There are a number of events that rise well above background extinction without reaching the scale of the "Big Five" major mass extinctions. They are often called the minor mass extinctions. When they are added to the "Big Five," there are 18 significant extinction events that range in age from Cambrian to Neogene. They occur on average every 30 million years –– an immensely long time compared to human history, but relatively frequent on a geological time scale that goes back 4.5 billion years.The chart above shows the percentage of groups of species (genera) of marine animals that disappear at each of the 18 extinction events. Extinctions in the Cambrian Period (left side of chart) are as great in terms of percent losses of genera as any of the major events. Even so, they are not considered to be major mass extinctions for two reasons. First, most of the losses are from only one major group of animals, the trilobites, whereas the "Big Five" are marked by extinctions of genera from many major groups. Second, the number of different kinds of marine animals (diversity) in the Cambrian Period was far less than at the times of the "Big Five." This means that the total number of extinctions (versus the percentage) was also much lower than in the Cambrian (in very simple terms, 60 percent of 100 is less than 60 percent of 500). The minor extinctions of the Cambrian Period were still significant events and at least one was felt worldwide.