The Paleogene is the first period of the Cenozoic. It begins approximately 65 million years ago with the extinction of the Triassic and ends 23 million years ago.
We can divide the Paleogene into three stages:
- First stage, begins 65 million of years ago and ends 56 million of years ago.
- Second stage, begins 56 million of years ago and ends 34 million of years ago.
- Last stage, starts 34 million of years ago and ends 23 million of years ago.
The Paleogene is the stage that follows the extinction of the non-avian theropod dinosaurs, the ornitischians, pterosaurs and marine reptiles, as well as the large trilobites and nautiloids.
The marine animals could be eradicated by the meteorite, but not all of them. In fact, there was a decrease in global water levels and an increase in the surface temperature of the oceans (from 5 to 8ºC) eliminating not only trilobites and Nautilus but also several groups of invertebrates such as stromatoporoid sponges (large reef-formers in the Paleozoic that become extinct in the Mesozoic).
It is also the time when early mammals begin to diversify and expand throughout the world, which is no longer formed by Pangaea but by two continents: Laurasia and Gondwana.
The Paleocene is the time of transition from the Mesozoic to the Cenozoic, with a mass extinction that changed the course of history, so the planet and animals had to acclimatize to the new conditions.
At first there was a cooling and cooling after the Triassic extinction by the fall of the asteroid, but in a relatively short time the temperature increased. This caused the global temperature to be close to subtropical in many points of the globe and warm in the areas near the poles, so that the variations between latitudes were almost zero, something similar to what happened with the oceans.
From this period we can highlight large forests of cycads, palms and ginkgos that we can observe thanks to the charcoal records, and that grew not only in the equatorial areas of the planet but also near the poles due to that warm-temperate climate that existed.
Along with the disappearance of the dinosaurs and large reptiles, the Earth was prepared for a new group of animals to take over. Mammals, at first small like mice (but not mice) that ran away from the dinosaurs, could now populate the whole planet.
The early mammals were not like most of the current placental mammals, but they were monotremes, like platypus. That is, the first mammals laid eggs, they were oviparous.
The condition of keeping the offspring in the womb and then giving birth (viviparous condition) that most of the so-called placental mammals present would appear thousands of years later, although even in the Paleocene the first marsupials and also the first placental mammals such as rodents, primates and the ancestors of equids begin to radiate.
In the Paleocene there are also some descendants of dinosaurs, birds or avian theropods that survived the cataclysm, and that had their chance to dominate the Earth at the beginning of the Paleocene like the Andalgalornis, although they were not so lucky.
The Eocene is described as “the warmest period on Earth since 500 million years ago”, that is to say, the global temperature was rising and there would have been no such values since the Cambrian Explosion and the diversification of the metazoans.
The forest areas with a tropical-subtropical climate were increasing, reaching the poles that were previously warm-temperate. There were only a few extensions of what we could call savannah (although they do not resemble today’s) somewhat arid in some inland areas in both the Northern and Southern Hemispheres.
In addition to these almost constant temperatures around the globe, the surface temperature of the oceans is said to have increased to over 20°C, a very high figure for such bodies of water.
One of the important data from the Eocene, in terms of plate tectonics, is the separation of Australia and Antarctica. This data is important because it explains why there are marsupials almost exclusively in Australia and placental ones did not arrive until it was colonised by humans in the 19th century.
There was a bridge between America-Antarctic-Australia that allowed animals to pass between these 3 continents, so that monotremes and marsupials could pass from America to Australia and vice versa, but when placentals originated, they travelled to America but not to Australia because the connection had been broken, allowing them not to be exterminated by placentals.
At the end of the Eocene and during the Oligocene, the connection between Antarctica and America was broken, and many mountain ranges originated all over the world. The movements of the tectonic plates promoted the placement of the continents as we know them today, but at this very moment the Earth began to cool and to give a new glaciation.
This cooling caused Antarctica to begin to freeze and not only did snow fall at polar latitudes but also increasingly closer to the equator, but initially only snow fell at high altitudes such as mountain peaks.
This cooling would be more pronounced in the Northern Hemisphere where it would begin to freeze during the later millennia (at the end of the Neogene) forcing much of the environment to become a large tundra or thick conifer forest that could withstand the coming cold winter (but it would not be until the Quaternary, almost 30 million years later).
Paleogene and the origin of cetaceans
Cetaceans, like whales and dolphins, are known to have a terrestrial origin. Their origin dates back to the beginning of the Paleogene, with totally terrestrial specimens that gradually adapted to marine life.
Many researches have been done about it, every step of the evolution from the terrestrial to the aquatic environment is known, such as the transformation of the hind legs into a powerful tail and the transformation of the front legs into strong fins, as well as the change of the position of the nostril from a frontal to a dorsal position, but it is not known what could have made these animals go to the aquatic environment.
As can be seen in the previous image, today’s cetaceans have a common ancestor with another animal that lives in an aquatic environment, the hippopotamus. Thanks to an increasingly rich fossil record, it could be seen that the ancestor of cetaceans was a terrestrial artiodactyl (animal with an even number of hooves) that adapted to the aquatic environment, probably because it found an empty niche to which it could make a lot of use.
The difference between the specialization in hippopotamus or cetacean could be where the adaptation to the water started, if in the open sea (cetacean) or in the river (hippopotamus), but, as indicated before, it is not very clear how the transition occurred.