HOW FOSSILS FORM

Fossils of hard mineral parts (like bones and teeth) were formed as follows:

Some animals were quickly buried after their death (by sinking in mud, being buried in a sand storm, etc.).

Over time, more and more sediment covered the remains.

The parts of the animals that didn't rot (harder parts likes bones and teeth) were encased in the newly-formed sediment. This process results in a heavy, rock-like copy of the original object - a fossil. The fossil has the same shape as the original object, but is chemically more like a rock! Some of the original hydroxy-apatite (a major bone consitiuent) remains, although it is saturated with silica (rock). 

DATING INDIVIDUAL FOSSILS 

Paleontologists use many ways of dating individual fossils in geologic time. The oldest method is stratigraphy, studying how deeply a fossil is buried. Dinosaur fossils are usually found in sedimentary rock. Sedimentary rock layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing them into rocks. Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed. Generally, deeper rocks and fossils are older than those found above them. 

Dating a fossil in terms of approximately how many years old it is can be possible using radioisotope-dating of igneous rocks found near the fossil. Unstable radioactive isotopes of elements, such as Uranium-235, decay at constant, known rates over time (its half-life, which is over 700 million years). The half-life of carbon-14 is 5,568 years. That means that half of the C-14 decays (into nitrogen-14) in 5,568 years. Half of the remaining C-14 decays in the next 5,568 years, etc. This is too short a half-life to date dinosaurs; C-14 dating is useful for dating items up to about 50,000 - 60,000 years ago (useful for dating organisms like the Neanderthal man and ice age animals). 

Radioisotope dating cannot be used directly on fossils since they don't contain the unstable radioactive isotopes used in the dating process. To determine a fossil's age, igneous layers (volcanic rock) beneath the fossil (predating the fossil) and above it (representing a time after the dinosaur's existence) are dated, resulting in a time-range for the dinosaur's life. Thus, dinosaurs are dated with respect to volcanic eruptions. 

THE TRIASSIC PERIOD Fact or fiction? 

Roughly 248 million years ago (mya), the Permo-Triassic extinction occurred. This is the largest extinction known. About 95% of all species and about 60% of the genera died out, including many animals (like the trilobite). The cause of the Permian extinction might have been global cooling, volcanic eruptions, or a crease in the continental shelf area during the formation of Pangaea. This catastrophic extinction and continental rearrangement opened the way for the rise of the dinosaurs and mammals. 

There were no dinosaurs at the beginning of the Triassic, but there were many amphibians and some reptiles and dicynodonts (like Lystrosaurus). During the early Triassic, corals appeared and ammonites recovered from the Permian extinction. Seed plants dominated the land; in the Northern hemisphere, conifers flourish. Glossopteris was the dominant southern hemisphere tree during the Early Triassic period. 

THE JURASSIC PERIOD Fact or fiction? 

At the beginning of the Jurassic, the Earth's continents were still jammed together, forming the supercontinent Pangaea, but they were beginning to drift apart. There had been a minor extinction at the end of the Triassic period, which gave rise to an abundance of dinosaurs in the Jurassic. The climate was hot and dry and at the beginning of the Jurassic, strongly seasonal. 

The dinosaurs dominated the near-tropical Earth during the Jurassic, and many new groups appeared. The gigantic sauropod dinosaurs, like the Diplodocus and Apatosaurus, diversified. Carnivorous theropods, like Allosaurus and Compsoenathus. were abundant. Bird-like dinosaurs also flourished. 

About 140 million years ago, during the late Jurassic period, the flowering plants (angiosperms) evolved, and would soon change the face of the Earth. 

THE CRETACEOUS PERIOD Fact or fiction? 

The Cretaceous period was the heyday of the dinosaurs. Huge carnivores like Tyrannosaurus rex and Giganotosaurus appeared, as did Triceratops and many, many others. There was a tremendous diversity in dinosaur species. Mammals were flourishing, and flowering plants developed and radically changed the landscape.

The breakup of the supercontjnent Pangaea into separate continents was underway. The separation of Laurasia and Gondwana was complete. In the first half of the Cretaceous, temperatures were warm, seasonality was low, and global sea levels were high (no polar ice!). At the end of the Cretaceous, there were severe climate changes, lowered sea levels, and high volcanic activity .

The Cretaceous period ended 65 million years ago with the extinction of the dinosaurs and many, many other prehistoric life forms. This mass extinction was the second-most extensive in the history of the Earth.