A Deep Dive into Dinosaurs
The Triassic, Jurassic, and Cretaceous Periods
Dinosaurs lived a very long time ago. They did not live with people. They lived during a time called the Mesozoic Era. The Mesozoic Era lasted about 186 million years. Scientists divide this time into three parts: the Triassic, Jurassic, and Cretaceous periods.
The first period was the Triassic Period. It began about 252 million years ago. The world looked very different then. All the land was joined together in one giant supercontinent called Pangaea. The first dinosaurs were small. Many were about the size of a dog or a small horse.
During the Triassic, dinosaurs were not yet the most important animals on land. Other reptiles lived beside them. Over time, dinosaurs slowly became more successful. By the end of the Triassic, many other reptiles had disappeared. Dinosaurs were ready to grow and spread.
The second period was the Jurassic Period. It began about 201 million years ago. The land started to break apart into smaller continents. The climate was warm and wet in many places. Plants like ferns and tall conifers grew in thick forests.
In the Jurassic, dinosaurs became bigger and more diverse. Some plant-eating dinosaurs, like Brachiosaurus, grew as tall as a four-story building. Meat-eaters like Allosaurus hunted other dinosaurs. There were many different shapes and sizes. This time is sometimes called the “golden age” of dinosaurs.
The third period was the Cretaceous Period. It began about 145 million years ago. The continents were moving closer to where they are today. New kinds of plants called flowering plants appeared. Insects and animals changed along with them.
The Cretaceous had famous dinosaurs like Tyrannosaurus rex and Triceratops. Dinosaurs lived all over the world, even near the poles. There were tiny feathered dinosaurs and huge horned ones. Dinosaur diversity was very high during this time.
At the end of the Cretaceous, about 66 million years ago, a large asteroid hit Earth. This event changed the climate. Many living things died, including all non-bird dinosaurs. Today, scientists believe birds are the living descendants of some small theropod dinosaurs.
Understanding when dinosaurs lived helps us practice deep time thinking. Deep time means thinking about millions of years. A geologist studies rocks and layers of Earth to understand these long stretches of time. Fossils found in rock layers help scientists know which dinosaurs lived in which period.
One important fossil hunter was Mary Anning. She lived in England in the early 1800s. She found important marine reptile fossils along the cliffs. Her work helped scientists understand ancient life and rock layers.
Scientists still ask big questions. One big question is: What caused dinosaur diversity to explode in the Jurassic? Some scientists think changes in climate helped. Others think new plant life allowed plant-eaters to grow larger, which helped meat-eaters grow too. Scientists continue to study fossils to learn more.
Vocabulary
1. Era
Definition: A very long period of time in Earth’s history.
Sentence: The Mesozoic Era lasted millions of years.
2. Period
Definition: A smaller part of an era.
Sentence: The Jurassic Period came after the Triassic.
3. Pangaea
Definition: A supercontinent where all land was joined together.
Sentence: During the Triassic, the continents were part of Pangaea.
4. Diversity
Definition: Having many different kinds of something.
Sentence: Dinosaur diversity increased in the Jurassic.
5. Descendant
Definition: An animal or person that comes from an earlier one.
Sentence: Birds are descendants of some dinosaurs.
6. Geologist
Definition: A scientist who studies rocks and Earth’s layers.
Sentence: A geologist looks at rock layers to understand deep time.
Comprehension Questions
Literal (Right There)
- What are the three periods of the Mesozoic Era?
- During which period did flowering plants appear?
- About how many million years ago did the Cretaceous Period end?
Inference (Think About It)
4. Why might breaking apart of Pangaea have helped dinosaurs spread?
5. How could new plant life in the Jurassic have helped dinosaur diversity grow?
Writing Prompts
- Imagine you are a geologist finding fossils from one of the three periods. Which period are you studying? What clues in the rocks help you know the time period?
- Choose one of the three periods. Describe what the land, plants, and dinosaurs might look like. Write 3–6 sentences using at least two vocabulary words.
