Equable means a reduced annual cycle of temperature and a lack of extremes, particularly the subfreezing temperatures in many regions. The Arctic and Antarctic had a temperate climate 20 degrees Centigrade warmer than the current temperature at the poles, while the equatorial climate was similar to today. Sixty percent of European mammals became extinct, to be replaced by primitive ancestors of many mammals that exist today. The evolutionary break may have been triggered by glaciation as water drawn out of the oceans to create the ice packs 33 million years ago lowered the sea level which opened land bridges and allowed Asian species to migrate into western Europe, replacing existing species. Zachos and his colleagues were able to confirm the land bridge hypothesis by linking the marine isotope signal of glaciation with the deposits of sediment debris dropped out at sea by melting icebergs.
Scientists are now able to coordinate a common timeline using events on land and in the ocean.
To accurately reconstruct Earth's past climate one must work in the polar oceans. Then Zachos measures the oxygen isotope ratio in the shells of these marine plankton. The temperature of the water determines the relative amount of each form taken up by the organism. By analyzing shells from deep sea cores from around the globe, Zachos constructs maps of sea surface temperatures that are used by Sloan and other climate modelers to reconstruct ancient climates. Scientists are puzzled by what might have created this anomaly in the climatic record, and use models to come up with likely solutions.