Geologic Time Basics
This is the longest unit of geologic time.
Eon
When organisms cannot adapt to rapid environmental changes, this usually happens to them.
They become extinct
This is the oldest era of the Phanerozoic eon.
Paleozoic Era
These are the three periods of the Mesozoic Era.
Triassic, Jurassic, and Cretaceous
These are the two periods of the Cenozoic Era.
Tertiary and Quaternary
This geologic time unit is a subdivision of an era.
Period
This term describes the extinction of many species in a relatively short period of time.
Mass extinction
This era is often called the “Age of Invertebrates” in its early part.
Early Paleozoic
During the Mesozoic, this process broke up Pangaea and eventually led to the formation of the Atlantic Ocean
The breakup of Pangaea
This is the most recent epoch of the Cenozoic Era.
Holocene
This term describes a sudden appearance of many new, complex life-forms in the fossil record at the start of the Paleozoic.
The Cambrian explosion
Iridium found in a global clay layer is evidence for this event that likely caused the extinction of the dinosaurs.
A meteorite impact
This nickname is given to the Middle Paleozoic because of the dominance of fish.
Age of Fishes
Dinosaurs are distinguished from present-day reptiles because they walked with their legs in this position relative to their hips.
Legs directly below their hips (upright posture)
During the Pleistocene ice age, sea level dropped because of this.
Glaciers trapped large amounts of water
This type of isolation occurs when a population is separated by a physical barrier such as a mountain range or ocean.
Geographic isolation
These were the first life-forms on Earth.
Unicellular organisms
This supercontinent formed during the Late Paleozoic.
Pangaea
These marine reptiles of the Mesozoic had small heads, long necks, and flippers.
Plesiosaurs
These are large mammals that lived during the Cenozoic, many of which went extinct at the end of the Pleistocene.
Mega-mammals
The geologic time scale is primarily based on this evidence recorded in rock layers.
The fossil record
These Paleozoic animals are significant because they were the first organisms to have hard body parts preserved in the fossil record.
Trilobites
This combination of events—formation of Pangaea, volcanic eruptions, and possibly a meteorite impact—caused the largest mass extinction at the end of the Paleozoic.
The Permian mass extinction
This era, beginning about 65.5 million years ago, is known as the “Age of Mammals.”
Cenozoic Era
According to scientists, this is the primary cause of present-day global warming.
Human use of fossil fuels