Article by: Adam Manning
Edited by: J. D. Dixon and Harry T. Jones
When we last left off in the Cretaceous, the most famous and dramatic mass extinction of all time had just occurred. The K-Pg mass extinction, triggered by an asteroid striking the earth 66 million years ago (Ma), wiped out 75% of all species. But as always, life always finds a way to bounce back, this time in the Cenozoic Era. The Cenozoic began 66 Ma and lasts all the way to today; you, reading this right now, are in the era. It is split into three periods, the earliest of which was the Paleogene.
The Paleogene lasted from 66 Ma to 23 Ma, and is split into three epochs: the Paleocene (66.0 – 56.0 Ma), the Eocene (56.0 – 33.9 Ma), and the Oligocene (33.9 – 23.0 Ma). The different epochs of the Cenozoic Period were inconsistently used in scientific literature for decades until the International Commission on Stratigraphy (ICS) began to officially define stratigraphy in 1969. The epochs of the Palaeogene were formally determined in 1978.
The Paleocene Epoch immediately followed the K-Pg mass extinction, and like the eventual outcome of most mass extinctions, the Earth and its ecosystems had to recover. In the aftermath of the K-Pg mass extinction, all of the large animals on land had gone extinct, and most had gone extinct in the oceans. This left niches that had previously been filled by the dinosaurs and marine reptiles open for new species. Poised to take advantage of this were the mammals, which began to radiate out and thrive. For example, the earliest elephant (Proboscidea), Eritherium azzouzorum from Morocco, 60 Ma, emerged during this time. The mammals were not the only animals who adapted into these new-found niches; the Terror Birds (Phorusrachidae) also appeared in South America in the Paleocene, 62 Ma, and the 13 m long giant snake Titanoboa cerrejonensis also stalked what is now Columbia, South America.
Following the Paleocene Epoch was the Eocene, when things got hot for the diversifying survivors. At the beginning of the Eocene was a sudden and rapid warming event, known as the Paleocene-Eocene Thermal Maximum (PETM). During this time, temperatures rose by approximately 6oC due to a large influx of methane gas into the atmosphere. The rapid global environmental change at this time accompanied the first appearance of the modern mammal orders of Primates, Artiodactyla and Perissodactyla in the fossil record, and with the artiodactyls there is also the emergence of the archaeocete whales 54 Ma in the Early Eocene of Pakistan, the ancestors of today’s whales.
During the later Eocene, there was a slow but steady overall drop in atmospheric CO2 due to a lack of volcanic activity and the uplift of mountains. This caused temperatures to decrease slowly and eventually triggered glaciers to form on Antarctica. This occurred at the boundary between the Eocene and the Oligocene and had widespread effects on the rest of the planet. As the climate continued to cool in the Oligocene, the climate also became drier as more water was trapped in the growing glaciers, and grasslands started to dominate ecosystems. Animals had to adapt to this new tough form of vegetation and the lack of cover in the wide open spaces, and the modern herbivore groups we see today like cattle and horses began to thrive. One mammal did this exceptionally well: Paraceratherium transouralicum, a giant, hornless rhinoceros that weighed in at 15 tonnes, making it one of the biggest land mammals ever and probably as big as land mammals can get biologically.
As the Palaeogene came to a close and the Neogene began 23 Ma, temperatures continued to drop as mountain ranges like the Himalayas continued to uplift, and the world began its transition into one of the most famous periods in Earth’s history, the ice ages.
 The Paleocene of Europe by Gabriel Ugeuto.
 The Eocene of North America by Mauricio Antón.
 Paraceratherium by Mauricio Antón.
Information References and Further Sources
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 Chart drafted by K. M. Cohen, D. A. T. Harper, P. L. Gibbard, and J.-X. Fan (c) International Commission on Stratigraphy, March 2020. To cite: Cohen, K. M., Finney, S. C., Gibbard, P. L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. Accessed 7th May 2021. Click Here.
 Frieling, J., Svensen, H. H., Planke, S., Cramwinckel, M. J., Selnes, H., and Sluijs, A. (2016). ‘Thermogenic methane release as a cause for the long duration of the PETM’, Proceedings of the National Academy of Sciences of the United States of America, 113 (43), pp. 12059-12064. Accessed 7th May 2021. Click Here.
 Fortelius, M., and Kappelman, J. (1993). ‘The largest land mammal ever imagined’, Zoological Journal of the Linnean Society, 108 (1), pp. 85-101. Accessed 7th May 2021. Click Here.
 Gehler, A., Gingerich, P. D., and Pack, A. (2016). ‘Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite’, Proceedings of the National Academy of Sciences of the United States of America, 113 (28), pp. 7739-7744. Accessed 7th May 2021. Click Here.
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 Gingerich, P. D. (2006). ‘Environment and evolution through the Paleocene–Eocene thermal maximum’, Trends in Ecology & Evolution, 21 (5), pp. 246-253. Accessed 7th May 2021. Click Here.
 Lyson, T. R., Miller, I. M., Bercovici, A. D., Weissenburger, K., Fuentes, A. J., Clyde, W. C., Hagadorn, J. W., Butrim, M. J., Johnson, K. R., Fleming, R. F., Barclay, R. S., Maccracken, S. A., Lloyd, B., Wilson, G. P., Krause, D. W., and Chester, S. G. B. (2013). ‘Exceptional continental record of biotic recovery after the Cretaceous–Paleogene mass extinction’, Science, 366 (6468), pp. 977-983. Accessed 4th June 2021. Click Here.
 Odin, G. S., Curry, D., and Hunziker, J. C. (1978). ‘Radiometric dates from NW European glauconites and the Palaeogene time-scale’, Journal of the Geological Society, 135, pp. 481-497. Accessed 7th May 2021. Click Here.
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 Sen, S., Antoine, P-. O., Varol, B., Ayyildiz, T., and Sözeri, K. (2011). ‘Giant rhinoceros Paraceratherium and other vertebrates from Oligocene and middle Miocene deposits of the Kağızman-Tuzluca Basin, Eastern Turkey’, Naturwissenschaften, 98 (5), pp. 407-423. Accessed 4th June 2021. Click Here.