It is thought that approximately 90% of plant species went extinct during the Carboniferous Period, approximately 320-290 million years ago. Which of the graphs represent the changes that likely occurred in carbon dioxide and oxygen levels? The solid black line depicts CO2 levels. The y-axis represents levels of oxygen or carbon dioxide. The dashed line represents O2 levels. The left side of the x-axis represents the time of the extinction event. As you move to the right, the x-axis represents years since the extinction event.