I posted the below on the Nature online comment system (that people can register for).
A New Name for a New Potential Dinosaur Clade in a New Era of Phylogenetic-Taxonomic Instability within Dinosauria
Baron et al. (Nature 543, 501-506; 2017) and Holtz (Nature 545, 30; 2017) suggest radical redesignations of the old dinosaur group titles Saurischia and Pachypodosauria. They do so in order to deal with the plausible albeit not certain phylogentic results from the former’s major reordering of dinosaur relationships. Concerning the well-known Saurischia the Baron et al. redefinition will result in major confusion among the public. This is a pertinent issue because the International Commission of Zoological Nomenclature has ruled in favor of adjusting the generic titles of dinosaurs and other forms in a manner that helps preserve popular familiarity with the name, sometimes in ways that are a technical stretch1. It may also prove highly unstable because the scarcity and generalized anatomy of basal dinosaur fossils mean that sorting out the links between major dinosaur clades is likely to remain unsettled indefinitely, which risks the contents of Saurischia frequently flipping back and forth in contending studies. So while I concur with Holtz that Saurischia should not be redefined as per Baron et al., instability may also afflict his redefined Pachypodosauria, the original meaning of which is in any case much different from the Holtz proposal – it originally included sauropodomorphs and large but not small theropods. It is therefore proposed that the dinosaur clade containing Diplodocus carnegii but not Triceratops horridus be assigned its own particular title, Paxdinosauria, in recognition of the remarkably long reign of the group, an existence only terminated by the accident of an extraterrestrial impact. In this arrangement when the evidence indicates paxdinosaurs are a real clade then saurischians and pachypodosaurs disappear, and sauropodomorphs are a subset of paxdinosaurs rather than saurischians.
Predentata2 is usually considered a junior synonym of Ornithischia. But no designation specifically covers all dinosaurs in the clade that includes Triceratops horridus and possessed the predentate bone at the tip of the lower jaws so distinctive to the group, so it is proposed that this apomorphy plus clade group be assigned the title Predentata. In this scheme Ornithischia, as defined by Baron et al., remains a distinct node-based clade.
The future possibility of differing results concerning the relationships of basal dinosaurs means that the taxonomic positioning of herrerasaurs, Eoraptor,Eodromaeus, Daemonosaurus and Tawa are subject to frequent, major alterations. Although technically unavoidable if this occurs it will be inconvenient, so for more general purposes it may often be useful to informally refer to these early forms as basodinosaurs, which may become a formal clade if some or all of these forms prove to be their own clade distinct from Saurischia, Ornithischia, Ornithoscelida, Paxdinosauria, Phytodinosauria.
The Baron et al. results somewhat undermine the taxonomic utility of Theropoda, a node based clade that although real may or may not include some or all tetradactyl basodinosaurs, with the types of the latter in Theropoda being additionally uncertain. There is a need for a label that unambiguously includes all bird footed tridactyl theropods that either possessed a pes in which metatarsal 1 did not contact the distal tarsals, or descended from such theropods, and belong to the clade that includes Neotheropoda. Apomorphy plus clade Avepoda3-5 accurately describes and contains the distinctively avian configuration of the feet of the only dinosaur group with living examples. Because Neotheropoda6 is a node based clade based on the tridactyl taxa known when it was redefined7, it does not and cannot include all tridactyl theropods that are within the clade that includes avians, and is therefore a derived subset of Avepoda that must exclude the most basal three toed theropods, rather than being the senior synonym of the latter.
Some major divisions of Dinosauria such as Theropoda, Avepoda, Neotheropoda, Sauropodomorpha, Sauropoda, Ornithischia, and Predentata are well substantiated monophyletic clades, and are likely to remain so. Other, higher rank divisions such as Saurischia, Ornithoscelida, Paxdinosauria and Phytodinosauria are markedly more speculative at this time, and are likely to remain that way for a considerable period, and perhaps forever.
Baron et al. claim that the standard split of dinosaurs into Saurischia and Ornithschia has been “universally accepted” in recent decades, but the reality of Saurischia has been challanged or outright rejected by some researchers3-5,-6,8.
1. Paul, G. S. A revised taxonomy of the iguanodont dinosaur genera and species. Cretac. Res. 29, 192-216 (2008).
2. Marsh, O. C. The typical Ornithopoda of the American Jurassic. Amer. J. Sci. 48, 85-90 (1894).
3. Paul, G. S. Dinosaurs of the Air. (The Johns Hopkins University Press, 2002).
4. Paul, G. S. The Princeton Field Guide to Dinosaurs. (Princeton University Press, 2010).
5. Paul, G. S. The Princeton Field Guide to Dinosaurs, 2nd Ed. (Princeton University Press, 2016).
6. Bakker, R. T. Dinosaur Heresies. (William Morrow and Company, 1986).
7. Sereno, P. A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria. Neues Jahrb. Geol. Paläont. Abh. 210, 41-83 (1998).
8. Paul, G. S. Predatory Dinosaurs of the World. (Simon and Schuster, 1988).