The First Dinosaurs
Homepage > Dinosaur Articles - The First Dinosaurs
Dinosaurs first appeared during the Early Triassic Period about 230 million years ago. Their ancestors include reptiles of the diapsid group. After their appearance, they would rule the Earth for over 165 million years. Dinosaurs were the dominant animals during this time. But they were not the only animals around. Other vertebrates lived along their side. Some of these prehistoric animals lived not only on land, but also in water and on the air.
The first dinosaurs were small, lightweight animals. They ate meat and plants, and were probably fast runners. Interesting Fact [The earliest known dinosaur is Eoraptor lunensis. Eoraptor lived 228 million years ago in South America. It was a small, primitive dinosaur that measured 3 feet (1 meter) in length. Its name means "dawn raptor". It had hollow bones and was a fast runner.]
In order to see where the dinosaurs came from, we have to first look at
their closely related early diapsid animals. Among the diapsid were a
number of animals that scientists refer to archosaurs (ruling reptiles).
The dinosaur family tree begins with the earliest of the archosaurs,
a group of large crocodile like animals, with sprawling legs to their
sides. They were known as the Thecodonts. 
The
Thecodonts were meat eating animals with long jaws and long tails.
Erythrosuchus was an archosaur, an ancestor of the dinosaurs. This carnivore grew up to 16 feet (5 meters) in length.
With the emergence of the thecodonts, a major stage in the evolution of dinosaurs was complete. But it was a long way before the true dinosaurs appeared. Early thecodonts walked like modern crocodiles do today. They had their legs to the sides, and used a sprawling motion while walking.
Later thecodonts walked on an upright position and were able to run, but they still used four legs to walk. Smaller, lightly built thecodonts were able to run on their hind legs for short distances. For some walking on two legs became a normal practice. Dinosaurs came from these group. Not all reptiles changed from the sprawling gait to the semi-erect and erect gaits.
Crocodiles are still sprawlers today.