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Harry Seeley came up with a classification system for dinosaurs — Saurischians (lizard-hipped) and Ornithischians (bird-hipped) — that is still used in paleontology. The only modern descendants of dinosaurs are birds. Recent discoveries like the fossils of proto-birds that have been uncovered in China only make that theory stronger. All other kinds of dinosaurs died out in the Cretaceous-Paleogene (K-Pg) extinction event which was probably caused by the impact of a large asteroid or comet near what is now Chicxulub, Mexico 66 million years ago.
Modern reptiles are the descendants of animals which were the cousins of dinosaurs. Despite the English translation of Saurischians as “lizard-hipped”, scientists are fairly sure that all Saurischian dinosaurs died out in the K-Pg event, and only some Ornithischian species survived and evolved into modern birds. Or, as one of my favorite cartoonists said, “Birds aren’t descended from dinosaurs, they are dinosaurs.” (XKCD Birds and Dinosaurs)