Saurischia from the Greek
sauros (σαυρος) meaning 'lizard' and ischion
(ισχιον) meaning 'hip joint') is one of the two orders, or basic
divisions, of dinosaurs. In 1888, Harry Seeley classified
dinosaurs into two orders, based on their hip structure.Saurischians ('lizard-hipped') are distinguished from the ornithischians ('bird-hipped') by retaining the
ancestral configuration of bones in the hip.
All carnivorous dinosaurs (the theropods) are saurischians, as are one
of the two primary lineages of herbivorous dinosaurs, the sauropodomorphs. At the end
of the Cretaceous Period, all non-avian saurischians became extinct. This is referred to as the Cretaceous-Tertiary extinction
event. Avians (modern
birds), as direct descendants of one group of theropod dinosaurs, are
considered to be a sub-clade of
saurischian dinosaurs in phylogenetic classification.
Saurischians are distinguished from ornithischians by their three-pronged pelvic
structure, with the pubis
pointed forward. The ornithischians' pelvis is arranged with the pubis rotated backward, parallel with
the ischium, often also with a
forward-pointing process, giving a four-pronged structure.
The ornithischian hip structure is superficially similar to that of birds, which led Seeley to name them
"bird-hipped dinosaurs," though he did not propose any specific
relationship with birds. He termed saurischians "lizard-hipped" dinosaurs
because they retained the ancestral hip anatomy also found in modern lizards.
However, as later study revealed, the hip structure possessed by modern birds
actually evolved independently from the "lizard-hipped" saurischians
(specifically, a sub-group of saurischians called the Maniraptora) in the Jurassic Period. In
this example of convergent evolution, birds developed hips
oriented similar to the earlier ornithischian hip anatomy, in both cases
possibly as an adaptation to a herbivorous or omnivorous diet.
In his paper naming the two groups, Seeley reviewed previous classification
schemes put forth by other paleontologists to divide up the traditional Order
Dinosauria. He preferred one that had been put forward by Othniel Charles
Marsh in 1878, which divided dinosaurs into four Orders: Sauropoda, Theropoda, Ornithopoda, and Stegosauria (these names are still used today in
much the same way to refer to suborders or clades within Saurischia and Ornithischia).
Seeley, however, wanted to formulate a classification that would take into
account a single primary difference between major dinosaurian groups based on a
characteristic that also differentiated them from other reptiles. He found this
in the configuration of the hip bones, and found that all four of Marsh's orders
could be divided neatly into two major groups based on this feature. He placed
the Stegosauria and Ornithopoda in the Ornithischia, and the Theropoda and
Sauropoda in the Saurischia. Furthermore, Seeley used this major difference in
the hip bones, along with many other noted differences between the two groups,
to argue that "dinosaurs" were not a natural grouping at all, but rather two
distinct orders that had arisen independently from more primitive archosaurs.This
concept that "dinosaur" was an outdated term for two distinct orders lasted many
decades in the scientific and popular literature, and it was not until the 1960s
that scientists began to again consider the possibility that saurischians and
ornithischians were more closely related to each other than they were to other
Although his concept of a paraphyletic Dinosauria is no longer accepted by
most paleontologists, Seeley's basic division of the two dinosaurian groups has
stood the test of time, and has been supported by modern cladistic analysis of relationships
alternate hypothesis challenging Seeley's classification was proposed by Robert T. Bakker in
his 1986 book The Dinosaur Heresies. Bakker's
classification separated the theropods into their own group and placed the two
groups of herbivorous dinosaurs (the sauropodomorphs and ornithischians)
together in a separate group he named the Phytodinosauria ('plant
Phytodinosauria hypothesis was based partly on the supposed link between
ornithischians and prosauropods, and the idea that the former had
evolved directly from the later, possibly by way of an enigmatic family that
seemed to possess characters of both groups, the segnosaurs.However, it
was later found that segnosaurs were actually an unusual type of
herbivorous theropod saurischians closely related to birds, and
the Phytodinosauria hypothesis fell out of favour.
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