Chapter 18 Fossils and Geologic Time

428 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
atmosphere. However, like the even more catastrophic extinction
at the end of the Paleozoic, the cause of this mass extinction
is still under debate.
Cenozoic Era: The Age of Mammals
The word Cenozoic means “recent life.” The Cenozoic Era
began 65 million years ago and continues to the present. With
the extinction of dinosaurs, mammals evolved as the dominant
group of vertebrate animals. Mammals inhabit nearly
every terrestrial environment. Whales, dolphins, and seals
inhabit the seas, and bats take to the air. The first humans
evolved in the late Cenozoic. The oldest human fossils were
found in Africa and are about 2 to 4 million years old.
In New York State, the most dramatic geological events of
the Cenozoic Era were probably the repeated advances of
continental glaciers. Glaciers nearly covered the state until
about 15,000 years ago. The formation of the glaciers resulted
in a drop in sea level that exposed a land connection between
Asia and North America. This allowed humans to migrate
from Asia into North America. Unlike earlier eras, the Cenozoic
is the only era with no specific end. The age of mammals
is expected to continue into the foreseeable future.
ACTIVITY 18-5 GEOLOGIC TIME LINE
Use a strip of paper about 5 meters long to make a time line of
the following events. Develop a scale that indicates how much
time is represented by each unit of distance. According to your
scale of time, plot each event where it belongs on the strip.
Origin of Earth and solar system
Oldest known rocks on Earth
Oldest microfossils
Fossil organisms become abundant
Earliest traces of terrestrial (land) organisms