Don't you know how the biological heirarchy works? It's like a family tree. When an organism evolves into seperate organisms they are placed on a new taxonomic rank. The superclass tetrapoda evolved into reptiles and amphibians. Reptiles then evolved into sub-classes which evolved into some other stuff which eventually evolved into birds. Then birds (and mammals) get a promotion and get bumped to the same level as reptiles even though they are their decendants; rather like drawing your brother on the same level as your great-grandfather on a family tree because he looks different to the rest of the family.
The reason we are not in the same class as giant sea scorpions is because our ancestors split from theres into a different phylum (right at the begining of animal life) much before classes became defined.
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