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The dinosaurs roamed the Earth from the Upper Triassic Period to the close of the Mesozoic era and the Cretacous Period (about 150 million years). The most plausible explanation for the extinction of the dinosaurs is the impact of a large extraterrestrial object in the Gulf of Mexico.

All dinosaurs are diapsid archosaurian reptiles. They are believed to have evolved from a group of reptiles known as the Ornithodira from the Archosauria. Ornithodira includes all the pterosaurs (flying reptiles), the Dinosauromorpha (Lagosuchus, dinosaurs, and birds), and the genus Scleromochlus (an ancestor to the pterosaurs).

The name “dinosaur” was established by Sir Richard Owen in 1842. Dinosaur comes from the Greek words deinos and sauros (meaning “fearfully great” and “lizard” respectively). They are within the Class Reptilia, and have some anatomical and physiological similarities to many modern reptiles, particularly toward large lizards and alligators. Dinosaurs are divided into two orders, Saurischia (“reptile-hipped”) and Ornithischia (“bird-hipped”) within the Superorder Archosauria. The saurischians have a pelvis with a forwardly directed pubis bone, where as the ornithischian pubis is parallel to the ischium.

Pelvis of the two orders of dinosaurs. (A) Saurischia (“reptile-hipped”); and (B) Ornithischia (“bird-hipped”)
Abbreviations: a, acetabulum; ap, anterior process of pubis; il, ilium; is, ischium; and p, pubis.

Dinosaur background information provided by Andrew R. Milner;
Hanman's Fossils and Minerals
info@hanmansfossils.com http://www.hanmansfossils.com