One of the problems with the scientific definition of the word "dinosaur" is that biologists and paleontologists tend to use much drier, more precise language than your average dino enthusiast on the street. So while most people intuitively describe dinosaurs as "large, scaly, dangerous lizards that went extinct millions of years ago," experts take a much narrower view.
In evolutionary terms, dinosaurs are the land-dwelling descendants of ancient archosaurs, egg-laying reptiles that survived the Permian Extinction (or appeared shortly afterward). Technically, dinosaurs can be distinguished from other prehistoric creatures descended from archosaurs based on a handful on anatomical quirks. Chief among these is posture: dinos had either an upright gait (much like modern birds), or, if they were quadrupeds, a fairly stiff, straight-legged style of walking (unlike modern lizards, whose limbs splay beneath them when they walk). Beyond that, the distinguishing features become rather arcane, such as an "elongate deltopectoral crest on the humerus."
Saurischians and Ornithischians
The dinosaur family is divided into two main groups. To vastly simplify the story, starting about 250 million years ago a subgroup of archosaurs split off into two types of dinosaurs, distinguished by the structure of their hip bones. Saurischian ("lizard-hipped") dinosaurs included predators like T. Rex and huge herbivores like Apatosaurus, while ornithischian ("bird-hipped") dinosaurs consisted of assorted other herbivores, such as Stegosaurs. (Confusingly, most scientists now believe that birds descended from saurischian, not ornithischian, dinosaurs!)
What Is (or Isn’t) a Dinosaur?
You may have noticed that the above definition of dinosaurs refers to "land-dwelling" creatures, which technically excludes ocean-dwelling lizards like Kronosaurus, and flying lizards like Pterodactylus, from the dinosaur umbrella (these and other species are also sometimes omitted for anatomical reasons). For the purposes of this site, though, there’s no reason to be as rigorous as an evolutionary biologist: here we discuss all sorts of ancient lizards, whether they can technically be classified as "dinosaurs" or not.
