Waterland

Waterland is set in the Bay of Bengal, 100,000,000 years into the future. The episode focusses on three species.
Toraton - a descendant of the tortoise and the largest creature ever to walk the Earth. They can weigh up to 120 tonnes.
Swampus - a descendant of the octopus which has become amphibious and formed a symbiotic relationship with a plant to house its young. They are very intelligent.
Lurkfish - a fish that can fire up to 1000 volts to stun prey. They are fish and ambush predators.
The episode shows that at this point of the future, volcanoes are belching out copious amounts of greenhouse gases causing the planet to overheat. It shows how swampus breed and how toraton have evolved to cope with their new size.