The Creatures of the past are not looking like them we see now.
The most of them were destroyed by a large meteor and a thorough change of the Earth's surface with numerous volcanic eruptions to follow and the movement of the tectonic plates on a large scale.
But some of them survived until today, but outside we can say why they did it.
Because when you think of Darwinian evolution, where it is the strongest that survive and multiply, so it seems a little strange to see some of these creatures, they are not some creatures that immediately would be able to survive from about 300 million ago and up until today, so why?
Why did none of the very large animals (Dinosaurs) not survive eks. those there lived in the water?
Or some of the many offspring that was after the very large animals?
Could it be that there was just a handful of those who had been hiding in caves or other places like the animals that survived did?
No, they were all exterminated both adults and kids.