Silesaurus opolensis here was a type of dinosauriform – a reptile very closely related to the ancestors of true dinosaurs, but not quite actually a dinosaur itself.
Living in Poland during the Late Triassic (~230 million years ago), it was a quadrupedal animal roughly the size of a large modern dog, about 50cm tall at the shoulder (1’8″) and 2m long (6’6″). The front of its lower jaw was toothless and covered with a keratinous beak, and there may have been a corresponding much smaller beak at the very tip of its upper jaw, too.
It was originally thought to be a herbivore, but coprolites full of insect remains suggest it was probably more of an omnivore, possibly foraging by pecking in a convergently similar manner to its distant bird cousins.
In fact, one of those pieces of Silesaurus poop was recently found to preserve a new species of tiny beetle in incredible detail.
Pisanosaurus mertii from the Late Triassic of Argentina (~228-216 mya).
Known only from a partial skull and a few pieces of its skeleton, this 1m long animal (3′3″) is usually considered to the be the earliest known member of the ornithischian dinosaurs – but some recent studies have thrown that into question, suggesting that it might not even be a dinosaur at all.
Instead it may have been a member of the silesaurids, Triassic dinosauriforms that were close cousins to the dinosaurs but not quite true members of the group. Small and lightly-built, silesaurids had long front limbs and may have been at least partially quadrupedal, and some showed evidence of herbivory with beaks at the tips of their snouts.
Of course, if the “oldest ornithischian” is actually a silesaurid, we’re left with no fossil record at all for ornithischians until the start of the Jurassic. And that brings back up the controversial question of where they actually originated…