What is a dinosaur? Here’s the definition from Dictionary.com:
Any chiefly terrestrial, herbivorous or carnivorous reptile of the extinct orders Saurischia and Ornithischia, from the Mesozoic Era, certain species of which are the largest known land animals.That’s a decent description of a dinosaur, but it’s not the scientific definition.
Paleontologists don’t group species together according to when and where they lived, but based on shared characteristics, usually anatomical features seen in fossils. Dozens of features distinguish dinosaurs from other reptiles, such as bones of the lower arm being longer than the upper arm. When Paleontologists combine and compare such characteristics between various reptiles, they can create a tree of evolutionary history that reveals the relationships between different groups.
Several groups of prehistoric creatures are often mistaken for dinosaurs, even though evolutionary trees show that they’re distantly related. One group doesn’t even consist of reptiles. To help you tell extinct reptiles apart, here is a simple guide to the major features of dinosaurs and ‘other saurs’, arranged in order of when the animals appeared millions of years ago (MYA).
Link to guide