Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina

Abstract

Pisanosaurus mertii was originally described on the basis of an incomplete skeleton from the early Late Triassic (Carnian) of northern Argentina. It is consistently regarded by most authors as a very basal ornithischian, the sister group of remaining members of the clade. The referral to Ornithischia is based mainly on tooth-bearing bones and tooth morphology. On the other hand, the postcranium is recognized as strikingly plesiomorphic for ornithischians, and even for dinosaurs. The recent description of non-dinosaurian dinosauriforms of the clade Silesauridae having ornithischian-like dentition invites a review of the phylogenetic affinities of Pisanosaurus. In this regard, an overview of the holotype specimen allows a reanalysis of previous anatomical interpretations of this taxon. The phylogenetic analysis presented here suggests that Pisanosaurus may be better interpreted as a member of the non-dinosaurian Silesauridae. It shares with silesaurids reduced denticles on the teeth, teeth fused to maxilla and dentary bone, sacral ribs shared between two sacral vertebrae, lateral side of proximal tibia with a fibular flange, and dorsoventrally flattened pedal ungual phalanges. The present analysis indicates that Pisanosaurus should be removed from the base of the Ornithischia and should no longer be considered the oldest representative of this dinosaurian clade.

Keywords:

• Pisanosaurus mertii
• Ornithischia
• Silesauridae
• Late Triassic
• Argentina

Acknowledgements

We thank the late Jaime Powell, Rodrigo González, Carolina Madozzo Jaén and Pablo Ortíz (Instituto Miguel Lillo) for their help during the revision of the collections of the Instituto Miguel Lillo. We also thank F. Novas, M. Ezcurra, G. Lio, F. Brissón Egli and N. Chimento (Museo Argentino de Ciencias Naturales) for comments and discussion on the phylogenetic relationships of basal ornithischians and Pisanosaurus. We are especially indebted to J. F. Bonaparte for fruitful discussions and sharing unpublished data on the Pisanosaurus finding, and to Trevor Worthy for his help with the English language. We thank reviewers R. J. Butler, B. Parker and P. M. Barrett for helpful and enlightening comments.

Supplemental data

Supplemental material for this article can be accessed at: https://doi.org/10.1080/14772019.2017.1352623