Triassic dinosaurs

The earliest dinosaurs are known from the Late Triassic Period of the Mesozoic Era, about 230 million years ago.

By the end of the Triassic Period, the three major groups of dinosaurs had appeared, the theropods, carnivorous dinosaurs that would later include T. rex; the sauropods, which would come to include the huge plant-eaters of the Jurassic Period, and the ornithischians, smaller, more lightly built dinosaurs.

The fossil record of Triassic dinosaurs is poor relative to the Jurassic and Cretaceous Periods, comprising largely fragmentary and incomplete material, so the discovery of several nearly complete skeletons of a new Late Triassic theropod dinosaur from New Mexico is an important contribution to understanding the early evolution of the dinosaurs and helps to clarify the relationships among other Triassic dinosaurs.

Sources:

http://www.nsf.gov/news/special_reports/tawa/index.jsp

http://www.universityofcalifornia.edu/everyday/pscience/dinosaurs.html