Riojasaurus lived about 225 to 219 million years ago. It was one of the earliest of the large herbivores, or the great, plant-eating dinosaurs. We know a lot about this dinosaur because we have found the bones of more than 20 different creatures. It was a heavily-built dinosaur, about 30-36 feet long. That’s about as long as a school bus! It grew very large for such a primitive, or simple, dinosaur. Riojasaurus walked on four legs instead of two. It needed all four legs to hold up its big, heavy body.
By studying the bones in its ankle, we believe that Riojasaurus was not able to stand up on its back legs to look for food in trees. It probably couldn’t run on its back legs either. The back legs were only slightly bigger than the front legs. The front legs were a little bit shorter, and this meant that its head was close to the ground. This was good for eating the ferns and other plants that grew low. Riojasaurus also had a long, flexible neck, so it could reach up into tree branches for food if it needed to. Paleontologists believe that these dinosaurs developed long necks because the low-growing plants were being eaten by too many other smaller herbivore dinosaurs. When creatures change like this to survive, we call it evolution.
Its skull was very small and light. It had to be: if its head were heavier, Riojasaurus wouldn’t be able to hold it up on its long neck. A small skull usually means a small brain. This means that Riojasaurus was not a very intelligent dinosaur. It had spoon-shaped, serrated teeth that looked like small saws. The vertebrae, or bones in its neck and back, were hollow. This made it much lighter and easier to move. The bones in its legs and arms, however, were very thick and solid. An almost complete Riojasaurus skeleton was found in northwest Argentina, in a place called “La Rioja”. Its name means “lizard from La Rioja”.