PALEOGENE, PALEOCENE, PALEOZOIC: WHAT DOES "PALEO-" MEAN IN THE GEOLOGIC TIME SCALE?

Why do we have a geologic time scale with the units identified by words? Because we need to talk and write to each other and make clear what part of the five and a half billion year history of Earth we mean. "Paleo-" is supposed to mean ancient. But how old is ancient? The geologic time scale is confused on that point: "paleo-" appears in three of its parts with different ages.

As we have better understood geologic time, we have changed the names in the time scale to reflect our improved understanding of which rocks formed when, abandoning Primary and Secondary when we realized that rocks assigned to one of those units might really belong to the other and changing boundaries as better stratigraphic sections became available to define them. But with some of the recent changes, we have lost the identifying clarity that a label is supposed to produce. With the advent of Paleogene, we now have three similar terms, not readily separable (especially by people just learning the time scale), used to indicate three different parts of geologic time. The confusion was bad enough with Paleocene and Paleozoic. I have to speak carefully and cautiously to make the Paleocene-Paleogene distinction clear, and my students don't always hear or see the difference. Some students get confused enough that they abandon trying to learn the time scale at all, more than failed to learn it when we still had the distinctive Tertiary. Admittedly, that statement is based on around 80 students per year taking my course in Historical Geology. Students used to be able to keep the Paleozoic era and the Paleocene epoch clear in their minds, but now I'm finding all three "Paleo-" words get confused with each other.

There have been two reasons for abandoning Tertiary that I have learned of: to create a more even division of the 63 million years or so between the Cretaceous and the Pleistocene and to stop using Tertiary as we have stopped using Primary and Secondary. I accept the first of those reasons (not the second), but I think we need a completely different word in place of Paleogene, one readily distinguished from the other names of the time scale and with a distinctive initial to be used in abbreviations on geologic maps.