<P> Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading . The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas . Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea . Temperatures continued to increase and began to stabilize . Humidity also increased with the proximity of water, and deserts retreated . </P> <P> The climate of the Cretaceous is less certain and more widely disputed . Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet . Average temperatures were also higher than today by about 10 ° C. The circulation of oxygen to the deep ocean may also have been disrupted . For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as "black shale". </P> <P> Not all of the data support these hypotheses, however . Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either . Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient . </P> <P> Different studies have come to different conclusions about the amount of oxygen in the atmosphere during different parts of the Mesozoic, with some concluding oxygen levels were lower than the current level (about 21%) throughout the Mesozoic, some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous, and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous . </P>

Which era is sometimes called the age of the dinosaurs