Barrett, Paul M. Department of Palaeontology, The Natural History Museum, London, United Kingdom.
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One of two major monophyletic clades of Dinosauria, the other being Ornithischia. Saurischian dinosaurs take their name (meaning “lizard hipped”) from the structure of their pelvis. In saurischians, the pubis bone points downward and forward from the hip joint, and the ischium extends downward and backward, so that the two bones are separated by a wide angle in side view (see illustration). This feature allows the majority of saurischians to be easily distinguished from ornithischian dinosaurs (in which the pubis has rotated so that it points downward and backward and thus lies alongside the ischium). However, the saurischian condition is actually a retention of the primitive reptilian condition (which is also present in crocodilians, squamates, as well as many extinct groups) and, therefore, cannot be regarded as a unique feature of saurischians. To add to the confusion, some saurischian dinosaurs (various advanced theropod groups and their direct descendants, the birds) have independently evolved an ornithischian-type pelvis. However, Saurischia can be satisfactorily defined as a monophyletic clade of animals on the basis of many other detailed anatomical features shared by all members of the group, largely pertaining to the structure of the hand and vertebral column, such as the possession of a stout, robust thumb and the presence of incipient lamination on the vertebrae. See also: Dinosauria; Ornithischia
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