The effect of character and outgroup choice on the phylogenetic position of the Jurassic dinosaur Chilesaurus diegosaurezi

Abstract

The systematic position of the unusual Late Jurassic dinosaur Chilesaurus diegosaurezi remains an area of real uncertainty. Originally thought to be an unusual theropod, Chilesaurus has, since its first discovery, been suggested to either be a sauropodomorph dinosaur, an early diverging, ‘transitional’ ornithischian, or a more ‘derived’ neornithischian dinosaur, close to the heterodontosaurs. However, despite these recent fluctuations, the original placement within Theropoda remains the most prevalent phylogenetic hypothesis. This study looks to critically assess these various competing hypotheses regarding Chilesaurus and to analyse various aspects of the anatomical datasets that have placed it within Theropoda, Sauropodomorpha and Ornithischia. The choice of anatomical characters in past studies is considered and, crucially, so too is the choice of operational taxonomic units. By revising the datasets used in the initial analyses of Chilesaurus, one cause of this phylogenetic uncertainty has been clearly identified — outgroup choice. The total absence of members of Ornithischia from the predominantly saurischian-taxon-based datasets used in the original phylogenetic analyses of Chilesaurus is here shown to have had a substantial effect on the systematic position that was first recovered for this taxon. By scoring a selection of basal ornithischian taxa into the primarily saurischian datasets previously used, this study has recovered a different position for the Chilesaurus within Dinosauria, different internal topologies within Theropoda, and even a different arrangement of three major dinosaurian groups. The inclusion of an ornithischian outgroup in an otherwise predominantly saurischian focused dataset has produced results that again suggest that Chilesaurus could be an early diverging member of Ornithischia. What is more, this result was achieved without the addition of any classical ornithischian-like characters to the data. Whilst the results of this analysis do not by any means end the debate on the phylogenetic position of Chilesaurus, or on the fundamental relationships within Dinosauria, they do serve to highlight the current instability of such cryptic taxa in phylogenetic analyses, as well as the need for more comprehensive taxon sampling in such analyses generally. The results further highlight the potentially adverse effects of the omission of certain outgroup taxa on the overall topologies recovered within clades.