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Quaternary Period

Mean temperature: 14 °C
Atmospheric oxygen: 20.8%

The Quaternary Period is the current and most recent of the three periods of the Cenozoic Era. It is divided into two epochs: the Pleistocene and the Holocene (which began 11,700 years ago). The Quaternary is typically defined by the cyclic growth and decay of continental ice sheets driven by Milankovitch cycles and the associated climate and environmental changes that occurred. The last expansion of the polar ice sheets took place about 12,000 years ago.

The Quaternary is a period of the repeating glaciations, which are defined by the previously mentioned Milankovitch cycles. The angle of Earth’s axial tilt with respect to the orbital plane varies between 22.1 and 24.5 degrees over a cycle of about 41,000 years. The current tilt is 23.44 degrees, roughly halfway between its extreme values. Increased tilt increases the amplitude of the seasonal cycle, providing more solar radiation in each hemisphere’s summer and less in winter – in other words, climate at higher latitudes becomes warmer and stops glaciation. The tilt last reached its maximum 10,700 years ago and now is in the decreasing phase of its cycle and will reach its minimum in 9,800 years. The current trend of decreasing tilt will promote milder seasons, as well as an overall cooling trend.

The Quaternary is the modern period, and it represents our current fauna and flora. However, periodic glaciations had a huge impact on the terrestrial biota, frequently destroying plant and animal species. After the last ice age, many large Pleistocene mammals such as saber-tooth cats, mammoths, and mastodons became extinct worldwide.

Neogene Period

Mean temperature: 14 °C
Atmospheric oxygen: 21.5%

Neogene is a period that spans 20.45 million years. It is sub-divided into two epochs, the earlier Miocene and the later Pliocene. The Isthmus of Panama connected North and South America later in the Pliocene, cutting off the Atlantic Ocean from the warm ocean currents from the Pacific, and leaving only the Gulf Stream to transfer heat to the Arctic Ocean. The global climate cooled considerably over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary period that follows.

During this period, mammals and birds continued to evolve into roughly modern forms, while other groups remained relatively unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of this period and later spread into Eurasia. Tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore greatly diversified, and herbivorous mammals evolved alongside it, creating the many grazing animals of today, such as horses and antelopes.

Paleogene Period

Mean temperature: 18 °C
Atmospheric oxygen: 26%

The Paleogene is the first period of the Cenozoic Era. This period consists of Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (about 55 million years ago) was marked by the Eocene Thermal Maximum, one of the most important periods of global change during the Cenozoic. The global climate during the Paleogene began a cooling and drying trend, periodically disrupted by warm periods. The trend was caused partially by the formation of the Antarctic Circumpolar Current, which significantly lowered oceanic water temperatures.

Mammals began a rapid diversification during this period; some mammals evolved into large forms that would dominate on land, while others would adapt to marine life. Those that took to the oceans became modern cetaceans, while those that took to the trees became primates. Birds, which were already well established by the end of the Cretaceous, also experienced adaptive radiation, as they took over skies free of the now-extinct pterosaurs. As the climate cooled, tropical plants became less numerous, and their range was restricted to equatorial regions.

Cretaceous Period

Mean temperature: 18 °C
Atmospheric oxygen: 30%

The Cretaceous is the last period of the Mesozoic era, ending with a large mass extinction – the famous comet that destroyed the dinosaurs. It was a period with a relatively warm climate, resulting in high sea levels that created numerous shallow inland seas.
During the Cretaceous the South Atlantic Ocean opened. India separated from Madagascar and raced northward on a collision course with Eurasia. Notice that North America was connected to Europe, and that Australia was still joined to Antarctica.

The Cretaceous period ended with a massive extinction event, known as Cretaceous-Paleogene extinction. Numerous plant and animal species on Earth were wiped out, and all the non-avian dinosaurs became dead. With the exception of some cold-blooded species like turtles and crocodiles, no tetrapods weighing more than 25 kilograms survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opened the Cenozoic Era that continues today.

Jurassic Period

Mean temperature: 16.5 °C
Atmospheric oxygen: 26%

The Jurassic is the second period of the Mesozoic era, also known as the Age of Reptiles. The supercontinent of Pangea began to break apart in the Middle Jurassic. In the Late Jurassic the Central Atlantic Ocean was a narrow ocean separating Africa from North America. Eastern Gondwana had begun to separate from Western Gondwana.

About 20 million years after the Permian-Triassic extinction, dinosaurs diverged from their archosaurian ancestors and began to challenge the reigning archosauromorphs and therapsids. At first, they were not the dominant terrestrial animals, but after the Triassic-Jurassic extinction, which wiped out many of their competitors, they rose to prominence and occupied all the main niches for the next 135 million years.

During the Jurassic period, the dominant marine vertebrates were fish and marine reptiles like ichthyosaurs (marine reptiles that resembled today’s dolphins), plesiosaurs (long-necked marine reptiles), pliosaurs (a version of the plesiosaur with a shorter neck and longer head), and crocodiles. Numerous turtles could be found in lakes and rivers.

Triassic Period

Mean temperature: 17 °C
Atmospheric oxygen: 16%

The Triassic began in the wake of Permian-Triassic extinction event, which left the Earth’s biosphere impoverished. Life did not recover until the middle of this period.
The supercontinent of Pangea, mostly assembled by the Triassic, allowed land animals to migrate from the South Pole to the North Pole, and warm-water faunas spread across the Tethys Sea.

The Permian-Triassic extinction devastated both terrestrial and marine life. Biodiversity rebounded in the Triassic, as the surviving species repopulated empty terrain, but these were short lived. At first, 90% of the big terrestrial vertebrates belonged to a single genus of therapsids – Lystrosaurus – a sure sign that ecosystems were badly damaged as biodiversity was low. Diverse communities with complex food-web structures took another 30 million years to re-establish themselves. By the Middle Triassic, big therapsids were replaced by archosaurs (the ancestors of dinosaurs, crocodiles, and birds), and the dinosaurs began to establish themselves.

Permian Period

Mean temperature: 16 °C
Atmospheric oxygen: 23%

The Permian is the last period of the Paleozoic era. It ended with the largest mass extinction in Earth’s history. The world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. Vast deserts covered Western Pangea during the Permian, as reptiles spread across the face of the supercontinent.

The Permian began with the Carboniferous flora still flourishing. In the Middle Permian a major transition in vegetation began. The swamp-loving lycopod trees, such as Lepidodendron and Sigillaria were progressively replaced by the more advanced seed ferns (like Glossopteris) and early conifers, gingkoes, and cycads.

Carboniferous Period

Mean temperature: 14 °C
Atmospheric oxygen: 32.5%

The name Carboniferous means “coal-bearing” and it is the period when all the major coal deposits are formed. During the Early Carboniferous the Paleozoic oceans between Euramerica and Gondwana began to close, forming the Appalachian and Variscan mountains. An ice cap grew at the South Pole, and four-legged vertebrates evolved in the coal swamps near the Equator.

The Carboniferous is famous for its vast rainforests of Lepidodendron, Sigillaria, and Calamites, which were the biggest rainforests in Earth’s history. Giant arthropods and insects appeared due to the high atmospheric oxygen levels and lack of predators. Terrestrial vertebrates became more abundant – amphibians became very diverse, and after the Carboniferous collapse reptiles started to prosper.