Dinosaur From Utah Was Top Predator, Ruled Over Early Tyrannosaurs
November 22, 2013

Dinosaur From Utah Was Top Predator, Ruled Over Early Tyrannosaurs

[ Watch the Video: New Species Of Carnivorous Dinosaur Found In Utah ]

Lawrence LeBlond for redOrbit.com - Your Universe Online

Tyrannosaurus rex has long been considered the apex predator of its time. But this large meat-eating theropod’s earlier relatives did not celebrate such stature, based on new evidence by a multi-institutional team of researchers who have found a new contender to fill the role of top dog during the early-to-mid-Cretaceous period.

Researchers from The Field Museum, the North Carolina Museum of Natural Sciences (NCMNS) and North Carolina State University (NCSU) have found the fossil of a giant predatory dinosaur that ruled the landscape some 98 million years ago, long before tyrannosaurs, such as T. rex, could assume the role of apex predators.

The new dinosaur species, called Siats meekerorum, was discovered in the Cedar Mountain Formation of Utah. Publishing the findings in the Nov. 22 issue of Nature Communications, the team describes this new predator as a 30-foot-long, four-ton behemoth that was undoubtedly the top predator in its ecosystem.

The dinosaur gets its genus name, Siats, from a mythical man-eating monster from the Ute Native American people. Its species name, meekerorum, is in honor of the Meeker family for their support of early career paleontologists at the Field Museum.

The specimen was discovered in 2008 by Lindsay Zanno, director of paleontology at NCMNS/NCSU. Zanno made this discovery while being part of a Field Museum expedition studying the 100-million-year-old rocks of the Cedar Mountain Formation. The remains of S. meekerorum were found scattered about and it took two summers' worth of work to collect and clean all the fossils in the collection.

S. meekerorum is a species of carcharodontosaur, which is a group of meat-eaters that include some of the largest predators ever discovered. S. meekerorum is only the second species to have been discovered from North America – the other being the Acrocanthosaurus, which roamed the North American landscape nearly 10 million years earlier; this specimen was discovered in 1950.

"It's been 63 years since a predator of this size has been named from North America," said Zanno. "You can't imagine how thrilled we were to see the bones of this behemoth poking out of the hillside."

Peter Makovicky, from the Field Museum of Natural History in Chicago, assisted Zanno in the discovery and collection of the fossils. Together, they made the distinction that, despite its massive size, the fossils were from that of a juvenile. The two researchers surmise that the adult may have reached the size of Acrocanthosaurus. And despite these two predatory dinosaurs dominant rule over the ecosystem 100 million years ago, they were still smaller than T. rex, which would have weighed twice as much as these predators when it roamed the countryside 30 million years later.

Although Siats and Acrocanthosaurus are both members of the carcharodontosaurs, they belong to different subgroups. Siats is a member of Neovenatoridae, a slender-bodied group of carcharodontosaurs. Neovenatorids have also been found in Europe, South America, China, Japan and Australia. But this is the first time a neovenatorid has been found in North America.

Previously, paleontologists had not known what dinosaur species was the top meat-eating predator during this period.

"Carcharodontosaurs reigned for much longer in North America than we expected," said Zanno.

She explained that Siats actually fills a gap of more than 30 million years in the fossil record. She said that the role of top predator had changed hands at some point from the Early Cretaceous carcharodontosaurs to the Late Cretaceous tyrannosaurs.

A lack of fossil evidence left paleontologists scratching their heads on when this change occurred. It was not known if tyrannosaurs outcompeted carcharodontosaurs or if carcharodontosaurs had gone extinct long before tyrannosaurs were able to take over as apex predator. With this new discovery it is clear that Siats’ large size would have prevented smaller tyrannosaurs from taking over as top predator in the food chain.

"The huge size difference certainly suggests that tyrannosaurs were held in check by carcharodontosaurs, and only evolved into enormous apex predators after the carcharodontosaurs disappeared," said Makovicky.

"Contemporary tyrannosaurs would have been no more than a nuisance to Siats, like jackals at a lion kill. It wasn't until carcharodontosaurs bowed out that the stage could be set for the evolution of T. rex," added Zanno.

Evidence of carcharodontosaurs reigning over the tyrannosaurs of the time was also found in tooth size. Fossilized teeth from tyrannosaurs in the Cedar Mountain Formation indicate that those living alongside Siats were much smaller in stature and could not compete for an apex predatory position. So it is likely Siats, as well as other carcharodontosaurs of the period had died out long before tyrannosaurs could fill that role.

Also, experts had previously believed that continental rifting during the Cretaceous caused isolation between dinosaur faunas on different continents, and the faunas of the early-to-mid-Cretaceous in North America were especially isolated. But with the latest findings, that theory is rapidly changing.

"Siats is just the tip of the iceberg; our teams are unearthing a lost dinosaurian ecosystem right here in the badlands of western North America," Zanno said.

"Since Siats, we have made more exciting discoveries, including two new species of dinosaur, which will further illuminate how North American Cretaceous dinosaurs are related to those on other continents," said Makovicky.

At the time of Siats’ reign, the landscape was lush with abundant vegetation and water supporting a wide variety of plant-eating dinosaurs, turtles, crocodilians, as well as giant lungfish. This ecosystem also included early tyrannosaurs and several species of feathered dinosaurs that have yet to be described by paleontologists.

"Stay tuned," said Zanno. "There are a lot more cool critters where Siats came from."


Image Below: This is an illustration of Siats meekerorum. Credit: Jorge Gonzales