SVP 2018: The clade ‘Ornithoscelida’ tested

Mortimer et al. 2018
reevaluate taxa and scores for the proposed clade ‘Ornithoscelida’ (Baron, Norman and Barrett 2017) and find it less parsimonious than alternatives. The authors, “find hundreds of questionable scores, many characters are correlated with each other, score for multiple variables at once, or are formed in such a way that potential homology is masked.” 

After repairing scores,
the authors report, “none of these experiments supported Ornithoscelida over Saurischia.” Their results show:”that phylogenetic analysis of morphological data is highly vulnerable to typographic errors and other accidental, unsystematic misscores in data matrices; both quantity and quality of scores are important.”

We looked at
the Ornithoscelida hypothesis earlier here, here, here, and here. It is not supported by the large reptile tree (LRT, 1313 taxa, subset Fig. 1), which supports the hypothesis of a Theropoda-Phytodinosauria dichotomy splitting Dinosauria after the Herrerasaurus clade.

Importantly
the outgroup taxa to the Dinosauria must be recovered correctly. At present few to no other studies have included a robust selection of bipedal crocodylomorphs, which nest as the sister group to the Dinosauria in the LRT. Together only Dinosauria and Crocodylomorpha make up the Archosauria in the LRT.

Figure 4. Subset of the LRT focusing on the Phytodinosauria. Three sauropods are added here.

Figure 4. Subset of the LRT focusing on the Phytodinosauria.

Scoring errors
are found in all (yes, all) phylogenetic analyses, including the LRT. One way to ‘eyeball’ whether an analysis is close to recovering actual evolutionary events is to look at every node for a gradual accumulation of derived traits. Key to this ideal is the inclusion of a sufficient number of relevant taxa. The LRT provides a good guide for taxon selection. As it grows larger it becomes a self-healing cladogram where imprecisely nested taxa reveal themselves. Some scores, when corrected, cement relationships. Other scores crack them apart.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Mortimer M, Gardner N, Marjanovic D and Dececchi A 2018. Ornithoscelida, phytodinosauria, saurischia: stesting the effects of miss cores in matrices on basal dinosaur phylogeny. SVP abstracts.

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