By Will Dunham WASHINGTON (Reuters) – Dinosaurs long dominated Earth’s land ecosystems with a multitude of forms including plant-eating giants like Argentinosaurus, meat-eating brutes like Tyrannosaurus and weirdos like Therizinosaurus, with its Freddy Krueger-like claws. But the origin of dinosaurs – precisely when and where they first appeared – remains a bit of a puzzle. Researchers are now