Cenozoic Era: Paleogene Period 

6623 Ma

The Paleogene: mammals rising

The Paleogene started with a bang, quite literally, a very, very big one. The lower boundary of the period is placed at 66 million years ago, pinned on a layer of rock that can be found nearly worldwide containing a high abundance of a unique element: iridium. What makes iridium so distinctive? It typically isn't found in such high abundances on Earth, but it is found in meteorites—which gave rise to the hypothesis that a massive impact event, where a large extraterrestrial body crashed into the Earth, was responsible for the disappearance of dinosaur fossils coincident with this boundary. What is usually just referred to as the "iridium anomaly" is accompanied by significant additional lines of evidence for a major impact, including tektites, shocked quartz, and of course the recognition of the crater itself. The impact occurred at the northern margin of the Yucatán Peninsula in southern Mexico, known as the Chicxulub crater. and it is one of the largest impact structures known on Earth (second behind only the 2-billion-year-old Vredefort impact in South Africa). The Chicxulub crater's size is approximately 200 kilometers (120 miles) in diameter and 20 kilometers (12 miles) in depth, and the impactor was a C-type asteroid approximately 10 kilometers (6 miles) in diameter (or in other words, huge!). This event caused myriad effects, including megatsunamis, release of sulfurous gases and an immense amount of dust into the atmosphere, climatic crisis, and ultimately a collapse of the food chain. Like the Permian-Triassic extinction 190 million years earlier, this was a massive enough event that geologists signify it not just as a period boundary, but also as an era boundary. But, like any extinction event, species that were able to adapt to the rapidly changing conditions were able to survive and diversify. And in this case, it was the mammals. Shown here are several novel mammals, including Hyaenodon (no, confusingly not directly related to modern hyena), rodent-like multituberculates, and of course a herd of Coelodonta antiquitatis, the woolly rhinoceros (yes, this time actually related to modern rhinoceros!).