Such changes have not been graceful. The grand theme in life's past six hundred million years has been peaceful coexistence, occasionally violently disrupted by major episodes of extinction that appear to have eliminated seventy-five percent or more of all species in some cases. Such events--the most famous terminating the Paleozoic and Mesozoic Eras (when the dinosaurs finally died out)--took upward of a million years to accomplish. But, once again, such long periods show up as dramatically sudden turnovers if the rock record is taken literally. One can go to Upper Cretaceous marine deposits in Denmark and Italy, for example, and trace the continued existence of typical Cretaceous species of marine life right up to the point where they all seem to drop out. Just above the last Cretaceous fossil, in chalky limestones of the same character and consistency, one finds a smaller array of rather different species. The cast of characters is always different, but the outline of the story is always the same; for real change to accrue in life's past six hundred million years, wholesale housecleaning always preceded a reproliferation. The newly recovered biota (animals and plants) always looked somewhat different from its predecessor: some lineages would escape extinction and, finding new opportunities, themselve proliferate into a host of new species--a group which a paleontologist, coming along millions of years later, would designate as an entire new family or order. Rather than stately progression, the gross history of life shows a mixture of status quo and revolution. But the order is there: those organisms we thank are more advanced that others (bees and ants, say, over dragonflies) show up in the expected order as fossils: dragonflies are fairly common in lake deposits of 280 million years ago, while ants and bees don't show up until the Mesozoic, over one hundred million years later.
Eldredge 1982:48-49
When we obtain absolute dates from the cores (usually by using oxygen isotopes), we always find that the date of the base of the "Jaramillo event"--one of the pole-switching episodes--always yields a date of about 980,000 years ago.
Eldredge 1982:103-104
Charles Schuchert, an eminent geologist at Yale in the early twentieth century, published an Atlas of Paleogeographic Maps toward the end of his productive career. Thumbing through these maps, anyone can see that today we are in a relatively unusual period of earth history: the continents today are abnormally dry. The more usual condition by far is for the seas to be flooded over most of the continental interiors. . .
Eldredge 1982:109