Explanation

Placoderms, Archeopteryx, and Therapsids are true fossils representing extinct organisms.

**Placoderms:** Ancient armored fish from the Devonian period, well-known for their fossilized remains.

**Archeopteryx:** A transitional fossil that shows characteristics of both dinosaurs and birds, marking a key step in avian evolution.

**Therapsids:** Synapsids from the Permian and Triassic periods, regarded as ancestors to mammals.