The Sandworms are the dominant life form on Arrakis/DE.
For many centuries it was believed that Sandworms were responsible for the desertification of Arrakis. However, many scientists later believed that sandworms were not the cause, but adapted out of that event.
According to Arna Satorinia, before becoming the gigantic water-aversive creature, the sandworm evolved from a small marine bottom-burrowing organism. The worm therefore was able to survive not only the cataclysmic event (49.7 Ma ago) that depleted all oxygen from the planet, but also the gradual evaporation of the original oceans. While most life had undergone mass extinction, the worm was given time to adapt, without being bothered by predators or competition for food.
By the time desertification was complete, the sandworm had become perfectly adapted to an arid environment, albeit now water-aversive.
Although the worm cannot withstand sudden contact with water, in theory, given some Ma of gradual change, the sandworms would evolve once again into marine-dwelling creatures.
The water needed for Pardot Kynes/DE terraforming project was lethal to sandworms; their death rate was much more higher than the rate of vegetation expansion. Combined with oxygen loss due to chemical reaction caused by the planet's high geological activity, the amount of oxygen dropped dramatically.
The last sandworm observed, was seen dying in 10402.