File:Geologic time and mass extinctions.png

The Quaternary period
I checked several reputable online resources before creating this image. The Illinois State Geological Survey glossary entry for the Cenozoic era cautions "Some authors do not include the Quaternary, considering it a separate era." The Geology department at Eastern Illinois University defines the Quaternary as the "Period of geologic time from about 2 million years ago until the present," while the United States Geological Survey website says it covers the last 1.6 million years.

If the Quaternary period was shown at the same scale as the other periods it wouldn't be visible on this chart (less than 1 pixel high), so I placed it on a color gradient.

Margin of error for dates
The table below shows the discrepancies for the dates of the five major mass extinctions from a small selection of sources. To avoid making the image too clutered, I included just two date ranges for the mass extinctions whose dates vary most widely between the different sources.

The author of the Sixth Mass Extinction article, paleontologist Dr. Niles Eldredge, is Curator-in-Chief of the permanent "Hall of Biodiversity" exhibition at the American Museum of Natural History. The ActionBioscience.org website belongs to the American Institute of Biological Sciences.