Siats comes from the middle of a 30-million-year gap in the fossil record of North American large predatory dinosaurs, during which the top predator role changed hands from carcharodontodsaurians in the Early Cretaceous to tyrannosaurs in the Late Cretaceous.
The lack of fossils left paleontologists unsure about when this change happened and if tyrannosaurus outcompeted carcharodontosaurs, or assumed that niche following carcharodontosaur extinction. The upper Cedar Mountain Formation is currently one of the only rock units known to produce fossils of both groups.
Teeth of tyrannosaurs from the Cedar Mountain Formation indicate that the tyrannosaurs living alongside Siats were much smaller in stature and did not compete for the apex predator role.
"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.
Zanno added, "Finding Siats in the 30-million-year predator gap tells us carcharodontosaurians reigned supreme in North America for much longer than anyone had expected."
Until recently, it was thought that continental rifting in the Cretaceous caused isolation between dinosaur faunas on different continents, and the early-to- middle Cretaceous faunas of North America were particularly isolated. That picture is rapidly changing through discoveries such as Siats.
"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
|Contact: Nancy O'Shea|