In the Jurassic period, 200-145 million years ago, the average median surface temperature was 61.7 degrees Fahrenheit. When it gave way to the Cretaceous, the temperature rose to 64.4 degrees Fahrenheit. Pretty warm, no doubt, but to find the hottest chapter in Earth's geologic history, we must fast-forward eleven million years after the Mesozoic-Cenozoic Extinction event, to a period called the Paleocene-Eocene Thermal Maximum, where the average median surface temperature spiked up to 73 degrees Fahrenheit, so warm that global seasonality would have been reduced. But as recently as 49 million years ago, the hot days gradually but surely began to cool down, shrinking the global rainforests until they were eventually confined within the latitudinal boundaries of Cancer and Capricorn. It wouldn't become completely apparent until 33.5 million years ago, when the cooling resulted in the extinction of 20% of all plant and animal life, an event scientists called Grande Coupure, the Great Break. In this alternate scenario, the Jurassic period ended with the Cretaceous Thermal Maximum, where the average median surface temperature spiked from 61.7 degrees Fahrenheit to 73 degrees Fahrenheit. This number would be the average median surface temperature for the next 100 million years (excluding the Mesozoic-Cenozoic Extinction Event). But for now, let's focus on the point of transition when the spike in temperature started, 145 million years ago. Would this sudden rise in temperature result in a Mesozoic equivalent to the Grande Coupure, or a genuine mass extinction? And would this put the evolution of the modern orders of birds and mammals at an earlier starting date?