Birds are the most diverse and speciose group of terrestrial vertebrates. Scientists had noted the anatomical similarities of birds and dinosaurs long before their close evolutionary history was understood. The discovery of fossil feathered species such as Archaeopteryx gave support to the theory that birds are directly descended from dinosaurs. This is no longer merely a theory – although most dinosaurs famously went extinct 65 million years ago, fossil evidence shows that modern birds evolved from a theropod dinosaur leneage towards the end of the Jurassic Period. They were thriving in the Cretaceous ecosystems and are the only dinosaur lineage to have survived to the modern day. Birds are relatively closely related to crocodiles and together form a clade called the Archosaurs. Since crocodiles are more closely related to birds than they are to lizards, birds can be classified as reptiles.