The main reason dinosaurs got so big is because they were reptiles. One fun fact about reptiles (snakes, lizards, etc..) is that they keep growing as long as they live. Human beings quit growing after about 18 years. But reptiles just keep on growing. Remember that the Bible says before Noah's flood people lived to be 900 years old (some even older). Can you imagine what would happen to an animal that kept growing for 900 years? It would get huge!
Scientists have found other animals that got much larger too. Beavers that were eight feet long! Cockroaches that were 18 inches long! Even human skeletons have been found of people that were 12 feet tall! So it's easy to see how other animals would get very big too.
The " . . . " means there is more to the verse,
you can read it in your Bible.
