Ancient creatures can be verified by residual fossils: the older the older, the more the difference between its morphology and the existing species. At the same time people can also discover the relationship between species and species from fossils.
Archaeopteryx has sharp teeth, feet three toes have claws, long bone tail, these are dinosaur features, and it also has feathers, wings, and modern birds similar, so is considered a dinosaur evolution to the birds of the bridge. And later also found feather dinosaur fossils, confirmed that birds are evolved from the dinosaurs.
The neck of the giraffe will be extended over time. Researcher’s call this ancient beast remains