Fossils show that the first fish occurred in the Ordovician Period which began about 460,000,000 years ago. These fish were jawless, and the most primitive of all. Their mouths were a simple opening, suited to feeding on the tiny animals that lay hidden in the mud.
Next came fish with jaws. Jaws allowed fish to explore various food sources and to explore various food sources and to feed more efficiently.
Early fish with jaws are called placoderms. The jaws actually evolved from a set of gill arches that were present in the jawless fish. Gill arches are the bony supports of the gills.
From these placoderms came our present-day fish, the sharks and bony fish.
A group of a mammal-like reptiles preceded the appearance of the dinosaurs. These early mammals gradually disappeared, however, during the Triassic Period and were replaced by the true mammals.
It is hard to decide exactly which of these extinct animals was a reptile and which was a mammal. It is quite probable that the latter reptiles had hair and other mammal-like characteristics.
The first mammals were small and were vulnerable to all the fierce dinosaur predators.
Once the dinosaurs died out the mammals were able to develop properly and evolve.
Eventually the mammals grew into forms that were almost as gigantic as their dinosaur predecessors.