Updated March 14, 2015 with a new restoration of the Pisanosaurus pelvis.
Pisanosaurus (Casamiquela 1967, Bonaparte 1976, Late Triassic) has been considered a basal ornithischian for over 50 years. With its beveled teeth creating one long occlusal surface and the hint of a missing predentary, it’s a cladistic lock. The pelvis has been problematic. It doesn’t really show any ornithischian traits — but then what is preserved is only the part that surrounds the acetabulum. The illustrated data with regard to in situ vertebrae vs. reconstructed vertebrae did not match with regard to number. Casamiquela described lumbar and caudal vertebrae and no cervicals. Bonaparte described cervicals with a skull on one end (Fig. 1), but no lumbars or caudals.
The proximal outgroup is Daemonosaurus, known only from some long cervicals and a complete skull with long teeth. Prior to that, basal sauropodomorphs and eoraptorids were ancestral. Evidently, ornithischians had a late start.
I recently revised my own reconstruction (Fig. 1) after realizing what I thought were the traced and unlabeled “humerus and manus” near the front half of the skeleton, beneath the anterior dorsals, were actually the tibia and pedal elements, which were better illustrated elsewhere. Embarrassingly, some things just take time to sink in.
So here (Fig. 1) is Pisanosaurus revised. It has more neck verts (more than any other ornithischian) and no preserved forelimbs. The pelvis could be saurischian, like the neck vert number, as shown, or with the pubis retroverted or something in between.
Nesbitt (2011) and Brusatte et al. (2010) derived Pisanosaurus and its sister Heterodontosaurus from Lesothosaurus and kin and these from Lewisuchus, which also gave rise to silesaurids, sauropodomorphs and theropods. By contrast, in the large reptile tree, Lewisuchus and silesaurids are basal archosaurs, more primitive than crocs. Theropods were basal dinosaurs and ornithischians arrived late, which matches the chronology better. With that long cervical series and sauropod-ish pelvis of Pisanosaurus, that also makes more sense.
Bonaparte JF 1976. Pisanosaurus mertii Casamiquela and the origin of the Ornithischia. Journal of Palaeontology 50(5):808-820.
Brusatte SL, Benton MJ, Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Casamiquela RM 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina. Ameghiniana 4 (2): 47–64. translated to English here.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. 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.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
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