What is a dinosaur?
Dinosauria (dinosaur in Greek) was first proposed by a British paleontologist Richard Owen in 1842, which means 'fearfully great reptiles' in Greek. While closely examining the great reptilian fossils of the Mesozoic that were found in England at that time, he found characters common to three types of giant ancient terrestrial reptiles that do not belong to lizards nor crocodiles (ornithischian Iguanodon, theropod Megalosaurus, ankylosaur Hylaeosaurus), and suggested a new name, dinosaur, for the group (Benton 2000, Owen 1842).
Unlike the present general reptiles, a typical characteristic of dinosaurs is a sacrum composed of three or more fused vertebrae, a perforate acetabulum, and a ball-shaped femoral head. In addition, unlike crocodiles and lizards, knee joints and ankle joints are located directly below their bodies. Ultimately, thanks to their sturdy pelvis and skeletal leg structure, the dinosaurs were able to support their weight effectively, and were the advanced reptiles of the Mesozoic that could walk upright in the land like mammals (Benton 2004, Fastovsky & Weishampel 2009).
Pelvic and leg joints for erect stance
The dinosaur hind legs can not spread sideways due to the structures ranging from the pelvic joint to the ankle joint, and the range of movement is limited only in the anteroposterior direction. Therefore, unlike lizards, the dinosaur hind legs can extend straight down from the body and naturally stand upright.
Fused vertebrae of the dinosaur
Maiasaura
Swimming dinosaurs?
A common mistake in describing the range of dinosaurs is the inclusion of pterosaurs, ichthyosaurs, and plesiosaurs in dinosaurs. The distinctive feature of Owen's 19th-century dinosaur definition at the time was the 'unique pelvis and leg structure that was constructed to stand upright and walk as a Mesozoic land reptile'. Although pterosaurs, ichthyosaurs, plesiosaurs, and Mosasaurus are all related as dinosaurs with common ancestors (Diapsida), their poor pelvis or limb structure does not fit the definition of a dinosaur. They were reptiles of the sky and of the sea that evolved from primitive reptiles before dinosaurs appeared.
Pteranodon
