Geologic Timeline

Fossils  From the fossil record, paleontologists learn:  the structure of ancient organisms  their environment  the ways in which the organisms lived.

Fossils and Extinct Species  Fossils are the most important source of information about extinct species  Species that have died out  The fossil record is incomplete so there are many organisms that have died out that we do not know about.

Types of Fossils  Many fossils are just fragments of an organism— teeth, pieces of a jawbone, or bits of leaf.

Types of Fossils  Most fossils are preserved in sedimentary rock.  Sedimentary rock usually forms when small particles of sand, silt, clay, or lime muds settle to the bottom of a body of water.  As sediments build up, they bury dead organisms that have sunk to the bottom.

Types of Fossils  As layers of sediment continue to build up over time, the remains are buried deeper and deeper.  Over many years, water pressure gradually compresses the lower layers and turns the sediments into rock.

Paleontologists  Researchers who study fossils to learn about ancient life.

What Fossils Can Reveal  By comparing body structures in fossils to body structures in living organisms, researchers can infer evolutionary relationships and form hypotheses about how body structures and species have evolved.

What Fossils Can Reveal  Bone structure and trace fossils, like footprints, indicate how animals moved.

What Fossils Can Reveal  Fossilized plant leaves and pollen suggest whether the area was a swamp, a lake, a forest, or a desert.  When different kinds of fossils are found together, researchers can sometimes reconstruct entire ancient ecosystems.

Geologic Time Scale  Geologists and paleontologists have built a time line of Earth’s history called the geologic time scale.

Hadean Eon  About 4600 mya (million years ago) – 4000 mya  Mya – million years ago

Archean Eon  About 4000 mya – 2500 mya

Proterozoic Eon  About 2500 mya – 542 mya

Cambrian Period  Inside the Paleozoic Era  About 542 mya – 488 mya

Ordovician Period  Inside the Paleozoic Era  488 mya – 444 mya

Silurian Period  Inside the Paleozoic Era  444 mya – 414 mya

Devonian Period  Inside Paleozoic Era  416 mya – 359 mya

Carboniferous Period  Inside Paleozoic Era  359 mya – 299 mya

Permian Period  Inside Paleozoic Era  299 mya – 251 mya

Triassic Period  Inside Mesozoic Era  251 mya – 200 mya

Jurassic Period  Inside Mesozoic Era  200 mya – 146 mya

Cretaceous Period  Inside Mesozoic Era  146 mya – 65.5 mya

Paleogene Era  Inside Cenozoic Era  65.5 mya – 23 mya

Neogene Period  Inside Cenozoic Era  23 mya – 1.8 mya

Quaternary Period  Inside Cenozoic Era  1.8 mya - present

Life on a Changing Planet  Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth’s history.  In turn, the actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.

Life on a Changing Planet  Earth and its climate has been constantly changing, and organisms have evolved in ways that responded to those new conditions.  The fossil record shows evolutionary histories for major groups of organisms as they have both responded to changes on Earth and how they have changed Earth.

Physical Forces  Climate  Temperatures  Geologic Forces  Volcanic activities  Wind and ocean currents

Physical Forces  The theory of plate tectonics explains how solid continental “plates” move slowly above Earth’s molten core—a process called continental drift.

Biological Forces  The activities of organisms have affected global environments.  For example, Earth’s early oceans contained large amounts of soluble iron and little oxygen.

Biological Forces  During the Proterozoic Eon, however, photosynthetic organisms produced oxygen gas and also removed large amounts of carbon dioxide from the atmosphere.