When Dinosaurs Lived

Fun facts about the Triassic, Cretaceous, and Jurassic periods

By Don Lessem
  • Grades: 3–5, 6–8

Q: What was the difference between the Triassic, Cretaceous, and Jurassic periods?
A: We define these periods as measurements of time and communities of life. The Triassic was a time of a single world continent, when the first pterosaurs took flight and dinosaurs evolved along with many other animals. In the Jurassic, both plant-eating and meat-eating dinosaurs grew enormous. The world broke into two continents. In the Cretaceous, flowering plants and many insects evolved. Duck-billed dinosaurs and horned dinosaurs developed. T. rex ruled in these last dinosaur days.

Q: How do you know which period certain dinosaurs lived?
A: Usually you can tell the time when the dinosaur lived by the age of the rock it is in. You tell the rock's age by small fossils of plants and little animals that we already know the age of. If it is volcanic rock, you can tell its age by chemical testing. Sometimes we can tell the age of the rock and the fossils in it within 100,000 years of the actual time, even if it happened 300 million years ago.

Q: Which time period had the most dinosaurs?
A: Most dinosaurs we know come from the end of the Cretaceous period. But dinosaurs probably lived in great numbers in all three periods of dinosaur time: Jurassic, Triassic, and Cretaceous. We haven't found as many dinosaurs from the other two periods because the rocks from that time aren't near the surface in places we have looked. We know more dinosaurs from the Cretaceous period than any other. That's just luck, since more rock from that time is now handy for excavating. We know about 175 kinds of dinosaurs from just the last 25 million years of dinosaur time. That's about half the total from all 165 million years they were around.

Q: Can you describe the Cretaceous period?
A: The Cretaceous was the last of the three periods of the Mesozoic Era, and dinosaurs lived in all three. The Cretaceous was a time when the seas were higher than ever before or after in Earth's history. The modern continents were taking shape and the first butterflies, bees, and flowers appeared. Duck-billed dinosaurs, horned dinosaurs, armored dinosaurs, giant meat-eaters like T. rex, and nasty little ones like the raptors all lived in the Cretaceous. It started 144 million years ago and ended with the extinction of the dinosaurs and many other large animals, 65 million years ago.

Q: What type of dinosaur was most abundant near the end of the Cretaceous period?
A: The fossils most found at the end of the Cretaceous are duck-billed dinosaurs like Edmontosaurus and horned dinosaurs like triceratops. They traveled in huge herds in the North American West. At any time, predators are much more rare, which is one reason we don't have a lot of T. rex — only about 15 reasonable skeletons (more than 1/3 full) in 100 years.

Q: Which Mesozoic period had the smallest dinosaur?
A: The smallest dinosaurs were probably from the late Triassic and early Jurassic. That's where we find the smallest ornithischian dinosaurs so far. Dinosaurs got biggest in the late Jurassic and Cretaceous.

Q: What is the most dangerous dinosaur period?
A: The most dangerous dinosaur period of the three was probably the Cretaceous, since the biggest, meanest predators lived then, including T. rex which came right at the end of dinosaur time.

Q: How hot was it when the dinosaurs lived?
A: The temperature varied, but generally it was hotter than today, with only a little snow in some polar places like Australia (in those days) once in a while.

Q: Where did the velociraptor live and when?
A: Velociraptor lived in Mongolia about 80 million years ago. Two years ago, it lived as a robot and computer animation for a short time in Hollywood during the making of Jurassic Park.

Q: In what period did gigantosaurus live?
A: It lived in the Late Cretaceous, about 90 million years ago.

Q: Why weren't people alive when dinosaurs lived?
A: People evolved from earlier mouse-sized mammals that were around all through dinosaur time but didn't get any bigger than a house cat. Only after the dinosaurs disappeared 65 million years ago did mammals get larger and more varied. Eventually, the primate mammals, the ape family, emerged and led to one very smart ape about one million years ago — us.

Q: Did the woolly mammoth live when people were here?
A: Yes, the woolly mammoth lived when the first Americans, Indians, were on this continent. They also lived in Asia. The native people hunted the mammoths with spears and other tools. They hunted them so well that they may have been the reason these and other giant animals from 15,000 years ago or more went extinct. Dinosaurs went extinct long before people came along.

  • Subjects:
    Archaeology, Dinosaurs, Habitats and Ecosystems