Moving with the times

The water was once zingily refreshing with its salt tang. Now it grew hot, thick, and swam with noxious gases. The fish was aware of his body’s mass for the first time, as he churned and struggled through the roiling current. His gills flapped in helpless desperation. Oxygen. Must have oxygen.
He struggled on, compelled by his choking cells. The water grew murkier as he passed a decaying plesiosaur, whose flesh flaked away to create a turgid soup. Normally he would rejoice at the sight – one less predator was no bad thing – but instead he felt primitive horror as he skirted through the gloom. Most of the plesiosaur’s skeleton was looming above the water line. The sea was almost gone.
The fish felt his brain would burst. He had to act. With instinctive panic, he swam into bubbling mud. He opened his gills to the air. Breathed. Evolved.