Earth's Paleocene Epoch

The Paleocene Epoch followed the Cretaceous-Tertiary Extinction and lasted from around 65 million years ago until 55 million years ago. It is the first period in the modern Cenozoic Era, also known as the Age of Mammals. It is during the Paleocene that mammals and birds began to fill the ecological niches that had been left by the demise of the dinosaurs, pterosaurs, and large marine reptiles.

Geographically, the Earth began to resemble its present state. The supercontinent of Pangaea was broken up and the continents continued to drift toward their modern positions as we know them today. Temperatures in the early Paleocene were cooler than those of the Cretaceous Period, and there was minimal glaciation at the poles. As the Paleocene wore on, the climate became much more warm and humid across the globe, with cool temperatures at the poles. Sea levels dropped worldwide, exposing more land for terrestrial ecosystems. Unfortunately, the Paleocene is a period of poor fossil record. Most of what remains of organisms in rock formations are teeth and small collections of bones.

In the oceans, the large marine reptiles that had dominated the top predatory food chains had gone extinct at the end of the Cretaceous Period. More opportunistic hunters like sharks began to assert their dominance in the absence of these large predators. As groups of marine organisms like ammonites went extinct, new groups began to fill the niches and Earth's ocean ecosystems began to resemble those that exist today. Squid and other cephalopods with soft bodies began to replace the shelled cephalopods that had dominated the Mesozoic middle food chains.

During the Mesozoic, mammals had been restricted to the ecological niches not filled by the larger terrestrial mammals. In other words, most of the mammals up to this point had been small, forest-dwelling insect eaters or herbivores that moved at night in order to avoid daytime predators. But where there is a gap in an ecosystem, an organism will fill it. The tops of the food chains had been left open and the mammals were poised to grow bigger, meaner, and more diverse. Mammals were able to evolve and adapt very quickly due to their high rate of reproduction and high numbers of offspring. Mammals belong to three separate groups that continue to exist today. Placentals are mammals that gestate their young inside a placenta contained within the body and give birth to young that are more or less functional in a few days. Marsupials give birth to tiny, embryonic babies that are reared in a pouch that contains the mother's teats. Modern day kangaroos are marsupials. The final group are the most bizarre and rare of the mammals. Monotremes lay eggs instead of giving birth to live young. The placentals would go on to become the most successful group of mammals in the Paleocene Period and beyond. By the end of the Paleocene, mammals had still not gotten much larger than a small bear, but this was only their first step into a larger world which they had come to dominate.

Reptiles survived the extinction in smaller forms and began to spread across the globe due to the warm global climate. Many of the reptiles we know today were making strides in the Paleocene such as crocodiles, turtles, snakes, and lizards. Some dinosaurs may have survived early into the Paleocene according to limited fossil records, but did not last for long before dying off. Birds were continuing to diversify after the demise of the large, reptilian pterosaurs. These relatives of dinosaurs were also colonizing land and filling many of the predatory niches that were left open. Giant predatory birds such as Europe's gastornis appeared in the late Paleocene and reached heights of over six feet, sporting a giant, bone-crushing beak. These giants would occupy the top of the food chains in many regions for five to ten million years before going extinct during the Eocene Epoch.