Nomenclature revisions (part 4)

Today’s blog will tag on the heels of “Nomenclature revisions (parts 1, 2 and 3) to highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades). Today we’ll restrict our scope to the Ornithosuchia.

Ornithosuchia – retained with a revision
Gauthier (1986) defined Ornithosuchia as the taxon that included extant birds and all extinct archosaurs that are closer to birds than they are to crocodiles. This definition is retained, with a revision. Gauthier’s Ornithosuchia included Ornithosuchidae + Ornithodira. Since pterosaurs were included within Ornithodira, Gauthier’s Ornithosuchia is redundant with Reptilia. Given a new node-based definition that deletes Ornithosuchidae and Pterosauria, the new Ornithosuchia is proposed to include TurfanosuchusTriceratops, their last common ancestor and all of its descendants. The outgroup is Crocodylomorpha.

Ornithodira – redundant
Gauthier (1986) defined “Ornithodira” as all forms closer to birds than to crocodiles. Here this definition is redundant with Ornithosuchia.

Sereno (1991) defined “Ornithodira” as the last common ancestor of the dinosaurs and the pterosaurs, and all its descendants. Here this definition is redundant with Reptilia.

Avesuchia – paraphyletic and redundant
Benton (1999) defined “Avesuchia/crown-group Archosauria” as the taxon comprising “Avemetatarsalia” and “Crurotarsi” (and sister taxa of “Crurotarsi” that are closer to Crocodylia than to Aves), and all their descendants. Because the definition included parasuchians, pterosaurs and Lagerpeton, here this created a paraphyletic clade redundant with Reptilia.

Avemetatarsalia – paraphyletic and redundant
Benton (1999) defined Avemetatarsalia as all “avesuchians/crown-group archosaurs” closer to Dinosauria than to Crocodylia. That definition is redundant with Ornithosuchia. Avemetatarsalia was meant to include Scleromochlus + pterosaurs + dinosauromorphs, but here that clade is paraphyletic (or redundant with Reptilia).

Dinosauriformes – no utility, paraphyletic
Novas defined Dinosauriformes as the most recent common ancestor of Marasuchus (Lagosuchus), Dinosauria and all descendants. Since Marasuchus is derived within the Theropoda here, that definition is now redundant with Dinosauria.

Benton (2004) redefined Dinosauriformes as Neornithes and all ornithodirans closer to Neornithes than to Lagerpeton. Since “Ornithodira” is now redundant with Reptilia (see above) and Lagerpeton now nests outside Euarchosauriformes, Benton’s definition has no utility.

Dinosauromorpha – no utility, paraphyletic
Sereno (1991) defined the Dinosauromorpha as all “Ornithodira” closer to Neornithes than to Pterosauria. Since the Pterosauria is far removed from the Dinosauria, this definition has no utility.

Sereno (1991) did not fix the problem when he stated the clade consisted of Passer and all species closer to Passer than to Pterodactylus, Ornithosuchus and Crocodylus. Sereno (1991) also provided a node clade definition: the last common ancestor of Lagerpeton, Lagosuchus, Pseudolagosuchus and the Dinosauria (including Aves) and all its descendants. Here Sereno’s definition is redundant with Archosauriformes. Removing the proterochampsid, Lagerpeton, from the definition creates a monophyletic clade, but one that would be redundant with Dinosauriformes (= Dinosauria). At present there are no non-dinosaur members to populate the Dinosauriformes or the Dinosauromorpha, since Crocodylomorpha is now the sister taxon of the Dinosauria.

Dinosauria – retained and expanded
Holtz and Padian (1995) defined the Dinosauria as all descendants of the most recent common ancestor of Triceratops and Passer, the sparrow. Standing firm, this definition still includes all taxa traditionally considered dinosaurs. It also adds members of the Poposauridae, PisanosaurusSilesaurus and Lotosaurus, a clade here labeled the Paraornithischia. Traditional clade names and inclusion lists for Theropoda, Sauropodomorpha and Ornithischia are retained.

Saurischia – no utility, paraphyletic
Seeley (1888) classified dinosaurs into two orders based on pelvis morphology. Here, with the Phytodinosauria, this division is polyphyletic and has lost its usefulness.

Phytodinosauria – resurrected
Bakker (1986) coined the term “Phytodinosauria” for a clade including Sauropodomorpha + Ornithischia. Here testing supports this clade.

Paraornithischia – new clade
The new clade Paraornithischia is proposed to include EffigiaLotosaurus, their last common ancestor and all of its descendants. This clade of apparent herbivores (none have sharp, serrated teeth and some are toothless) demonstrates a variety that hints at a wider radiation of undiscovered forms, all currently restricted to the Middle and Late Triassic. The clade also includes PisanosaurusShuvosaurus/Chatterjeea and Silesaurus. This clade consists only of herbivores, many of which had a predentary, paired predentaries or something like it that may have been fused to the often toothless dentaries.  While it may be tempting to consider this the clade basal to Ornithischia, at present moving this branch to the base of the Ornithischia adds four steps. More taxa will bring greater resolution.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

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Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Benton MJ 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. London: Phil Trans Roy Soc B, 354: 1423–1446.
Benton MJ, Clark JC 1988. Archosaur phylogeny and the relationships of the Crocodylia. In Benton MJ editor. The phylogeny and classification of the tetrapods, 295–338. Syst Assoc, Sp Vol 35A, Clarendon:Oxford.
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Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
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Walker AD 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Phil Trans R Soc London. Ser B Bio Sci 248 (744): 53–134.


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