Time Marches On Chapter 3 Section 5 p Vocabulalry: 1. geologic time scale 2. eon 3. era 4. period 5. epoch 6. extinction

Approximately 1,500 fossil bones of dinosaurs that lived about 150 million years ago are exposed in the quarry wall of Dinosaur National Monument in Utah and have been excavated by paleontologists. In terms of the Earth’s history, 150 million years is little more than 3% of the time our planet has existed. It is a little less than 4% of the time represented by Earth’s oldest know rocks.

Leaf fossil Dragonfly fossil Turtle fossilFish fossil Green River formation is home to many well preserved fossils. One of the best places in North America to see Earth’s history recorded in rock layers is in Grand Canyon National Park. Over the course of six million years, the Colorado River has eroded countless layers of rock. These layers represent almost half, or nearly 2 billion years, of Earth’s history.

Geologists study 4.6 billion years of Earth’s history! To make their job easier, they have created the geologic time scale which divides Earth’s 4.6 billion year history into distinct intervals of time. The geologic time scale accounts for Earth’s entire history. It is divided into four major parts called eons. Dates given for intervals on the geologic time scale are estimates. EON – largest division of time ERA – 2 nd largest division of time, divided into periods PERIOD – 3 rd largest division, divided into epochs EPOCH – a subdivision of geologic time

The Appearance and Disappearance of Species At certain times during Earth’s history, the number of species has increased and decreased dramatically. An increase in population happens because of a sudden increase or decrease in competition among the species. On the other hand, if the population decreases in a relatively short period of time, it could be a mass extinction. Extinction is the death of every member of a species. Gradual events, such as global climate change and changes in ocean currents, can cause mass extinctions. Hallucigenia, named for it “bizarre and dream like quality,” was one of numerous organisms to make its appearance during the early Cambrian period. It was present when the number of marine species greatly increased.

The Paleozoic Era – Old Life Marine life flourished at the beginning of the Paleozoic era. The oceans became home to a diversity of life. There were few land organisms. By the middle of this era, all groups of modern plant life appeared. By the end of the era, amphibians and reptiles lived on the land and there were many insects. The Paleozoic era ended with the largest mass extinction in Earth’s history. Some scientists believe that ocean changes were a likely cause of this extinction which killed nearly 90% of all marine species. This era lasted from about 542 million to 251 million years ago.

The Mesozoic Era – The Age of Reptiles The Mesozoic era began about 251 million years ago. The Mesozoic is known as the Age of Reptiles, because reptiles, such as the dinosaurs inhabited the land. During this time, reptiles ruled. Small mammals appeared about the same time as dinosaurs, and birds appeared late in the era. At the end of the Mesozoic era, about 15% to 20% of all species on Earth, including the dinosaurs, became extinct. Global climate change may have been the cause.

The Cenozoic era began about 65.5 million years ago and continues to the present. This era is known as the Age of Mammals. During this era, mammals had to compete with dinosaurs and other animals for food and habitat. Unique traits, such as regulating body temperature and bearing young that develop inside the mother may have helped mammals survive in the environmental changes that probably caused the extinction of the dinosaurs. The Cenozoic Era – The Age of Mammals

Geologic Eras Song

Quiz Time! 1. How much geologic time is represented by the rock layers in the Grand Canyon? 2 billion years 2. What is the geologic time scale? It is a scale that divides Earth’s 4.6 billion year history into distinct intervals of time. 3. What is the largest division of time on the geologic time scale? Eons 4. During which era did plants start to appear on land? Paleozoic era

5. Why is the Mesozoic era known as the Age of Reptiles? Reptiles, including dinosaurs, were the dominant organisms on land. 6. The unit of geologic time that began 65.5 million years ago continues to the present is the: A. Holocene epoch B. Cenozoic era C. Phanerozoic era D. Quaternary period B Science Humor… A try, try, tryceratops! What do you call a dinosaur that doesn’t give up?