What is the most significant difference between precambrian and paleozoic life-forms?

Life forms in Precambrian period lived in water and thus were soft bodied while in  Paleozoic  period life forms originated on land also and thus they became hard bodied.

Explanation:

Most of the species in Pre - Cambrian period were having soft body while the species in Paleozoic era have a hard body.  

Precambrian life forms originated in oceans while in the  Paleozoic era life forms diversified and evolved on land as well.  

There were no well structured plants and trees in the Precambrian period while the plant and trees evolved in Ordivician period of the Paleozoic era.

the answer is d.) homologous; have mammary glands

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