How does a fox exist in a mostly carbon dioxide environment.

You will observe the fox is looking at the Rover thru an opening in the mudstone wall, the theory is such life that survived the Solar Flares which were concurrent with the Permian Catastrophe on Earth 252 million yrs ago, which vaporized Mars' atmosphere causing its seas to boil away, retreated underground where indoor type vegetation which does not require sunlight to photosynthesize, absorbs CO2 and breathes out oxygen purifying the air and enabling the animals to survive.