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Ornithoscelida
Ornithoscelida
is a clade that includes various major groupings of dinosaurs. An order Ornithoscelida
Ornithoscelida
was originally proposed by Thomas Henry Huxley but later abandoned in favor of Harry Govier Seeley's division of Dinosauria
Dinosauria
into Saurischia
Saurischia
and Ornithischia. The term was revived in 2017 after a new cladistic analysis by Baron et al.

Contents

1 Huxley's concept 2 The modern Ornithoscelida
Ornithoscelida
hypothesis 3 See also 4 References

Huxley's concept[edit] Thomas Henry Huxley
Thomas Henry Huxley
originally defined the term in an 1869 lecture as a group comprising two subgroups: the large and heavy-set Dinosauria and the newly discovered Compsognathus, which he placed in a new grouping Compsognatha.[1] The former were defined by their shorter cervical vertebrae, and the femur length exceeding tibia length, and the latter with longer cervical vertebrae, and the femur length shorter than tibia length. He noted that the characteristics of their bones showed many features akin to birds. The dinosaurs Huxley had divided into three families:

Megalosauridae: Teratosaurus, Palaeosaurus, Megalosaurus, Poekilopleuron, Laelaps, and Euskelosaurus
Euskelosaurus
(tentatively) Scelidosauridae: Thecodontosaurus, Scelidosaurus, Hylaeosaurus, Polacanthus
Polacanthus
(tentatively), and Acanthopholis Iguanodontidae: Cetiosaurus, Iguanodon, Hypsilophodon, Hadrosaurus, and Stenopelix
Stenopelix
(tentatively)

This classification quickly fell out of use, due to the dominant classification system by Harry Govier Seeley
Harry Govier Seeley
that grouped dinosaurs into two main branches: Saurischia
Saurischia
and Ornithischia.[2] The modern Ornithoscelida
Ornithoscelida
hypothesis[edit] In the beginning of the twenty-first century, improved descriptions of the early ornithischians Heterodontosaurus
Heterodontosaurus
and Lesothosaurus
Lesothosaurus
vastly increased the available information on the origins of the Ornithischia. In March 2017, a paper in the journal Nature by Matthew Baron, David Norman, and Paul Barrett, published an analysis in which the theropod dinosaurs — no longer containing the Herrerasauridae — were more closely related to ornithischian dinosaurs than to the Sauropodomorpha, the group to which the sauropod dinosaurs belong.[3] Previous analyses had rather combined the Theropoda
Theropoda
with the Sauropodomorpha
Sauropodomorpha
into the Saurischia, to the exclusion of the Ornithischia. These groups had also been formally defined to reflect this. Using these standard definitions, the new results would have had the effect of bringing the Ornithischia
Ornithischia
within the Saurischia
Saurischia
and indeed the Theropoda; and the Sauropodomorpha
Sauropodomorpha
outside the Dinosauria. To avoid this, Baron and colleagues redefined all these groups. Proposing that the Ornithischia
Ornithischia
and Theropoda
Theropoda
were sister groups also meant that a new name was needed for the clade combining them. They named this new clade Ornithoscelida, defining it as "the least inclusive clade that includes Passer domesticus
Passer domesticus
and Triceratops horridus." This means that this node clade consists of the last common ancestor of the extant theropod Passer and the ornithischian Triceratops; and all its descendants. Huxley's old name Ornithoscelida was chosen because its meaning, "bird legs", well fitted the hindlimb traits of the clade.[4] The cladogram below shows the phylogeny from Baron et al. 2017:

Dinosauromorpha

†Marasuchus

unnamed

†Silesauridae

Dinosauria

Saurischia

†Herrerasauridae

†Sauropodomorpha

Ornithoscelida

†Ornithischia

Theropoda

A follow-up study, presented by Parry, Baron and Vinther (2017), demonstrated how, if using the same dataset, the Ornithoscelida hypothesis can also be recovered using a range of different phylogenetic analysis methods, including Bayesian maximum-likelihood. The same study, when analysing a modified version of the original Baron et al. (2017) dataset, also found some support for the Phytodinosauria
Phytodinosauria
hypothesis when using certain types of analysis.[5] The Ornithoscelida
Ornithoscelida
hypothesis has been challenged by a team of international researchers in November 2017, following a correcting of the original anatomical dataset produced by Baron and colleagues. This reworking produced the traditional model, with Ornithischia
Ornithischia
and Saurischia
Saurischia
recovered as sister-taxa. However, this traditional tree was only weakly supported and not statistically significantly different from the alternative Ornithoscelida
Ornithoscelida
hypothesis.[6][7] See also[edit]

Phytodinosauria Saurischia

References[edit]

^ Huxley, T.H. (1870). "On the classification of the Dinosauria
Dinosauria
with observations on the Dinosauria
Dinosauria
of the Trias". Quarterly Journal of the Geological Society of London. 26: 32–51. doi:10.1144/gsl.jgs.1870.026.01-02.09.  ^ Padian, Kevin (2017). "Palaeontology: Dividing the dinosaurs". Nature. 543 (7646): 494–495. doi:10.1038/543494a.  ^ https://www.cam.ac.uk/research/news/new-study-shakes-the-roots-of-the-dinosaur-family-tree ^ Baron, M.G., Norman, D.B., and Barrett, P.M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543: 501–506. doi:10.1038/nature21700 ^ Luke A. Parry; Matthew G. Baron; Jakob Vinther (2017). "Multiple optimality criteria support Ornithoscelida". Royal Society Open Science. 4 (10): 170833. doi:10.1098/rsos.170833.  ^ Max C. Langer; Martín D. Ezcurra; Oliver W. M. Rauhut; Michael J. Benton; Fabien Knoll; Blair W. McPhee; Fernando E. Novas; Diego Pol; Stephen L. Brusatte (2017). "Untangling the dinosaur family tree". Nature. 551 (7678): E1–E3. doi:10.1038/nature24011.  ^ Matthew G. Baron; David B. Norman; Paul M. Barrett (2017). "Baron et al. reply". Nature. 551 (7678): E4–E5. doi:10.1038/

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