Siats Meekorum and the Rise of Tyrannosaurus

Before Tyrannosaurus dominated North America, before the most iconic saurian theropod became the top predator of the Late Cretaceous, there was another king of the dinosaurs.

98 million years ago, Siats meekorum was the biggest theropod in North America.  Siats was discovered at the Cedar Mountain Formation in Utah, the homeland of the Ute tribe, and was named after a man-eating monster in their mythology.  It is known from only fragmentary remains, but it is known to be a part of the clade Megaraptora.  Megaraptorans are not, as their name might suggest, giant dromaeosaurs; instead, they are a subgroup of the family Neovenatoridae, related to allosaurs and carcharodontosaurs.

The only known skeleton of Siats is estimated to be from an animal about thirty feet long.  However, based on comparisons with more completely known megaraptorans, these bones are believed to be from a subadult.  When fully grown, Siats is estimated to have grown even bigger - up to forty feet in length.

(Personally, I believe the reconstruction of the animal above is incorrect due to its inclusion of feathers, as filamentous body coverings are currently only known from coelurosaurian theropods, which Siats is not.  However, if feathers originated earlier than I think among theropods, Siats was so closely related to coelurosaurs that it’s possible it had feathers as well.   But we’re not here to talk about feathers.)

If Siats really did grow to such lengths, it was comparable in size to Tyrannosaurus.  At the time when Siats lived, however, tyrannosaurs were very small.  Although no tyrannosaurs are known from the Cedar Mountain Formation, tyrannosaurs from elsewhere in the world at the same time - such as the Mongolian species Timurlengia - were around one-quarter the size of Siats.  So how did tyrannosaurs become the top predators in North America?  And what happened to Siats?

There are many gaps in the tyrannosaur fossil record, but I have a theory of my own.  Tyrannosaurs had more advanced brains than most other theropod dinosaurs, and since they began their evolutionary history as very small animals, they likely rose to the top of the food chain by outsmarting competing predators, not growing to massive sizes until they had established themselves as the most intelligent predators on the block.  When the sea levels fell during the Cretaceous, tyrannosaurs migrated across a land bridge from Asia to North America.  At that time, allosaurs and carcharodontosaurs were the top predators in America, but they were eventually outcompeted by the more intelligent tyrannosaurs.  Unable to stand up to the competition, allosaurs and carcharodontosaurs - like Siats - went extinct in North America, and Tyrannosaurus became the dominant predator on the continent.

Despite its relative obscurity, Siats is one of my personal favorite dinosaurs.  It’s sort of symbolic to me.  It represents a time and place that we know very little about, even in comparison to the rest of the Mesozoic.  Its remains are fragmentary, its life is mysterious, and the circumstances of its extinction are even more poorly understood.  It dominated the landscape for millions of years, before being slain to make way for a new age - like the mythical beast for which it’s named.

Perhaps this is an overly romantic view of the situation.  Even so, I find Siats worthy of appreciation, both as a real animal, and as a symbol of a time long since passed.

pretty-much-poseidon  asked:

Is it possible that the plates on the backs of Stegosaurs arose as defense against Allosaurs? Given the theory that Allosaurs used their heads more like axes rather than biting, swinging down towards the back/neck of their prey, the plates would get in the way of said swinging theropod head- or would they be too fragile to withstand that amount of force?

I mean, the plates weren’t all that strong (as can be seen from plates with bite marks in them; Carpenter et al., 2005), but I could see how they could serve as a bit of a deterrent in the case that aggressive display fails. There could be multiple selective pressures at play here - namely predation and display.

(Src: Greg Paul)

The Allosaur who wanted to be a Tyrannosaur.

The all new hotness from South America, Gualicho shinyae, a Neovenatorid with short arms and two-fingered hands.

So yes. It is an allosauroid with convergent traits with tyrannosaurids.

Link to the original paper, on free access:

New Dinosaur Had the T. Rex Look: Tiny Arms

Tyrannosaurus rex wasn’t the only meat-eating menace with teeny-tiny arms.

Like its distant relative, T. rex, a newly identified dinosaur, named Gualicho shinyae, sported small arms and hands with two clawed fingers.

“We’re slowly getting more information on this sort of pattern of limb reduction, and getting at this question of why tyrannosaurs and some other theropods shortened their forelimbs,” said study corresponding author Peter Makovicky, curator of dinosaurs at the Field Museum in Chicago. [See Images of the Tiny-Armed Gualicho shinyae]

G. shinyae belonged to a group called the allosaurs and lived about 90 million years ago, during the Cretaceous period. That’s a good 23 million years before T. rex entered the scene between 67 million and 65 million years ago.

Because they are only distantly related (both are considered theropods, which are bipedal and mostly carnivorous dinosaurs), the two species likely developed their tiny arms and two-digit hands independently due to similar evolutionary pressures — a process called convergent evolution, the researchers said.

Bony discovery

Study lead author Sebastia?n Apestegui?a, a paleontologist with the Natural History Foundation (AZARA) in Buenos Aires, Argentina, and a team of researchers discovered the dinosaur along the northern edge of a large reservoir in Patagonia, Argentina, in 2007. (In fact, five other predatory dinosaurs have also been found in that area, called the Huincul Formation.)

“Unfortunately, it’s one of those specimens that was discovered probably 50 years too late,” Makovicky told Live Science. “We have the hind limbs and the forelimbs, we have the section of the back and the tail, a little bit of the hips.” But they couldn’t locate the skull and much of the vertebral column, likely because of erosion.

However, based on what they found, as well as the anatomy of other allosaurus dinosaurs, they estimate that at its hips G. shinyae was about 6 feet (1.8 meters) tall, and that it measured about 25 feet (7.6 m) long and weighed about 1 metric ton (1.1 tons), Makovicky said.

“It’s not a giant animal by dinosaur standards, but not small, either, so [it’s] a midsize predator,” he said.

G. shinyae is also a bizarre mosaic of different dinosaurs. It looks a bit like Carcharodontosaurus, another carnivorous theropod, as well as Deltadromeus (delta runner), a carnivorous dinosaur with slender arms found in Africa, which is possibly a close relative, the researchers said. [Photos: Dinosaur’s Battle Wounds Preserved in Tyrannosaur Skull]

Divine inspiration     

Researchers named the dinosaur Gualicho shinyae after two women: The genus honors Gualichu, a goddess of animals who was revered by the Tehuelche people of Patagonia. When Europeans brought Christianity to the area, the newcomers reinterpreted Gualichu as a demonic entity, the researchers wrote in the study.

The paleontologists joked that Gualichu had put a curse on them during their fieldwork, after their truck flipped over (no one was seriously hurt), Makovicky said. The species name honors Akiko Shinya, who discovered the specimen.

The newfound dinosaur is an exciting find, said Thomas Carr, an associate professor of biology at Carthage College in Wisconsin, who was not involved in the study.

“It really provides a sharper focus on the whole phenomenon of forelimb reduction and finger loss in theropods,” Carr said. “It clearly occurs across many different lineages for different reasons and in different ways.”

It also shows that “tyrannosaurs’ [arms] really aren’t unusual,” Carr said. “It’s not just a one-off.”

The new study was published online today (July 13) in the journal PLOS ONE.

Original article on Live Science.

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