Cladistics is a method of classifying all organisms by a common ancestry, and it is based on the branching of the organism’s evolutionary family tree. Those that share common ancestors—and thus have similar features—fall into taxonomic groups called clades. Thanks to cladistic analysis, all dinosaurs were found to have many unique characteristics in common. In fact, these reptiles are defined as a monophyletic group descending from a common ancestor. And with the development of modern cladistic “testing,” true dinosaurs can be distinguished from their closely related, but non-dinosaur, contemporaries.

Using cladistic analysis, a reptile is a dinosaur if it has several specific characteristics in its fossilized skeleton, including some of the following: an elongated deltopectoral crest on the humerus; three or fewer phalanges in the fourth finger of the hand; the absence of a postfrontal bone; a crest on the tibia; three or more sacral vertebrae; a fully open hip socket; a ball-like head on the femur; and a well-developed ascending process on the astragalus, fitting on the front face of the tibia. In other words, identifying a dinosaur has a great deal to do with its skeletal anatomy. Although these characteristics are very technical, the main point is that scientists now have a clear test to determine if a fossil skeleton is truly that of a dinosaur.