Scientists may conclude to climate was very warm during a particular time if they discovered many fossils of what

The correct answer is - marine animals, amphibians, and reptilians.

The fossils of the animals can be very good indicator of what the climate has been in a particular period of the geological past. Some animals tend to thrive in warmer climates, some animals in colder climates, while some are typical for more moderate ones.

If the climate has been warmer, than it means that the sea levels would have been higher, thus larger water environment. That will result in an abundance of fossils of marine life. Also, the amphibians are enjoying the waters, thus they will be represented int he fossil records much more. The reptilians are a group of animals that are cold blooded, so whenever the climate is warmer they tend to thrive. Also, they diversify a lot in those periods and are becoming larger, thus the reptilian fossils will be abundant as well.