Complex sentence: A striking change from using atlatl to bow and arrow could be seen from strata 1 through 8 which contained only atlatl and dart items whereas strata 12 to 16 contained only bow and arrow items.
Split into simpler sentences: A striking change from using atlatl to bow and arrow could be seen when comparing the strata. From strata 1 through 8 there were only atlatl and dart items, whereas strata 12 to 16 contained only bow and arrow items.