Why where prehistoric animals so huge?

The reason why so many prehistoric animals — mastodons, mammoths (whose name means "huge") and many dinosaurs — were so big is something of a mystery. For a long time, environmental factors such as higher oxygen content in the air and greater land masses (i.e., more space) were thought to contribute to their large size. read more

No one knows for sure, but there are lots of theories. Being larger can provide many evolutionary advantages—bigger animals are less vulnerable to predators and can compete more assertively for resources. read more