The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 16 - page 1

Life of the Cenozoic

Pamela J. W. Gore
Georgia Perimeter College


The Cenozoic Era is sometimes called the Age of Mammals. During the Cenozoic, mammals came to dominate the Earth, much as reptiles had done during the Mesozoic.

A spectacular adaptive radiation of mammals near the beginning of the Cenozoic resulted in the appearance of mammals as diverse as bats and whales, descending from shrew-like mammalian ancestors in as little as 12 million years.

The appearance and evolution of primates led to the ancestors of humans by the Neogene. Homo sapiens appeared in the Pleistocene Epoch.

We know more about the life of the Cenozoic Era than we know about life of any other time. This is because the fossils are better preserved and have had less time to be destroyed, they are stratigraphically uppermost, and more accessible for study. In addition, Cenozoic fossils more closely resemble life today.

Biologic changes in the Cenozoic can be tied to changes in the environment and geographic change.

Changes in climate to cooler and dried conditions, led to the expansion of the grasslands, which influenced the evolution of herbivorous mammals.

Continental breakup as a result of plate tectonics, stimulated biological diversity. This resulted in distinct faunal radiations on separate landmasses, and in isolated marine basins.

Major events of the Cenozoic
Major events of the Cenozoic Era.

The Diversity of Life in the Cenozoic

The Cenozoic dawned with the aftermath of the extinction at the end of the Cretaceous Period of the Mesozoic Era. The extinction had affected vertebrates and invertebrates, on land and in the sea, causing the disappearance of many groups of organisms including:

  1. Dinosaurs
  2. Ammonoids
  3. Large marine reptiles (plesiosaurs and mosasaurs)
  4. Rudists (reef-forming bivalves)

There were also drastic reductions in the number and diversity of groups of organisms such as:

  1. Coccolithophores
  2. Planktonic foraminifera
  3. Radiolarians
  4. Belemnoids

At the beginning of the Cenozoic Era, diversity (as indicated by the number of genera) was much lower than it had been in the Cretaceous, as a result of the extinctions.

Recovery from the extinctions was rapid (explosive), and diversity quickly climbed to a level much higher than had ever existed previously.

Diversity of marine animals through geologic time
Diversity of marine animals through geologic time, as indicated by number of known fossil genera.

Following the terminal Cretaceous extinction, diversity of marine and terrestrial organisms increased sharply, and rose to present levels.

A slight drop in diversity in the Paleogene, shown in the graph above, marks an extinction event at the Eocene-Oligocene boundary, which was associated with dramatic worldwide cooling. Many species of marine molluscs, foraminifera, and ostracodes were affected. Marine organisms were affected more severely by the extinction than were terrestrial organisms.

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

March 3, 2006