The distribution of the percentage extinction for genera of marine animals throughout Phanerozoic shows that there has/have been great mass extinction(s) over the past 500 million years that claimed 40 percent or more of these genera.

Four great mass extinctions.

Explanation:

Phanerozoic era is a period of time that occured in over 542 millon years. It is an era in geological history that can be divided into 3 eras.

a. Paleozic era: This era occured between 542 to 251 million years ago

b. Mesozoic era: This era occured between 251 to 65 million years ago

c. Cenozoic era:This era pictures between 65 millon years ago till this present day.

There has been been four great mass extinctions over the past 500 million years that claimed 40 percent or more of these genera during the Phanerozoic era and they are:

a. Ordovician Mass Extinction

b. Devonian Mass Extinction

c. Permian Mass Extinction.

d. Cretaceous-Tertiary (or the K-T) Mass Extinction.

the element that provides strength to the exoskeletons of clams and oysters is phosphorus.

this is so because calcium carbonate is the main component in the shells. calcium carbonate is strengthened by phosphorus.

explanation:

the cell cycle is the final answer! : )