After the end of the first age, however, temperatures increased again, and these conditions were almost constant until the end of the period. The warming may have been due to intense volcanic activity which produced large quantities of carbon dioxide. Between 70–69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO2 pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between 21 and 23 °C (70-73 °F). Atmospheric CO2 and temperature relations indicate a doubling of pCO2 was accompanied by a ~0. 6 °C increase in temperature. The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics, further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.