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siats-tyrannosauroids-dinosaur.

The recently discovered dinosaur may have weighed more than four tons and stretched nearly the length of a school bus.

Illustration by Jorge A. Gonzalez

Brian Switek

for National Geographic

Published November 22, 2013

Tyrannosaurs reign as the most famous of all meat-eating dinosaurs. But they didn't always dominate, suggests the newly discovered bones of a massive carnivorous dinosaur that lived 98 million years ago.

Named Siats meekerorum (pronounced "See-atch"), the dinosaur discovered in eastern Utah by paleontologists was a previously unknown "apex," or top, predator that ruled long before North America's tyrannosaurs came to power. (See also: "Smallest Meat-Eating Dinosaur Discovered in North America.")

In the Nature Communications study published today, Lindsay Zanno of North Carolina State University and Peter Makovicky of Chicago's Field Museum of Natural History add to our knowledge of gigantic dinosaur predators prior to the days of Tyrannosaurus rex, which lived some 67 million years ago.

At full size, the two-legged carnivore may have weighed more than four tons and stretched nearly the length of a school bus.

The discoverers report that the dinosaur's first (or genus) name is a tribute to its predatory prowess. In the legends of Utah's native Ute tribe, "Siats" is the name of a voracious monster.

Black Bone Discovery

Zanno knew she had discovered a significant dinosaur as soon as she happened across a collection of black bone fragments sitting on the surface of eastern Utah's Cedar Mountain Formation.

Remains of large carnivorous dinosaurs are rare in these rocks. "We had no idea how much would be in the ground," Zanno says, "but we were stoked because we knew immediately we had a larger theropod and that it was going to fill in a huge gap in our understanding of theropod evolution on the continent."

The discovery team recovered a partial skeleton including vertebrae, parts of the hip, the lower leg, and toes.

At first, Zanno says, she expected that Siats would be something like the huge, sail-backed carnivore Acrocanthosaurus, but the new dinosaur turned out to be something else. Distinctive anatomic features on the bones mark Siats as a newly recognized type of predator called a neovenatorid, cousins of the earlier, well-known Allosaurus.

Similar to a previously discovered dinosaur called Neovenator, Siats would have sported a pointier, less blocky head than the big tyrannosaurs, and had relatively long, three-clawed arms, rather than short ones like those made famous by T. rex.

Incomplete Skeleton

Since the newly discovered dinosaur's known skeleton is incomplete and is from a juvenile, Siats' exact size at adulthood isn't entirely clear.

From estimates based on comparisons with more complete skeletons of other dinosaurs, Zanno says that "a juvenile Siats would have been, at minimum, about 30 feet long and around 9,000 pounds."

That's an impressive size that "still puts juvenile Siats as the third largest predator ever discovered in North America," Zanno says. The fact that the animal was not yet fully mature raises the likelihood that the adults were even bigger.

"Future material may reveal Siats grew up to be one of the biggest predators known around the globe," Zanno says.

Keeping Tyrannosaurs Down

The size of Siats is only part of the dinosaur's significance. "In the rock beds that contain the colossal bones of Siats, we also find the teeth of relatively tiny tyrannosaurs about the size of a large dog," Zanno says.

Early tyrannosaurs lived in the shadow of gigantic allosaurid carnivores like Siats. It was only after dinosaurs such as Siats disappeared, Zanno says, "that tyrannosaurs were free to evolve into the giant predators we know and love today."

University of Oxford paleontologist Roger Benson, who first recognized neovenatorids with colleagues in a 2010 study, agrees that Siats helps fill in a missing chapter in predatory dinosaur history.

Until now, there were "25 million years of missing data," Benson says, between allosaurid giants like Acrocanthosaurus and North America's huge tyrannosaurs.

With Siats at 98 million years old, he adds, the dinosaur "shows us that allosaurids stayed on top for at least another 10 million years." Exactly when and why dinosaurs like Siats gave way to the tyrannosaurs, however, relies on future Cretaceous finds from this final era of the Age of Dinosaurs.

Follow Brian Switek on Twitter.

17 comments
Gabriele Menefee
Gabriele Menefee

I'm just glad they aren't around today  But it would be fascinating to be able to have seen how they lived and interacted with the other creatures.  We would just be a little snack to a Tyrannosaurs that large.  It is almost unimaginable to my mind for something that big to have existed.  But bones don't lie. 

Vyel Nothiner
Vyel Nothiner

So maybe Tyrannosaurs weren't always so tough after all...

qi wang
qi wang

How amazing. The environment  can offer the needs for so gigantic creatures.

Jerold Te
Jerold Te

There's a lot to know about history

Verginia Georgieva
Verginia Georgieva

there are so questions 

so things that we dont know about the past,the history

Eric Paul
Eric Paul

Wait...so the article is claiming the new dinosaur, at full size, could be more than 4 tons (at least 8,000 lbs) and 30 ft. long.  But then they quoted a scientist that said the skeleton they unearthed was a juvenile at more than 4 tons (9,000 lbs) and 30 ft. long.  So, if what the scientist is saying is true, then an adult would be more around 5 tons and OVER 30 ft. long - probably 35-40 ft.  They need to be more clear in the article!

El Gabilon
El Gabilon

We should thank our lucky stars that this creature is not around today. Life is fragile. It is too bad that humans cannot find a way to live in peace and harmony so that we could enjoy life instead of knowing that at any instant, like the dinosaurs we could all be gone. As a matter of interest, had that super nova that exploded  been a little bit closer we would'nt be here today since it  would have blown our atmosphere out of existence and most of life, if not all on earth would have perished in minutes.

Cooper Nord
Cooper Nord

I'm not quite as well educated about dinosaurs as I'd like to be, or like many here, but this article reminded me why I want to do paleontology. Making pure, new discoveries; finding and pulling something out of the rock that has not seen the light of day for 98 MILLION YEARS. That's roughly 12,5000,000 (12.5 million, zeros just look cooler) lifetimes for average US citizens... That's pretty cool if you ask me. AND ON TOP OF ALL OF THAT, It's one of the more incredible stories of life on earth THAT WE KNOW OF. So there's always gonna be more to discover

I Thank You,

Coop Nord (young aspiring scientist)

Rayen Abidi
Rayen Abidi

I think it would be awesome to find another carcharodontosaurid coliving with t.rex or at least lythronax, gorgosaurus, daspletasaurus...

Thomas Holtz
Thomas Holtz

Given that megalosauroids (assuming <i>Sciurumimus</i> is indeed a megalosauroid) have fuzz, than the presence of fuzz-making placodes is inferred for all tetanurines, allosauroids included. However, what we can't say is how much of the body of any one of these taxa might have had fuzz and what had scales. Clearly a fair amount of the carcharodontosaurid <i>Concavenator</i> was scaly.


That said, no one has ever demonstrated that typical mudstones can even record the presence of protofeathers or feathers (or fur, for mammals). So we have a taphonomic blindspot: we can't use the positive presence of scales to dismiss the possibility that there was not also fuzz.

Alf M.
Alf M.

So It's the norm now to put feathers on Tyrannisauridae as well?

James Kirkland
James Kirkland

The recognition of an allosaurid in rocks this young is significant. My view is that the Mussentuchit preserves a mix of younger, more derived dinosaurs (tyrannosaurids and hadrosauroids) coming in from Asia with opening of the Alaskan "landbridge" about 100 Ma and older more basal taxa (basal iguanodonts like Tennontosaurus, small titanosauromorphs, and allosauroids). We had published on this in the late 1990's based on the extensive research collections of the microvertebrates generated by Rich Cifelli at the University of Oklahoma and published in the "Proceedings of the National Academy of Sciences" in 1997 and on the description of Eolambia in 1998.  This makes the Mussentuchit  even more interesting for research as it seems to record the immigration induced extinction resulting from the opening of the Asian migration corridor. Additionally, We have other large allosauroids in older Lower Cretaceous study (as old as Barremian (125+ Ma) that await additional research. So...., neoventorsaurids may have been hanging out in Utah for some time as was Acrocanthosaurus, while there were still viable connections to the rest of the world through Europe..

James Kirkland
James Kirkland

Tom; We'll get the in the coastal Upper Cretaceous (Cenomanian) Dakota Formation.

On the published reconstructions of Siats; hate the proto-feathers! Allosaurus had scales!

AND the co-occurring Tyrannosauridoids are bigger than a large dog.


Thomas Holtz
Thomas Holtz

So neovenatorids are now revealed in North America, too. Someday we might actually find the North American spinosaurids: our mysterious absence from the roster of Early/mid-Cretaceous theropods of the continent.

Wes Cox
Wes Cox

I have always thought that as well.  They we're lighter built but bigger than TRex.  If they mainly thrived in South America, it stands to reason that there had to be a North American variant of the species.

 @Rayen Abidi

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