However, the latest analysis identifies a Scottish specimen, called Saltopus, as the closest thing in the fossil record to what the hypothetical common ancestor might look like.
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The system places dinosaur species into two distinct categories, Ornithischia and Saurischia, based on an early observation that dinosaur hip bones displayed either a lizard-like pattern (Saurischia) or a bird-like one (Ornithischia).

The Ornithischia group includes the relatives of Stegosaurus and Triceratops, while the theropods (such as T. rex), sauropods (Diplodocus) and another meat-eating group called Herrerasauridae, all fell within the lizard-hipped Saurischia group. Counterintuitively, the dinosaur group that later gave rise to modern birds (theropods) falls on the lizard branch of the family tree.
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The results, published in Nature, suggest that theropods should be switched over to become an off-shoot of the same branch that led to Stegosaurus and Triceratops. The revised grouping of Ornithischia and Theropoda has been named the Ornithoscelida, reviving a name originally coined by the evolutionary biologist Thomas Henry Huxley in 1870.

https://www.theguardian.com/science/2017/mar/22/scottish-fossil-may-cause-radical-shakeup-of-dinosaur-family-tree-saltopus

The new tree fits more neatly with the observation that many meat-eating species and bird-hipped dinosaurs had feathers. The fact that previously they were in separate groups led some to speculate that all dinosaurs, including the long-necked sauropods were feathered. But there has been no fossil evidence for this, and it is a suggestion that never really took hold.

The latest work also indicates that dinosaurs evolved into meat-eaters on two separate occasions during their evolution and it even implies that the very first dinosaur was omnivorous and therefore ate both plants and meat.

http://www.bbc.co.uk/news/science-environment-39305750