If eras are marked by major changes in life forms, what distinguishes one period from the next?

Small jumps in species evolution, like size, strength, speed, color, etc.

Explanation:

Evolution distinguished both eras and periods. The thing is that eras involved bigger adaptation changes like the jump from unicellular to pluricellular beings. However, periods were marked by very different and smaller changes in species, like developing stronger muscles, lighter bodies, bigger jaws, smaller paws, etc. The periods always showed how different life forms improved their bodies with baby steps, instead of big jumps.

no, to many things have came in to factor