What were the first true dinosaurs like?
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Based on current fossil finds, the first true dinosaurs emerged in the Late Triassic period, between about 230 to 225 million years ago. They were small, agile, carnivorous reptiles whose unique characteristics, such as two-legged motion, enabled them to quickly dominate the available ecological niches.
Apparently, by the time dinosaurs evolved, they had already split, even at this early stage, into two major groups: the ornithischians and the saurischians. These two main groups of dinosaurs are based historically on their hipbone structure.