I keep seeing flying reptiles, like pteradon, in books as dinosaurs. And swimming dinosaurs too. But in a class the teacher said that all dinosaurs live on land, don't fly or live in the water. She also said that they have bones that come straight down from there hip bones and that makes them different from other reptiles. But then I've seen some in books that look like crocodiles and it says they're dinosaurs. Who is right? Are the books old or what? Thanks!
from Terri, age 12, Elk Grove, IL, USA; July 19, 2001

TOM: Some people get confused and think that all the strange reptiles of the Mesozoic (such as pterosaurs like Pteranodon or marine reptiles like the ichthyosaurs, plesiosaurs, and mosasaurs or crocodile relatives like Rutiodon or Postosuchus) are dinosaurs. However, that is not true. These creatures are not considered dinosaurs by paleontologists. Because of that, some books will say that "no dinosaur flew" and that "all dinosaurs lived on land". However, as you will see below, that isn't exactly true.

You are correct that dinosaurs are characterized by bones that come straight down from their hip sockets. There are a few other features found in all dinosaurs like that, and that is the reason that Sir Richard Owen (in 1842) first grouped some prehistoric reptiles into a new group which he named "Dinosauria" (the "fearfully great lizards").

Paleontologists (and scientists in general) now classify groups of animals based on patterns of common ancestry: that is, an animal belongs to a group because it descended from a certain ancestor. We can recognize patterns of common ancestry because of the presence of specialized features that evolved in that common ancestor: for example, the ancestor of all mammals had fur and gave milk, so all descendants of that ancestor give milk and have hair (although a few groups later lost hair as a new specialization, such as in whales).

Scientists now use the word "Dinosauria" for all the descendants of the most recent common ancestor of Iguanodon and Megalosaurus. In other words, an animal is a dinosaur if and only if it is a descendant of the last ancestor that Iguanodon and Megalosaurus had in common (or was that ancestor itself!). Based on our current knowledge of dinosaur anatomy, the ancestor of all dinosaurs had upright legs, and a partially opened hip socket, and a hand where fingers IV and V (the ring finger and pinky) were small. Later dinosaur groups had these features, although they may get modified along each particular line (for example, the meat-eaters lost their pinky and then their ring finger; the duckbills actually enlarged their pinky, and so on).

Paleontologists have discovered that birds are the descendants of advanced meat-eating reptiles, and that dromaeosaurs (raptors) were birds' closest relatives. Because birds are the descendants of meat-eating dinosaurs, they are also descendants of the most recent common ancestor of Megalosaurus and Iguanodon. That makes birds members of Dinosauria! Since some birds swim, and (of course) most birds fly, therefor there are some flying and swimming dinosaurs.