Department of Plant Biology and Department of Geological Sciences
Michigan State University
What follows is a simplified version of the Geological Time Scale that we will use for this course.
QUATERNARY PERIOD
-------------------------------------------------------------------------------------------------- 2 Ma
TERTIARY PERIOD
-------------------------------------------------------------------------------------------------- 66 Ma
------------------------------------------------------------------------------------------------- 144 Ma
JURASSIC PERIOD
------------------------------------------------------------------------------------------------- 208 Ma
TRIASSIC PERIOD
------------------------------------------------------------------------------------------------- 245 Ma
------------------------------------------------------------------------------------------------- 286 Ma
CARBONIFEROUS PERIOD
------------------------------------------------------------------------------------------------- 360 Ma
DEVONIAN PERIOD
------------------------------------------------------------------------------------------------- 408 Ma
SILURIAN PERIOD
------------------------------------------------------------------------------------------------- 438 Ma
ORDOVICIAN PERIOD
------------------------------------------------------------------------------------------------- 505 Ma
CAMBRIAN PERIOD
------------------------------------------------------------------------------------------------- 570 Ma
Note that the time scale starts as a RELATIVE TIME SCALE. Field observation and description of typical rocks, along with the fossils which are present. Period boundaries are placed based on significant changes in the fossils found. Very large-scale changes in the fossils define Era boundaries. With the advent of radiogenic dating, it became possible to transition to an ABSOLUTE time scale with definitive dates for the beginning and end of each period. The numbers shown are in millions of years prior to the present. The time scale is always arranged with the oldest eras/periods toward the bottom, to correspond with the stratigraphic (rock) sequence.
Note also that the classic geological time scale begins with the Cambrian Period of the Paleozoic Era (~570 Ma). Since the periods are based on fossils and no significant fossils were known prior to the Cambrian (in the 19th and early 20th centuries), that's where the scale begins. We now know the earth is ~4.5-4.6 billion years old, but the vast stretch of Precambrian time is documented primarily by microscopic fossils or, prior to the origin of life, by no fossils at all.
Ralph E. Taggart (taggart@pilot.msu.edu)