A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone

Abstract

The South American Late Triassic offers the most comprehensive window to the early radiation of dinosaurs. This is enhanced by the discovery of Sacisaurus agudoensis, a new dinosauriform from the Caturrita Formation of Brazil. Various morphological features suggest its close phylogenetic affinity to Silesaurus, and both may be basal ornithischian dinosaurs. Sacisaurus has a pair of elements forming the tip of its lower jaw, hypothesized to be equivalent to the ornithischian predentary. This suggests that during an initial stage of their evolution, those dinosaurs had a paired predentary, which later fused into a single structure. As an originally paired bone, the predentary is comparable to elements that more often form the vertebrate mandible, such as the mentomeckelian bone. Although synapomorphic for ornithischians, the predentary does not seem neomorphic for the group, but primarily homologous to parts of the symphyseal region of the lower jaw of other vertebrates.

Keywords:

• Sacisaurus
• Late Triassic
• ornithischia
• predentary
• Brazil
• Caturrita Formation

Acknowledgements

The material of Sacisaurus agudoensis was collected by the crew of FZB/RS during field trips funded by “Projeto Pró-Guaiba”. We thank Ana Maria Ribeiro for her curatorial assistance, and Adam Yates, Paul Barrett, Randall Irmis, and Sterling Nesbitt, for critically reviewing the MS. We are particularly indebted to R. Irmis and S. Nesbitt for discussing ideas and providing essential information that deeply improved this paper. MCL acknowledges the financial support granted by the Brazilian agency FAPESP. We also thank colleague Marcelo Trotta, who shared with us his insightful idea for the generic name.

Notes

^† [email protected]

^† [email protected]

Additional information

Notes on contributors

Jorge Ferigolo

† † [email protected]