Calculate the mean (average) persistence times for species with planktonic larvae and species with nonplanktonic larvae.

One of the life stages of invertebrate organisms that allows the species to disperse is the planktonic larval stage. The growth of the larvae occurs at adrift at sea. In contrast, nonplanktonic larvae develop at the benthic layer of the sea and do not show much dispersion.

The mean or average is calculated by adding given scores or data and dividing the sum of the data by the number of scores or data.

Mean persistence time of species with planktonic larvae equals sum of persistence time of planktonic larvae species divided by the total number of bars.

\(\frac{{6.12 + 6.12 + 6.12 + 1.08 + 1.44 + 6.12 + 0.17 + 7.92 + 2.16 + 6.12 + 6.12 + 7.20 + 3.24}}{{13}} = 4.6\)

Hence, the average persistence time of planktonic larvae species is 4.6 million years.

The mean persistence time of species with nonplanktonic larvae equals the sum of the persistence time of nonplanktonic larvae divided by total number of bars.

\(\frac{{1.80 + 1.80 + 6.12 + 1.08 + 2.52 + 2.52 + 0.72 + 1.44 + 1.44 + 4.68 + 2.88 + 1.80 + 1.80 + 1.80 + 1.80 + 1.08 + 1.08 + 1.08 + 1.08 + 1.08 + 1.08 + 1.08 + 0.72 + 0.72 + 0.72 + 0.72 + 2.16}}{{28}} = 1.77\)

Hence, the average persistence time of nonplanktonic larvae species is 1.7 million years.