Untamed Science has the skinny on Siats meekerorum, an early Cretaceous predator the size of a bus:
So imagine a world where the giant Siats is top dog, and ancestors of the T. rex (tyrannosaurs) about the size of great danes were running around. How adorable is that? It wasn’t until Siats and other Carcharodontosaurs went extinct that T. rex was able to evolve and eventually fill its niche in the ecosystem–the top predator most of us grew up learning about.
Siats comes from a Native American legend about a man-eating monster called the Siats. The scientists who unearthed the skeleton learned of the story in the area in Utah it was discovered, and they adopted the name to reflect this local lore. To be clear, Siats were NOT man-eaters, of course. They lived 100 MYA and were extinct by the time the T. rex flourished at the end of Cretaceous.
So you might be thinking, “why isn’t it called Siats-osaurus?” Well the simple answer is that Lindsay [Zanno director of Paleontology & Geology Research Laboratory at the North Carolina Museum of Nature Science,] named it and she’s bored with “-saurus.” Dinosaurs don’t have to have “–saurus” at the end of their name to be a dinosaur. Plus, paleontologists are so cool; they do what what they want.
Lots of video (including an interview with Zanno) at the link.