They are dead animals; probably sharing a bilological relationship with modern birds. Materialistic evolution is false -- it says that "you" are a product solely of your brain; in much the same way that your liver produces bile, your brain produces "you." When you die, you will cease to exist, or so say the atheistic materialists -- Death is an endless, dreamless sleep.
This view is abjectly false; we know this to be so because you, as a spiritual being united with a physical, corporeal body, have free will. Atoms do not have free will, neither do computers, as much as AI researchers try to make them "think." They never will; they will always be bound completely by the conservation laws of nature (energy, momentum, angular momentum -- miracles are simply "violations" of those laws.)
Dinosaurs existed for the same reason that stars existed and continue to exist, for the Glory of the One and Triune God. They were simply part of His Creation, which we can now admire from a "safe" distance, both in time and space. However, we live in a fallen world, that of men but also that of angels, and all of Creation, dinosaurs included, suffered from that.