Man-eating monster dino nearly wiped out T.rex

A newly discovered predatory dinosaur from Utah is so large that it may represent the biggest species of meat-eating dino that ever lived in North America.

The dinosaur, Siats meekerorum aka "Man Eating Monster," is described in the latest issue of Nature Communications. The specimen was just a juvenile, but conservatively it measured at least 30 feet long and weighed 9,000 pounds.

The name Siats pays homage to a human-chomping monster from legends of the Ute native tribe of Utah.

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Siats still had a lot of room to grow, and there is only a 4-inch difference between the estimated femur length of a juvenile Siats and an adult Acrocanthosaurus (the second largest predator in North America), so we think a safe estimate for an adult Siats is that it at least vied with Acrocanthosaurus (11,000 pounds) for the No. 2 slot,” lead author Lindsay Zanno told Discovery News.

“However, our upper estimate on Siats is a body mass larger than T. rex (currently in the No. 1 spot), so future material may reveal Siats grew up to be one of the biggest predators known around the globe,” added Zanno, who is director of the Paleontology and Geology Research Laboratory at the North Carolina Museum of Natural Sciences.

Zanno, who is also an assistant professor of biology at North Carolina State University, co-authored the paper with Peter Makovicky of the Field Museum of Natural History.

The researchers analyzed the remains for Siats, which were unearthed at the Dakota Formation in Emery County, Utah. The fossils reveal that the huge dinosaur lived approximately 98 million years ago at the dawn of the Late Cretaceous.

It was a carcharodontosaur, which refers to a type of enormous carnivorous dinosaur known, not just for their size, but also for their jagged, sharp teeth.

Zanno and Makovicky believe that the mere existence of these apex predators prevented tyrannosaurs at the time from evolving into even larger beasts. It was only when Siats and other carcharodontosaurs mysteriously died out that tyrannosaurs like T. rex emerged.

“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 said. “This is a clear indication that tyrannosaurs were living in the shadows and carcharodontosaurs like Siats reigned at the dawn of the Late Cretaceous in North America.”

She thinks it’s possible that Siats even ate these small tyrannosaurs, although it probably went after less aggressive prey, such as large, plant-eating long-necked dinosaurs and primitive duckbilled dinos.

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These and other animals, including crocodiles, turtles and mammals, all lived in a lush, wet coastal plain environment not far from a shallow sea that was then spreading over central North America. The sea divided the continent into smaller island landmasses.

Roger Benson, a paleobiologist at the University of Oxford, told Discovery News that the finding of Siats shows that extremely large predatory dinosaurs were present in North America at the time and that “their presence would have excluded tyrannosaurs from evolving to multi-ton sizes.”

Stephen Brusatte, a Chancellor’s Fellow at the University of Edinburgh’s School of GeoSciences said that it also raises the question: Why did Siats and its relatives die out?

“They were the largest predators for tens of millions of years, and then they suddenly seem to die out, which allowed tyrannosauroids to assume the apex role,” Brusatte told Discovery News. “But why?”

He continued, “This is a major question that we need to figure out, and continued exploration of middle-early Late Cretaceous rocks in North America should provide some important clues.”