Now called the Late Ordovician mass extinction (LOME), Earth’s first major mass extinction wiped out about 85 percent of all marine species as the ocean chemistry radically changed and Earth’s climate turned bitter cold. In between these creatures were the ancestors of gnathostomes, or jawed vertebrates. During these waves of mass extinction, most vertebrate survivors were confined to refugia, or isolated biodiversity hotspots separated by large areas of deep ocean. In these zones, surviving jawed vertebrates evidently had an advantage. According to the team, instead of wiping Earth’s ecological slate clean, the Late Ordovician mass extinction triggered a reset.