Oxygen Saga. All known "intelligent" forms of life requite oxygen, and the atmospheres of all habitable planets con tain at least 19% oxygen The history of oxygen on Arrakis is unique, including one proved oxygen-depletion catastrophe and one near-catastrophe, the latter occurring in historical times, making Arrakis worth special note. The major source of atmospheric oxygen is plant photosynthesis. If life evolves on a planet at all, rudimentary plant life is first seen in the fossil record about one-and-a-half to two billion years after planet formation Two billion years or so pass before the required 19% oxygen is reached. By this time advanced plant hfe forms are widespread. Fossil remains of lycopsids ami spheaopstds are especially common Arrakis followed this sequence, its atmospheric oxygen content reaching 19% about 900 million years ago. Arrakis followed die normal evolutionary sequence for the next 850 million years. By this time a great diversity of marine and land life had evolved Land-dwelling mammals and reptiles were abundant. However, the fossil record makes it clear that die very large reptiles found on some planets even today never evolved The oxygen catastrophe on Arrakis occurred about 49.7 million years ago Only a few sedimentary rocks are found less than 49 7 million years old, and their formation ceased about 48.5 million years ago. Since sedimentary rocks almost invariably require the presence of standing water in order to form the evidence shows that the oceans largely disappeared 49.7 million years ago The small, remnant oceans and all traces of standing water disappeared within the next million years.