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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
Harry Govier Seeley
's division of 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

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
Compsognathus
, which he placed in a new grouping Compsognatha . 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
Megalosaurus
, Poekilopleuron , Laelaps , and Euskelosaurus (tentatively) * Scelidosauridae : Thecodontosaurus , Scelidosaurus , Hylaeosaurus , Polacanthus
Polacanthus
(tentatively), and Acanthopholis * Iguanodontidae : Cetiosaurus , Iguanodon , Hypsilophodon
Hypsilophodon
, Hadrosaurus
Hadrosaurus
, and 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 .

THE MODERN ORNITHOSCELIDA HYPOTHESIS

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. Previous analyses had rather combined the Theropoda with the 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 within the Saurischia
Saurischia
and indeed the Theropoda; and the Sauropodomorpha outside the Dinosauria. To avoid this, Baron and colleagues redefined all these groups. Proposing that the Ornithischia and 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 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. The cladogram below shows the phylogeny from Baron et al. 2017:

Dinosauromorpha

Marasuchus
Marasuchus

unnamed

Silesauridae

Dinosauria

Saurischia
Saurischia

Herrerasauridae

Sauropodomorpha

Ornithoscelida

Ornithischia

Theropoda

SEE ALSO

* Phytodinosauria * Saurischia
Saurischia

REFERENCES

* ^ Huxley, T.H. (1870). "On the classification of the Dinosauria with observations on the Dinosauria of the Trias". Quarterly Journal of the Geological Society of London. 26: 32–51. * ^ Padian, Kevin (2017). "Palaeontology: Dividing the dinosaurs". Nature. 543 (7646): 494–495. doi :10.1038/543494a . * ^ 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

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Ornithoscelida
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