In 1887, Harry Govier Seeley was the first to subdivide dinosaurs into Saurischians and the Ornithischians based on the nature of their pelvic bones and joints. While the clade Saurischia is well represented in the Late Triassic, the record of the Ornithischia is certainly more problematic. Only a single Triassic ornithischian taxon was generally considered to still be valid: Pisanosaurus mertii, originally described by Argentinian paleontologist Rodolfo Casamiquela in 1967, based on a poorly preserved but articulated skeleton from the upper levels of the Ischigualasto Formation (Late Triassic).
The holotype and only known specimen (PVL 2577) is a fragmentary skeleton including partial upper and lower jaws, seven articulated dorsal vertebrae, four fragmentary vertebrae of uncertain position in the column; the impression of the central portion of the pelvis and sacrum; an articulated partial hind limb including the right tibia; fibula; proximal tarsals and pedal digits III and IV; the distal ends of the right and left femora; a left scapular blade (currently lost); a probable metacarpal III; and the impressions of some metacarpals (currently lost).
But Pisanosaurus shows some derived traits that resulted as unambiguous synapomorphies of the Silesauridae clade, and include: reduced to absent denticles on maxillary and dentary teeth; sacral ribs shared between two sacral vertebrae; lateral side of proximal tibia with a fibular flange; dorsoventrally flattened ungual phalanges; and ankylothecodonty, teeth partially fused to maxilla and dentary bone. The inclusion of Pisanosaurus within Silesauridae implies that this taxon does not constitute the oldest ornithischian. This is consistent with previous interpretations proposing that ornithischian radiation occurred after the Triassic–Jurassic boundary.
To explain the relatively low diversity exhibited by Ornithischia in the Late Triassic-Early Jurassic, several hypotheses have been proposed. One, suggests that Ornithischia is the sister-taxon of Neotheropoda (the least inclusive clade that includes Coelophysis and modern birds) within the clade of ‘traditional theropod taxa’. In this model, a ‘transitional’ ornithischian may possess some anatomical features of theropods that appear to be more like those in more derived than Eodromaeus murphi and Tawa hallae.
In a second hypothesis, Ornithischia is positioned as the sister-taxon to the coelophysids. In this model, Neotheropoda and Ornithoscelida would encompass the same set of taxa, but Ornithoscelida would, theoretically, take priority of Neotheropoda as it is the older name. In a third hypothesis, Ornithischia is positioned as the sister-taxon to the ‘other neotheropods’ not contained in the coelophysid clade.
Another hypothesis proposes that Ornithischia forms the sister-taxon of Averostra. Like Ornithischia, Averostra is only known from the Jurassic and Cretaceous Periods, and both share a number of anatomical features, such as fusion of the sacral neural. Another anatomical traits that could unite such a group include the possession of six or more sacral vertebrae; and the fusion of the sacral neural spines into a broad and continuous sheet, as in early ornithischians like Lesothosaurus diagnosticus and tetanuran theropods like Megalosaurus bucklandii. It’s worth mentioning the fact the earliest known unambiguous members of both Ornithischia and Averostra, are found in the same formation in South America: Laquintasaura venezuelae and Tachiraptor admirabilis.
It was suggested (Baron and Barrett 2017) that Chilesaurus diegosaurezi from the Late Jurassic, might represent the earliest diverging member of Ornithischia. Chilesaurus shows several characters typical of ornithischians. The features include a premaxilla with an edentulous anterior region; loss of recurvature in maxillary and dentary teeth; a postacetabular process that is 25–35% of the total anteroposterior length of the ilium; possession of a retroverted pubis; a pubis with a rod-like pubic shaft; a pubic symphysis that is restricted to the distal end of the pubis; and a femur that is straightened in anterior view. The unique combination of ‘primitive’ and ‘derived’ characters for Chilesaurus has the potential to illuminate the order in which traditional ornithischian synapomorphies were acquired.
The Phytodinosauria hypothesis suggest that Ornithischia is nested among the taxa traditionally termed as sauropodomorphs could also offer a solution to the problem of the lack of unambiguous ornithischians in the Carnian and Late Triassic in general.
Baron, M. G. (2017): Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?, Historical Biology, DOI: 10.1080/08912963.2017.1410705
Agnolín FL, Rozadilla S. (2017): Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina, Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1352623
Baron M. G, Barrett P. M. 2017, A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220
Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543, 501–506 (2017). doi:10.1038/nature21700
Barrett, Paul M.; Butler, Richard J.; Mundil, Roland; Scheyer, Torsten M.; Irmis, Randall B.; Sánchez-Villagra, Marcelo R. (2014). A palaeoequatorial ornithischian and new constraints on early dinosaur diversification, Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.1147
Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol & Stephen L. Brusatte, Untangling the dinosaur family tree, Nature 551 (2017) doi; oi:10.1038/nature24012
Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a