Although initially described by Rodolfo Magín Casamiquela in 1967 as "a good and rather primitive member of Ornithopoda" and given its own family (Pisanosauridae), Pisanosaurus mertii is now thought to be the earliest known ornithischian. With its discovery in the same Argentine Formation as Herrerasaurus (the earliest known saurischian), both of Dinosauria's main lineages were covered, which suggests that the first dinosaurs rose in South America during the Triassic and when the "great dying" wiped out the competition they began their opportunistic march to global domination from there.
Because of a weird suite of features, Pisanosaurus has been accused in the past of being a chimera of two different critters jumbled together, not least because its pubis (a hip bone) points forward like a saurischian rather than backwards like an ornithischian. However, its ridged teeth are typical of ornithischians, and they are set towards the inside of the jaws, leaving a broad shelf on the outside. That shelf probably anchored cheeks that prevented food loss while chewing and gave Pisanosaurus a distinct advantage over the competition who were still dropping their fodder all over the floor. Pisanosaurus also set ornithischians on the road to spectacular success by adopting a most efficient predator-avoidance system; the small size of its shoulder blade and forearm suggest it wasn't built for quadrupedal locomotion but for moving swiftly and evasively on two legs. That said, the latest living and most successful ornithischians moved on all fours, but they were built like Sherman tanks.