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Clade
Subclade

Taxon
Taxon
Ornithischia
Nominal Author
Seeley 1888
2nd Nominal Author
Taxon Status ACTIVE
Comments

Ornithischia was first defined by Padian and May (1993) using Triceratops and “birds” as specifiers. Sereno (1998) subsequently replaced “birds” with Neornithes, linking Ornithischia with Saurischia and Dinosauria as a node-stem triplet. Benton (2004) cited Sereno’s definition of Ornithischia in The Dinosauria, whereas Weishampel (2004) and Norman (2004) presented first-order alternatives in the same volume without explanation, the former using Tyrannosaurus in place of Neornithes and the latter using Iguanodon and Cetiosaurus as internal and external specifiers, respectively.

Presumably Norman’s choice of specifiers was based on their antiquity, although Megalosaurus, not Cetiosaurus, was the first named basal saurischian. Regardless, neither Megalosaurus nor Cetiosaurus are well understood, and both have functioned historically as repositories for disparate taxa. Megalosaurus, Cetiosaurus and Iguanodon, in addition, are less nested within Dinosauria than alternative specifiers.

The practice of selecting first-named or historically significant taxa for specifiers can decrease stability and clarity in definitions. For these reasons, the active definition is a first-order revision of the original definition (Padian and May 1993). It employs species as specifiers and, should sauropodomorphs prove to be more closely related to ornithischians than to theropods, maintains the taxonomic content of Ornithischia by adding Saltasaurus loricatus as a second external specifier.

Potential Synonomy
Predentata

Active Phylogenetic Definition
Active Definition
The most inclusive clade containing Triceratops horridus Marsh 1889 but not Passer domesticus (Linnaeus 1758) or Saltasaurus loricatus Bonaparte and Powell 1980.
Shorthand
> Triceratops horridus but not Passer domesticus or Saltasaurus loricatus.
Definitional Author
Sereno 2005
Definition Status checked
Definition Type STEM
Node-Stem Triplet STEM
Other Triplet Taxa
Dinosauria, Saurischia
Specifiers
Specifier(s) A
Triceratops horridus
Specifier(s) B
Specifier(s) C
Passer domesticus, Saltasaurus loricatus
Qualifiers
+Taxon
-Taxon
Datum
Taxonomic Content
Publication Year Unknown
Unknown
Unknown
1993

Inactive Taxon Status
Rejection Criteria
Critique
 

Definitional History #1
Earliest Record
Those dinosaurs closer to Triceratops than to birds.
Latest Record
Padian and May 1993:379
Range (My)
Range (My)
Triceratops, birds
Range (My)

Definitional History #2
Earliest Record
All dinosaurs closer to Triceratops than to Neornithes.
Latest Record
Sereno 1998:61
Range (My)
Range (My)
Triceratops, Neornithes
Range (My)

Definitional History #3
Earliest Record
All dinosaurs more closely related to Triceratops than to Tyrannosaurus.
Latest Record
Weishampel 2004:323
Range (My)
Range (My)
Triceratops, Tyrannosaurus
Range (My)

Definitional History #4
Earliest Record
All dinosaurs closer to Iguanodon than to Cetiosaurus.
Latest Record
Norman et al. 2004:325
Range (My)
Range (My)
Iguanodon, Cetiosaurus
Range (My)

Definitional History #5
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Definitional History #6
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Definitional History #7
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Definitional History #8
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Definitional History #9
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Definitional History #10
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