Which was the oldest dinosaur?

There are several contenders for oldest dinosaur at the moment. They all come from Argentina, in rocks that date back 225 million years, and include Eoraptor a small predator measuring about 1 metre in length, two larger predators named Herrerasaurus and Staurikosaurus and a very poorly known herbivore named Pisanosaurus. They have all been discovered in similarly aged rocks, so at the moment there is no single oldest dinosaur known.