A new relative of the enigmatic dinosaur Chilesaurus from the early Late Jurassic of Argentina and the interrelationships and early evolution of dinosaurs

• Applicant: Professor Oliver Rauhut, Ph.D.
• Subject Area: Palaeontology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548605118

Traditionally, the iconic reptile clade Dinosauria has been subdivided into Ornithischia (“bird-hipped dinosaurs”) and Saurischia (“lizard-hipped dinosaurs”), the latter including the Sauropodomorpha and Theropoda. This division has recently been questioned, with a new analysis finding evidence for close relationships of Ornithischia and Theropoda, in a clade called Ornithoscelida, to the exclusion of Sauropodomorpha. This has, unsurprisingly, sparked controversy, and several recent studies have pointed out that none of the possible hypotheses – Saurischia, Ornithoscelida, or a clade of Sauropodomorpha and Ornithischia to the exclusion of Theropoda, usually dubbed Phytodinosauria – is statistically significantly better supported than any of the others on the basis of currently published data. A taxon that has played an important role in these discussions is the enigmatic Chilesaurus from the latest Jurassic (Tithonian) Toqui Formation of Chile, which shows features that were thought to be synapomorphies of either Theropoda, Ornithischia, or Sauropodomorpha. Its inclusion in datasets created to establish early dinosaur interrelationships has lend additional support to the Ornithoscelida hypothesis. However, Chilesaurus is almost 100 million years younger than the estimated time of the original split of Dinosauria. Thus, the current datasets used for testing early dinosaur interrelationships, which are focused on a limited taxon sampling of early branching members of the different subclades, might not be well-suited to test the phylogenetic position of Chilesaurus. Recent fieldwork in the Oxfordian-Kimmeridgian Cañadón Calcáreo Formation of Argentina has yielded almost complete remains of a Chilesaurus-like new dinosaur. Being almost ten million years older and showing several more plesiomorphic characters in respect to Chilesaurus, this new taxon has great potential to more securely place ‘chilesaurs’ in dinosaurian phylogeny, and to improve our understanding of dinosaur phylogeny and the interrelationships of the three main lineages, Ornithischia, Sauropodomorpha and Theropoda, in general. Interestingly, the new taxon shows characters that were so far considered to be unique to the different main dinosaurian lineages, although often not to their earliest branching members, but to more derived subclades. This further highlights the need to increase both taxon and character sampling in studies of early dinosaur interrelationships. Thus, the objectives of the current project are a detailed study of the new, Chilesaurus-like dinosaur from Argentina, and an elaboration of a large, critically revised phylogenetic dataset that can be used to test the phylogenetic position of these enigmatic taxa, and the interrelationships of early dinosaurs in general, as well as the role that homoplasy and parallel evolution might have played in the early evolution of that clade. For this, a collaboration with several leading experts on early dinosaurs is planned.

• DFG Programme: Research Grants
• International Connection: Argentina, Brazil, Switzerland, United Kingdom
• Co-Investigator: Dr. Sebastian Höhna
• Cooperation Partners: Professor Dr. Richard Butler; Dr. Serjoscha Evers; Professor Dr. Max Cardoso Langer; Dr. Fernando Novas; Dr. Diego Pol