The Paleocene epoch, from 65.5 to 55.8 million years ago, marked the beginning of the Age of Mammals. The climate was hot and humid, with large areas of land inundated. The asteroid that hit Mexico’s Yucatan Peninsula wiped out most life on Earth, leaving niches for birds and mammals. Most mammals were small, with Xenartha branching off from the rest of the placental mammals. Grass had not yet covered the Earth, and most mammals were arboreal. Placental fauna dominated Africa, North America, and Eurasia, while marsupials dominated South America, Antarctica, and Australia.
The Paleocene epoch is the time on Earth between the Cretaceous-Tertitary extinction event, 65.5 million years ago, until 55.8 million years ago. Paleocene means “old (more) new” in Greek. This is a reference to the fact that the Paleocene was the beginning of the Age of Mammals, which continues today, but was too primitive to contain modern animal orders.
During the Paleocene, the climate was more like that of the Mesozoic (Age of the Dinosaurs) – hot and humid, with the tropics extending at least 45 degrees from the equator and temperate forests reaching to the poles. Large tracts of central Eurasia, North America and Southeast Asia were inundated. Antarctica was a temperate continent with marsupial fauna, still connected by the Antarctic Peninsula to South America. This sealed off Drake Passage and prevented the Antarctic Circumpolar Current from forming, which would have caused Antarctica to freeze about 22 million years after the end of the era.
In the early Paleocene, much of life on Earth was destroyed when a 6 km wide asteroid crashed into what is now Mexico’s Yucatan Peninsula. This kicked up a cloud of dust that covered the Earth for at least several years, killing off most photosynthesis-dependent plants and the animals that require them to live. The largest animals – the dinosaurs – were all wiped out, along with pterosaurs (flying reptiles), plesiosaurs, pliosaurs, mosasaurs (aquatic reptiles), and many plants and invertebrates. This has left open a wide variety of niches for the main surviving groups to exploit: birds and mammals.
Most mammals during the Paleocene were relatively small – less than 20 kg (44 lbs). Beginning as small rodent-like mammals, within 10 million years they had diversified into a few new orders, most of which are now extinct. Xenartha, the clade of mammals that includes anteaters, sloths and armadillos, is known to have branched off from the rest of the placental mammals about 60 million years ago. Marsupials and placentals had diverged from each other well before the start of the period, up to 130 million years ago. The ancestral forms of most living orders had arisen shortly before the Paleocene, including ungulates, insectivores, badger-like omnivores, etc.
Odd and even ungulates diverged towards the beginning of the Paleocene. Grass had not yet begun to cover the Earth, instead most of the planet was covered in tropical and temperate forests. As such, most mammals were arboreal. At the time South America, Antarctica and Australia were separated from the rest of the world’s land mass and were dominated by marsupial rather than placental fauna. Little is known about the animals that lived in Antarctica and Australia during the Paleocene: they are known to be placentals, the descendants of Mesozoic mammals on every continent (whose fossils are rare), and at least some of them were present-day predecessors marsupials from Australia and South America. Phorusracids, “terror birds” evolved towards the beginning of the Paleocene in South America and (probably) also lived in Antarctica, acting as apical predators. The marsupial or reptilian fauna of the time (except crocodiles) were oppressed by these avian killers.
Among the groups of placental mammals that thrived in Africa, North America, and Eurasia during this period are the primates, plesiadapids (ancestors of rodent-like primates), and condylarths (the ancestors of all ungulates, which were the first true predators of ‘apex of mammals and larger herbivores). The mammalian superorders of Afrotheria, Laurasiatheria, and Euarchontoglires had split about 20 million years before the start of the Paleocene, so evolution during the Paleocene was a further diversification of these orders, the earliest diversification available after the extinction of the dinosaurs.
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