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 tree also nests it close to Jeholosaurus, but closer to Pisanosaurus (see above) and a Triassic clade member, Daemonosaurus. This taxon has a standard ornithischian pelvis, a small skull, small hands and large premaxillary teeth. The chevrons were deep.

Thescelosaurus neglectus (Gilmore 1913. Boyd 2014 Late Cretaceous North America) was long known from a postcranial skeleton, later re-united with skull and mandible material. It nests with other basal ornithischians like Haya and Fona.

The hand PILs indicate a lack of flexibility around the longest of the short fingers, which acted as a weigh-bearing 'hoof' during occasional quadrupedal excursions.

Fona herzogae (Avrahami et al, 2024, Middle Cretaceous, North America) is a smaller relative of Thescelosaurus in the LRT.

Avrahami HM, Makovicky PJ, Tucker RT and Zanno LE 2024. A new semi-fossorial thescelosaurine dinosaur from the Cenomanian-age Mussentuchit Member of the Cedar Mountain Formation, Utah". The Anatomical Record. 307 (12): 3717–3781.

Bonaparte JF 1976. Pisanosaurus mertii Casamiquela and the origin of the Ornithischia. Journal of Palaeontology 50(5):808-820.

Boyd CA 2014. The cranial anatomy of the neornithischian dinosaur Thescelosaurus neglectus. PeerJ 2:e669; DOI 10.7717/peerj.669

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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.

Gilmore CW 1913. A new dinosaur from the Lance Formation of Wyoming. Smithsonian
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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
wiki/Fona
wiki/Thescelosaurus