Pisanosaurus mertii (Casamiquela 1967) Ischigualasto Formation, Late Triassic, ~222 mya, ~1 m in length, was thought to be the oldest known ornithischian. Unfortunately, only scattered pieces are known from this specimen. Here, derived from a sister to Daemonosaurus, Pisanosaurus is basal to all other Ornithischia. Haya is a sister taxon.

Distinct from Daemonosaurus, Pisanosaurus had a battery of short chopping teeth on its maxilla and dentary. The biting surfaces formed one long occlusal surface. No fangs were present. The mandibular fenestra was absent with a surangular at least twice as deep as the angular. A small coronoid process appeared.

The cervicals were shorter and smaller, but still numbered over ten, as in Pantydraco and Tawa.

The hind limbs remained primitive and whatvery little is known of the pelvis indicates the pubis had not yet retroverted. That may change with future discoveries. As you can see (above) the pubis and ischium appear to be reversed from the ornithopod pattern and the robust nature of the both elements are more similar to those of sauropodormorphs.

Haya griva (Makovicky et al. 2001, Santonian, Late Cretaceous) was considered a small basal bipedal ornithopod and a sister to Jeholosaurus. The large reptile treealso nests it close to Jeholosaurus, but closer to Pisanosaurus (see above, in the absence of ornithopods) and Daemonosaurus. This taxon has a standard ornithischian pelvis, a small skull, small hands and large premaxillary teeth. The chevrons were deep.

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Casamiquela RM 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina. Ameghiniana 4 (2): 47–64.

Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 200a. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.

Irmis RB, Parker WG, Nesbitt SJ and Liu J 2007b. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.

Makovicky PJ, Kilbourne BM, Sadleir and Norell MA 2011. A new basal ornithopod (Dinosauria, Ornithischia) from the Late Cretaceous of Mongolia. Journal of Vertebrate Paleontology 31 (3): 626–640.

Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218

Sereno P 1991. Lesothosaurus, "Fabrosaurids," and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11:168-197.

wiki/Pisanosaurus
wiki/Haya