A comparison of oxygen levels with the diversity of arthropods and limbed vertebrates during the interval of time shown in the previous figure. The far left column, which portrays the number of new terrestrial arthropod orders over the time interval from 450 million to 290 million years ago shows that during the time from 380 million to 350 million years ago, a time interval encompassing Romer’s Gap, there were no new orders. This time interval also coincides with either dropping- or low-oxygen values.

Romer’s Gap ended in the Carboniferous Period (split in two in America, where we call it the Mississippian and Pennsylvanian Periods), and its European name comes from the fact that the majority of coal now found on Earth dates from this time. During this time oxygen levels rose in spectacular fashion, and in the last intervals of the Carboniferous and continuing into the successive Permian Period, oxygen levels finally topped out at nearly 35 percent, creating a unique interval in Earth’s history, the subject of the next chapter.