[Image: S.H.Morgan’s cladogram of the animal groups that fall under Dinosauria. Somewhat outdated, but still useful for our purposes.]
Many of us grew up referring to any vaguely reptilian prehistoric animal as a “dinosaur”. In truth, that group is much more exclusive than you might think. Dinosaurs fall under one of two orders—Saurischia or Ornithischia—and share a more recent common ancestor with one another than with any of the following animals:
The first vertebrates capable of powered flight, pterosaurs ruled the Mesozoic skies long before the earliest birds appeared on the scene. Current thinking is that they shared a close relationship with dinosaurs in the group Ornithodira, but they themselves were not dinosaurs.
Many prehistoric crocodile relatives had erect limbs like dinosaurs, so perhaps it’s no wonder people get them confused. However, these animals evolved their erect stance independently of dinosaurs. A good rule of thumb to remember is that if it walks on four legs and looks like a crocodile, it probably isn’t a dinosaur.
[Image: Various prehistoric marine reptiles by Sergey Krasovskiy]
Unlike birds such as penguins, non-avian dinosaurs generally weren’t as big on the whole aquatic lifestyle thing as we once thought. The dolphin-like ichthyosaurs, long-bodied mosasaurs and snaky- or thick-necked plesiosaurs were more closely related to lizards than to dinosaurs.
[Image: Dimetrodon by Marco A. Pineda]
As synapsids, the often sail-backed pelycosaurs were more closely related to mammals than to dinosaurs. That’s right—creatures often marketed as dinosaurs actually occupy a branch on the animal family tree much closer to you and me!
This group includes modern mammals, so it should be pretty obvious why they’re not considered dinosaurs despite many of the early forms’ more reptilian appearances.
Plenty of other examples exist, but these critters are some of the most common culprits when it comes to being confused for dinosaurs. Remember, it can’t hurt to do your research before calling something a dinosaur!