a word. Huge.

T. mcraeensis joins the ranks of not only some of the largest of the tyrant reptiles that dominated ecosystems across Asia and North America in the final chapters of the age of dinosaurs, but also finding itself amongst some of the largest carnivores that ever exist on the level of that stature. 

It still looked a lot like T.rex overall and was very close to the same size. Because all members of its family are very consistent, we can infer that the missing arm and leg material looked typical for the family. Short but strong, two finger arms; and tall, very powerful legs. Though part of the feathered dinosaur group, Coelurosaurs, Tyrannosaurus mcraeensis likely only have a mane or short cape of feathers to offset heat concerns. Most of the body was covered in tiny, pebbly scales like those found on bird feet; with short horns and knobs on the brow and cheeks. The brow didn't stick up quite as much as Tyrannosaurus rex was known to have, making for a flatter-topped head.

It might have also had a different coloration given it lived more open habitat than its more famous cousin. Certain colors are more likely in dinosaurs than others; so yellows, blacks, grays, and rusty reds are possible.

close up of a diagram using the placeholder name "Alamotyrannus" by cisiopurple

#2 How Was This Discovered? 

The original fossil was discovered in 1983 in southern New Mexico. Tyrannosaur remains had been known from the region from Texas and North Mexico as well as neighboring states, mostly from loose teeth found in isolation,  but this was the first decently complete skeletal remains. Much of the back half of the skull, most of the lower jaw, and a few other assorted elements were found. Because the remains were all found together and fit with each other, it was deduced they were all from the same individual. 

However because they looked so extremely similar to specimens of Tyrannosaurus rex and the layer of rock it was recovered from hadn’t been tested rigidly to see how old it was, it was assumed this was just a southern living T.rex. And for roughly 40 years, that’s what it had to remain. A potential enigma in a museum's safe keeping.

#3 What’s the Name Mean? 

Not what a bunch of news articles misspelling it means. Tyrannosaurus mcraeensis has the same genus name as Tyrannosaurus rex, with “Tyrannosaurus” meaning “Tyrant Reptile”. Whenever you called a T.rex a “Rex”, you were referring to it by its species name.

A genus is more or less a label used to help clarify which kinds of animals are especially closely related and very similar to each other, so it can contain multiple species. The label of “mcraeensis” as the species name is in reference to the McRae Group, a rock layer from the fossil formation the specimen from New Mexico was found at.

This name was chosen because it was important to figure out this fossil wasn’t just a T.rex, since the McRae Group’s rock layers and the fossils inside them turned out to be older than the fossil layers T.rex is found in. This was discovered by both more rigid testing of the rocks for age and determining some of the animals present were also new species and not those that lived with Tyrannosaurus rex

#4 Why Did It Take So Long to Name? 

For a fossil found in the 1980s, it might seem really strange it took just a decade shy of half a century to fully study it and find out it wasn’t just a T.rex. Many paleontologists had long suspected that the specimen and other material from the south might be a new species or genus for a long time, even informally nicknaming it "Alamotyrannus" as a sort of placeholder name. However there are several reasons why it took so long to officially describe the fossil. 

1. Costs 

Research costs a lot, in both time and money. Many researchers have to fund their own projects either out-of-pocket and or by competing to earn grants. And with so many other fossils to study and catalog, what initially looked to be just another T.rex of unremarkable attributes or completeness just didn’t seem like a priority. There was a lot of other work to be done on more unknown subjects, so the need for intensive study just slipped through the cracks. 

2. Lack of Comparison 

Tyrannosaurus rex nowadays might have the highest number of specimens to its name out of every other big meat-eating dinosaur in its size class, but it wasn’t always that way. A majority of the well-studied work on both older and new specimens of T.rex came from the 1990s onwards. That’s when more complete specimens like “Sue” were available.  So, it wasn’t until relatively recently there were enough T.rex specimens to compare to the fossils found in New Mexico and see for sure if there were any noticeable differences between them that couldn't just be explained as individual variation inside a species.

3. Due Caution 

You might’ve heard multiple cases of dinosaurs that were once thought to be different species winding up to actually to be either two different sexes of the same species or different age groups. Like how juvenile Tyrannosaurus look noticeably different than the adults, so out of context they could be mistaken for two different types of animal. Scientists wanted to be sure that any differences they found in the New Mexico dinosaur vs. T.rex couldn’t be just explained as differences between individuals for things like age or random chance variation.

These are the kind of challenges researcher Sebastian G. Dalman and company had to work against. By doing an extensive comparison of the New Mexico specimen against multiple specimens of T.rex, alongside review of the rock layers the fossil was found at to gauge their age, they were able to figure out multiple subtle, but distinct traits to clearly show the difference between the species.


#5 Mcraeensis vs. Rex 

Because most of the time fossil animals are only known from the bones and only from a few individuals, telling different but similar species apart can sometimes be very tricky business. Anybody can tell the difference between a lion and a tiger when seeing the whole thing in life, but it takes some expertise to reliably tell the difference between the skeletons. Let alone individual bones.

Assorted Lion (Panthera leo) and Tiger (Panthera tigris) skulls, see how it is very hard to tell them apart without a expert knowledge? Highlight the text to see which is which Top right, bottom right, and bottom left are the lions

Now imagine how difficult it might be when you only have a few bones to work with, many times they are broken, and you don’t have a very big reference point to work with?

Fossils of Tyrannosaurus mcraeensis (light) and Tyrannosaurus rex (dark) compared, notice some differences exist even if they are slight?

So what exactly is the difference between the two species of Tyrannosaurus? One of the biggest differences is time and location. T.mcraeensis lived in the southern portion of Laramidia, a western half of North America at that time separated from the eastern half by water (remember the map from earlier?). T.rex meanwhile lived in the middle of the continent further to the north. 

T.mcraeensis existed between 72-70 million years ago whereas T.rex lived later between 68-66 million years ago. In terms of length and height the two were more or less the same. The main difference between them are some of the proportions and configuration of the skull bones.

Don’t be fooled by some click-bait articles however, this creature isn’t likely to usurp is younger relative T.rex for the top spot just yet. There is only one specimen known for sure so far which falls well within the middle of the size range for T.rex specimens. It’s not as big as some truly massive T.rex specimens like the “Sue” and “Scotty”.

With what limited material we have, T. mcraeensis might have been less robust than its more famous relative. So even at the same length and height they might’ve had a different weight.  Overall T.rex was somewhat bulkier, with a more prominent brow and less curved lower jaw. Its teeth were more pointed but thicker. T.mcraeensis was the opposite in all categories.

#6 What did it hunt?

The subtle differences between the Tyrannosaurus species might have something to do with what prey was available to them. It's possible T.mcraeensis was relying less on a crushing bite and more on rending prey with its teeth, explaining the flatter teeth better for carving but less so for puncturing bone. Make no mistake, it absolutely could still crush bone, it was just less extremely invested in it as T.rex. Tarbosaurus, a more primitive close relative which thrived on hunting large hadrosaurs and sauropods, had some similarities in the jaw structure with T.mcraeensis looking a bit like a halfway point between Tarbosaurus and Tyrannosaurus rex. Sauropods meanwhile were extremely rare in T.rex's range, which might have called for a different tool kit to evolve in T.rex.

If this new Tyrannosaurus hunted like Tarbosaurus, it might have focused on targeting the muscular areas of a prey animal like the tail, limbs, and neck. Rushing in, the jaws would snap down with more than enough force to stab the teeth into the tissue. Then, locking its jaws down on the target, the tyrant reptile would wrench back. The thinner, serrated teeth would slice and shred flesh and cause massive blood loss. Targeting thinner areas like horns, skulls, or hips could likely crack bone. And smaller game caught by surprise would likely wind up pulped instantly. Given that social behavior has been indicated in multiple large tyrannosaur species, it's also quite possible T.mcraeensis might have either lived in a social group or formed coalitions to hunt big game. Though more fossils will be needed to know this for sure!

A Tyrannosaurus encounters the giant titanosaur, Alamosaurus, by Zubin Erik Dutta. A very rare event for T.rex, every other day for T.mcraeensis

Still for the most part the two species had very similar attributes and abilities. All of the remarkable features you might’ve heard about T.rex over the years from its keen eyesight to its complex brain to its agile frame would all be found in the tyrant of the southern wilds. In the end, both Tyrannosaurus species were very similar and would be hard to tell apart. Equally formidable and deadly to their ecosystems as the near uncontested apex predators.

#7 What about Tyrannosaurus regina and Tyrannosaurus imperator?

A very infamous paper came out recently that attempted to reclassify Tyrannosaurus rex by splitting it into three different species. Click-baiting articles certainly didn't help the confusion. However, the evidence the paper used to support its claim was very spotty at best.

Numerous Tyrannosaur experts that reviewed the paper found that the criteria and traits, which the paper tried to use to separate all of the Tyrannosaurus specimens into three species, weren't consistent. It also was brought up the differences could be easily explained as differences between individuals of the same species depending on things like age.

That is why the designation of "Tyrannosaurus regina" and "Tyrannosaurus imperator" were not accepted across a majority of paleontologists, and aren't considered very likely to be anything more than just different individual T.rex. 

The two Tyrannosaurus species, by allotyrannosaurus

The difference with the New Mexico tyrannosaur is this classification essentially did everything right that the previous paper did wrong. The criteria to name a new species was used a lot more strictly, the physical differences were found to be consistent, and there was positive of a pretty substantial gap in time for it versus T.rex

Further review and study is just name of the game when it comes to paleontology, but for now it seems possible that T. mcraeensis will go down in history as the very first new species in the Tyrannosaurus genus since the genus was named over 100 years ago. 

 # 9 Did it evolve into T.rex?

Because it is older and looks almost like a halfway point between Tyrannosaurus rex and older types of tyrannosaurs like Tarbosaurus bataar, you and a lot of others might naturally wonder if Tyrannosaurus mcraeensis might in fact be the direct ancestor to T.rex? Was it perhaps the forerunner to the king? 

The experts that worked on the specimen don’t necessarily think so. These animals were specialized in their own way and because the fossil record is incomplete, it’s unlikely T.mcraeensis was the only Tyrannosaurus species alive in its time.

Instead of a direct ancestor, it might be that the Southern Tyrant was instead a close relative of the direct ancestor to T.rex, very similar and of the same “generation” but not identical. Think of it almost like an aunt or uncle in terms of relation to T.rex. Instead, their was likely a very similar species to a still unknown giant predator that eventually gave rise to the most famous dinosaur of all. 

  # 10 The Usurping Dynasty

What it does however show is that the family that gave rise to the largest of the tyrant reptiles had moved into North America earlier than it was previously thought. Contrary to some older notions of the largest tyrannosaurs evolving from pre-existing forms in the Americas like Gorgosaurus or Daspletosaurus, the lineage for both T.rex and T.mcraeensis hails back to the largest continents. That's right, North America's giant tyrants aren’t exactly homegrown, but descendants of giant predators that roamed Asia and migrated into the New World via land bridges. For many millennia before even the oldest species of Tyrannosaurus, giant and giant killers like Tarbosaurus and Zhuchengtyrannus ruled over eastern Asia. A chance extinction event struck the northern half of Laramida North America from around 72-68 million years disrupted the local ecosystems. The native tyrannosaurs like the lean, less massive Albertosaurus found their prey either going extinct from climate changes, forced to move into new regions, or being outcompeted by larger, more heavily armed herbivores from the south. Hunting a 3 ton Pachyrhinosaurus is one thing. It's a whole different matter to square up with a 8 ton, much more heavily armed Triceratops more adapted to the new ecosystem as well. And be it from directly Eastern Asia itself where Tarbosaurus once reigned or from the south in Tyrannosaurus mcraeensis' domain, the new crop of giant tyrannosaurs bigger than any seen before soon moved in. Old herbivores out, new herbivores in. Old carnivores out, new carnivores in. A minor extinction event cleared the way for T.rex’s reign in Laramidia during the last chapter in the age of dinosaurs. 

Tyrannosaurus invading the North and displacing a native Albertosaurus, by Rudolf Hima

What T.mcraeensis shows is that these Asiatic apex predators descended from animals similar to Tarbosaurus had moved into North America earlier than was previously thought, carving out a dominion all to themselves in the south before possibly the spread into the north. The story of life for the most famous dinosaurs of all has a new chapter.

Want more tyrannosaur content?

Tired of the same old Jurassic rip-offs in your dinosaur media? Care for some lesser known science in your storytelling?

Try Prehistoria the debut short novel by GojiCenter's in-house Paleontology consultant. Told from the perspective of a unique velociraptor species, the first half of the book is a journey back in time to show the lives of bygone beasts in the manner of documentaries like Walking with Dinosaurs and Dinosaur Planet.

In this case, a look back at the wilds of Mongolia during a time two different tyrannosaur species competed and conflicted with each other. With surprised, but real-life animal behaviors rarely seen in any media. Hear their story in the first half, and then browse the second half to learn the science behind the research and speculation. Jargon free, just the details.

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