Examples

The elephant's prehistoric ancestors were (mammoths/mammoth's).

Step 1. The plural noun is mammoths.
Step 2. No apostrophe is needed since the plural noun is not also possessive.

Most (scientists/scientists') best guesses are that mammoths lived two million years ago.

Step 1. The plural noun is scientists.
Step 2. An apostrophe is needed since the plural noun is also possessive. It modifies best guesses.

The diet of (mammoths/mammoth's), like that of elephants, was herbivorous.

Step 1. The plural noun is mammoths.
Step 2. No apostrophe is needed since the plural noun is not also possessive.

As a result, (elephants/elephant's) have teeth that are similar to the mammoth's.

Step 1. The plural noun is elephants.
Step 2. No apostrophe is needed since the plural noun is not also possessive.

Because of the northern climate's influence, (mammoths'/mammoths) ears were smaller, making them less likely to freeze.

Step 1. The plural noun is mammoths.
Step 2. An apostrophe is needed since the plural noun is also possessive. It modifies ears.