Ornithoscelida is a clade that includes various major groupings of
dinosaurs. An order
Ornithoscelida was originally proposed by Thomas
Henry Huxley but later abandoned in favor of Harry Govier Seeley's
Saurischia and Ornithischia. The term was
revived in 2017 after a new cladistic analysis by Baron et al.
1 Huxley's concept
2 The modern
3 See also
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. 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
Scelidosauridae: Thecodontosaurus, Scelidosaurus, Hylaeosaurus,
Polacanthus (tentatively), and Acanthopholis
Iguanodontidae: Cetiosaurus, Iguanodon, Hypsilophodon, Hadrosaurus,
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 and Ornithischia.
In the beginning of the twenty-first century, improved descriptions of
the early ornithischians
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
indeed the Theropoda; and the
Sauropodomorpha outside the Dinosauria.
To avoid this, Baron and colleagues redefined all these groups.
Proposing that the
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:
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 hypothesis when using certain types of analysis.
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
Saurischia recovered as sister-taxa. However, this traditional tree
was only weakly supported and not statistically significantly
different from the alternative
^ Huxley, T.H. (1870). "On the classification of the
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
^ 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.