The only honest answer is "I don't know." Groucho is right in stating that biblically and theologically death couldn't have existed before sin. It seems incompatible with Christ as our salvation, His death and resurrection. How do you answer that and remain a Christian?

The scientific theory is  we know how to measure elapsed time by our dating. I am not convinced of that. God is outside of time. It's also possible dinosaurs existed in another part of the globe at the same time or may have been part of another creation. Since the biblical term "earth" or "world" could have meant the known world, we can't know. That's just my opinion.