The geologic time scale n The earth’s history is divided into four eras: Precambrian - beginning when earth was formed (4.6 billion years ago) and ending 600 million years ago Cambrian - from 600 million years ago to 245 million years ago Mesozoic - from 245 million years ago to 65 million years ago Cenozoic - from 65 million years ago to now

Geological periods n The three most recent eras are divided into periods n Paleozoic Era Cambrian mybp until 510 mybp Ordovician mybp until 440 mybp Silurian mybp until 410 mybp Devonian mybp until 360 mybp Carboniferous mybp until 290 mybp Permian mybp until 245 mybp

Geological periods n Mesozoic Era Triassic mybp until 205 mybp Jurassic mybp until 145 mybp Cretaceous mybp until 65 mybp n Cenozoic Era Tertiary - 65 mybp until 1.8 mybp Quaternary mybp until now

The Fossil Record n Fossils form best when there is slow decomposition and rapid burial of plant or animal parts. n The extent of fossilization depends on the habitat and the structure of the organisms.

Pollen Leaves 1. A tree lives in a swampy habitat. The tree drops leaves, pollen, and seeds into the mud, where decomposition is slow. 2. The tree falls. The trunk and branches break up as they rot. Figure 24.1a upper

3. Flooding brings in sand and mud, burying the remains of the tree. 4. Over many years, the mountains erode and the swamp is filled with sediment. The habitat dries. Sand and gravel Buried material from swamp Bedrock Figure 24.1a lower

Intact The pollen was preserved intact because no decomposition occurred. Compression Sediments accumulated on top of the leaf and compressed it into a thin carbon-rich film. Cast The branch decomposed after it was buried. This left a hole that filled with dissolved minerals, faithfully creating a cast of the original. Permineralized The wood decayed very slowly, allowing dissolved minerals to gradually infiltrate the cells and then harden into stone. Figure 24.1b Four types of Fossils

Archaeopteryx—the first bird in the fossil record Figure 24.2 lower