I find it hard to believe that dinosaurs ever existed. They were never mentioned and totally skipped in the holy books (Old Testament, New Testament, Qur'an) although these books do contain stories as old as it gets (to Adam and Eve, Noah, The Great Flood, and Ibrahim). How can we know dinosaurs existed?
In ancient times Xenophanes (570-480 BC) wrote about fossil sea shells indicating that land was once under water
Dinosaurs is a very broad term and refers to a group of animals of which we have bones and entire skeletons.
Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, approximately 230 million years ago, and became the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65.5 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic era. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic, and consequently they are considered a type of dinosaur in modern classification systems.
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You can visit most museums and see or touch some fossils them if that convinces you.