2) trilobites are useful as index fossils, because each species existed for

Trilobites are found worldwide and evolved rapidly, making them easily distinguishable from trilobites of other time periods

primary succession occurs in essentially lifeless areas regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. secondary succession occurs

trachea, alveoli, lung capillaries, arteries