Dinosaurs are distinguishable from other reptiles by several characteristics. The most obvious, however is their posture. All dinosaurs, alive and extinct, would stand holding their limbs straight below them much like a mammal. We know this because their leg bones won't fit together any other way.
|Illustration of Diplodocus by artist, Mary Mason Mitchell from the late 1800s, showing what we now consider an incorrect sprawling posture.|
|Barosaurus, a dinosaur, standing with an erect posture.|
Other reptiles like squamates (snakes and lizards), testudians (turtles and tortoises) and crocodilians stand with their limbs out to the sides of their bodies in a sort of sprawling or semi-sprawling posture.
|Cuban Rock Iguana or Cyclura nubila, a modern lizard. See how the legs come out to the sides? Not a dinosaur.|
So no, animals like lizards and crocodiles, despite how they superficially resemble what many people believe dinosaurs look like, are not true dinosaurs themselves. This can also be determined by looking at their evolutionary histories (a bit more complicated to go over in one blog post!). Fun fact though: Crocodillians are more closely related to birds(dinosaurs) than they are to other reptiles like lizards, snakes and testudians. They still aren't actual dinosaurs though, just very close relatives.
What about all those other prehistoric animals? Things like the flying pterosaurs, marine plesiosaurs and the sail-finned Dimetrodon.
|Nope. This one is actually more closely related to us mammals than to dinosaurs. Its also way older than the oldest dinosaur by roughly a few million years.|
Its confusing at first, I know. But if you look at the defining characteristics of a true dinosaur, none of these guys meet all of them. And again, the whole different evolutionary history thing is important too. The point I want to bring across here is just because an animal is a big scaly prehistoric creature, doesn't mean it is automatically a dinosaur. There were lots of interesting prehistoric animals running around in the earth's past just like there are today. Dinosaurs didn't account for everything!
Holtz, Thomas R., and Luis V. Rey. Dinosaurs: The Most Complete, Up-to-date Encyclopedia for Dinosaur Lovers of All Ages. New York: Random House, 2007. Print.
Kenneth D. Angielczyk, Dimetrodon Is Not a Dinosaur: Using Tree Thinking to Understand the Ancient Relatives of Mammals and their Evolution Evolution: Education and Outreach, Volume 2, Number 2, 257-271