In 1842, Sir Richard Owen (1804–1892) assumed dinosaurs all descended from a common ancestor (in paleontological terms, they were a monophyletic group). This meant all dinosaurs shared some common characteristics. And when finally determined, the characteristics could be used to differentiate true dinosaurs from other organisms.

In 1887, Harry Seeley (1839–1909) discovered there were two major dinosaur groups: the saurischians and the ornithischians. This led scientists to assume there was more than one common ancestor. To further complicate matters, discoveries of diverse dinosaur species pointed to many ancestors. As a result of this, dinosaurs were seen as a group of reptiles with few characteristics in common. Because of these earlier scientific interpretations, dinosaurs no longer shared one set of common characteristics; thus, they were classified as a polyphyletic group, arising from many sources among the archosaurs. They became a polyphyletic group, arising from many sources among the archosaurs. Interpreted this way, there was no longer one set of common characteristics that dinosaurs shared.