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The Triassic, Jurassic, and Cretaceous periods were marked out by geologists to distinguish among various types of geologic strata (chalk, limestone, etc.) laid down tens of millions of years ago. Since dinosaur fossils are usually found embedded in rock, paleontologists associate dinosaurs with the geologic period in which they lived—for example, "the sauropods of the late Jurassic."

To put these geologic periods in the proper context, bear in mind that the Triassic, Jurassic, and Cretaceous don't cover all of prehistory, not by a long shot. First came the Precambrian period, which stretched from the earth’s formation to about 542 million years ago.

The development of multicellular life ushered in the Paleozoic Era (542–250 million years ago), which embraced shorter geologic periods including (in order) the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. It's only after all that we reach the Mesozoic Era (250-65 million years ago), which includes the Triassic, Jurassic and Cretaceous periods.

Ages of the Dinosaurs (The Mesozoic Era)

This chart is a simple overview of the Triassic, Jurassic, and Cretaceous periods, all of which were part of the Mesozoic era. In brief, this incredibly long period of time, measured in "mya" or "millions of years ago," saw the development of dinosaurs, marine reptiles, fish, mammals, flying animals including pterosaurs and birds, and a huge range of plant life. The largest dinosaurs did not emerge until the Cretaceous period, which started over 100 million years after the start of the "age of dinosaurs."

... Triassic: 237–201 mya ...

Archosaurs ("ruling lizards");

therapsids ("mammal-like reptiles")

...Jurassic: 201–145 mya ...

Dinosaurs (sauropods, therapods);

Early mammals; Feathered dinosaurs

...Cretaceous : 145–66 mya ...

Dinosaurs (sauropods, therapods, raptors, hadrosaurs, herbivorous ceratopsians);

Small, tree-dwelling mammals

Archosaur: Sometimes called “ruling reptiles,” this group of ancient animals included dinosaurs and pterosaurs (flying reptiles)

Therapsid: A group of ancient reptiles that later evolved to become mammals

Sauropod: Huge long-necked, long-tailed vegetarian dinosaurs (such as the Apatosaur)

Therapod: Two-legged carnivorous dinosaurs, including raptors and Tyrannosaurus

RexPlesiosaur: Long-necked marine animals (often described as similar to the Loch Ness monster)

Pterosaur: Winged flying reptiles that ranged from the size of a sparrow to the 36-foot-long

QuetzalcoatlusCycad: Ancient seed plants that were common during the time of the dinosaurs and are still common today

Early estimates of 300-year lifespans for the largest sauropods were based on comparisons with crocodiles and turtles, which have much slower metabolisms. The consensus is now that Apatosaurus and Diplodocus dinosaurs probably only lived for 70 or 80 years, which is about the same as an elephant today.

The Struthiomimus may have been the fastest dinosaur. It could run up to 60 miles per hour!

Troodon had a large brain for its relatively small size and was probably among the smartest dinosaurs. Its brain is proportionally larger than those found in living reptiles, so the animal may have been as intelligent as modern birds, which are more similar in brain size.

The short answer is we know of about 900 valid dinosaur species that existed.

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