In our second grade science textbook it states that if an animal either flew or swam it was not considered a true dinosaur. Is this true?
from Jill W., age 29, Hastings, NE, USA; July 16, 2001

TOM: Not exactly. This simplification has been used in the past as a way of teaching kids that the marine reptiles (plesiosaurs, ichthyosaurs, mosasaurs, etc.) and the flying pterosaurs are not dinosaurs. That is still true.

However, paleontologists define dinosaurs as all the descendants of the most recent common ancestor of Iguanodon and Megalosaurus. We now know that birds are descendants of that ancestor, and thus birds are technically a type of dinosaur. Since most birds obviously fly, then there are plenty of flying dinosaurs. Furthermore, both extinct (Hesperornis) and modern (penguins) birds swim, so there are swimming dinosaurs.