Friday 18 August 2017

Chilesaurus diegosuarezi: How an enigmatic Dinosaur sheds light on the evolution of the whole group.

Chilesaurus diegosuarezi is a Late Jurassic Dinosaur from the Toqui Formation of Chile first described in 2015. It shows an unusual combination of features, combining elements usually associated with Therapods, Protosauropods and Ornithischian Dinosaurs. At the time of its description it was considered that, on the balance of probability, it was a Tetanuran Theropod (the group that also includes Tyrannosaurids, Megalosaurids, Ornithomimids, Allosaurids, Maniraptorforms, and Birds), with a number of unTheropod-like features that derive from its adaptation to a herbivorous lifestyle, leading to convergent evolution of some features with the herbivorous Protosauropods and Ornithischians.

However, in a paper published in March this year (2017), a group of palaeontologists led by Matthew Baron of the Department of Earth Sciences at the University of Cambridge and the Department of Earth Sciences at the Natural History Museum, presented the results of a cladistic study of Dinosaur phylogenetics (computerised analysis of relationships within the group based entirely upon shared common features rather than assumed relationships), which appeared to overturn some long-held views on Dinosaur evolution, most notably that the Ornithischians had diverged from other Dinosaurs before the split between Theropods and Sauropods; Baron et al.'s analysis suggested that this was an inaccurate and antiquated view, based upon the value attributed to the structure of the hip joint by Victorian palaeontologists, and that in fact the Ornithischians were more closely related to the Theropods than the Sauropods, which were the first group to split away from other Dinosaurs, early in the history of the group.

In a new paper published in the journal Biology Letters on 15 August 2017, Mathew Baron and Paul Barrett, also of the Department of Earth Sciences at the Natural History Museum, re-investigate the phylogentic position of Chilesaurus diegosuarezi, with a view to using this enigmatic species to better understand the history of the whole group.

Using this methodology, Baron and Barrett conclude that Chilesaurus diegosuarezi is in fact an Ornithischian rather than a Tetanuran Theropod, although it is the closest known species to the base of this group, holding a sister-group relationship to the rest of the Ornithischia, which are all more closely related to one-another than to Chilesaurus diegosuarezi. This lends strong support to the theory that Theropods and Ornithischians are closely related, with the 'primitive' Chilesaurus diegosuarezi being an Ornithischian that retains a number of Theropod-like features,

 Simplified time-calibrated strict consensus tree in which Chilesaurus is recovered within Ornithischia. Silhouettes represent supraspecific taxa—from left to right: Sauropodomorpha, Heterodontosauridae, Genasauria and Neotheropoda. Baron & Barrett (2017).

Chilesaurus diegosuarezi lacks a number of features usually thought of as being diagnostic of Ornithischian Dinosaurs, such as the predentary bone at the anterior end of the lower jaw, which presumably precluded its inclusion in the Ornithischia in its original description. However under the new classification proposed by Baron and Barrett this is not a problem, as these features are presumed to have evolved after the ancestors of  Chilesaurus diegosuarezi split away from the rest of the group.

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