The Paleocene Period, mammals growing bigger and occupying new niches

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Article 29: The Early Paleocene sees the recovery of the Earth, after the mass extinction event at the end of the Cretaceous. During this period mammals grow bigger and occupy a wider variety of ecological niches.

The Paleocene Period of the Earth’s history lasts (only) 9 million years, beginning 65 and ending 56 million years ago.

It starts with the mass extinction event at the end of the Cretaceous. This is a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora.

K-T Boundary
The bull’s eye marks the location of the Chicxulub impact site. The impact of a 10 mile wide comet contributed to global climate changes that killed the dinosaurs and many other forms of life. By the Late Cretaceous the oceans have widened, and India approaches the southern margin of Asia.

The Early Paleocene sees the recovery of the Earth. The die-off of the dinosaurs leaves unfilled ecological niches worldwide.

It ends with the Paleocene-Eocene Thermal Maximum. This Eocene Climatic Optimum is a geologically brief interval (~0.2 million or 200.000 year), characterized by extreme changes in climate and carbon cycling.

65 Million years climate change
Credit: New Scientist

The climate becomes warm and humid worldwide towards the Eocene boundary, with subtropical vegetation growing in Greenland and Patagonia and crocodilians swimming off the coast of Greenland. Tropical conditions give rise to abundant marine life, including coral reefs. Warm seas circulate throughout the world, including the poles.

Tropical oceans are at 35°C, nearing body temperature.

Polar oceans are at 12°C, which is the temperature of the waters off San Francisco. Carbon dioxide concentrations exceed 800 parts per million.

These warm temperatures worldwide gives rise to thick tropical, sub-tropical and deciduous forest cover around the globe, the first recognizably modern rain forests, with ice-free polar regions covered with coniferous trees. With no large grazing dinosaurs to thin them, Paleocene forests are probably denser than those of the Cretaceous.

Flowering plants, angiosperms, first seen in the Cretaceous, continue to develop and proliferate and along with them co-evolve the insects that feed on these plants and pollinate them. Because of the climatic conditions of the Paleocene, reptiles are more widely distributed over the globe than at present.

Richard Norris of Scripps Institution of Oceanography at the University of California San Diego and colleagues want to use the Eocene 50 million years ago as a model for future possibilities. 

They point out in the journal ‘Science’ that carbon dioxide levels passed the 400 parts per million mark in May 2013 and that, if the world carries on burning fossil fuels at the present rate, the conditions of the greenhouse world of 50 million years ago could be here in just 80 years. And their message is: what happened once before could happen again.

Mammals have first appeared in the Triassic, evolving from advanced cynodonts and developing alongside the dinosaurs, exploiting ecological niches untouched by the larger and more famous Mesozoic animals: down in the insect-rich forest underbrush and high up in the trees.

These smaller mammals as well as birds, reptiles, amphibians and insects survive the mass extinction at the end of the Cretaceous. They diversify and spread throughout the world. In the late Paleocene, early owl types appear, such as Ogygoptynx in the United States and Berruornis in France.

While early mammals were small, nocturnal animals, that mostly ate soft plant material and small animals such as insects, the demise of the non-avian dinosaurs at the beginning of the Paleocene allows mammals to grow bigger and to occupy a wider variety of ecological niches. Ten million years after the death of the non-avian dinosaurs, the world is filled with rodent-like mammals, medium-sized mammals, scavenging in forests, and large herbivorous and carnivorous mammals, hunting other mammals, birds and reptiles.

Chriacus 2
Chriacus. Length including tail about 1m. Credit: Dinopedia. 

Chriacus had an estimated body mass of 5 to 10 kg. A nearly complete skeleton has been found in early Eocene rocks of Wyoming. As this skeleton shows, Chriacus was equally adept in the trees and on the ground. Among today’s mammals it can be compared best to members of the raccoon family and to the civets. Like most climbing mammals, Chriacus had powerful limb musculature, very mobile joints and feet bearing five digits with claws. The tail of Chriacus was long and robust.

Fossil evidence from the Paleocene is scarce and there is relatively little known about mammals of the time. Because of their small size (constant until late in the epoch), early mammal bones are not well preserved in the fossil record. Most of what we know comes from fossil teeth, a much tougher substance, and only a few skeletons.

Phenacodus is an extinct genus of mammals from the late Paleocene through middle Eocene, about 55 million years ago. It is one of the earliest and most primitive of the ungulate mammals.

During the Paleocene, the continents continue to drift toward their present positions. Supercontinent Laurasia separates into Europe, Greenland, North America and Asia. South and North America remain separated by equatorial seas; they join during the Neogene.

The components of the former southern supercontinent Gondwanaland continue to split apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa is heading north towards Europe, slowly closing the Tethys Ocean. India is on its way to Asia, which will lead to a tectonic collision and the formation of the Himalayas.

Thanks to Wikipedia, Paleocene, to Paleocene Mammals, and to New Scientist

ShantiShanti is a regular contributor
All essays of this series can be found in: At Home in the Universe
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