How do scientists know that different kinds of plants & animals lived in the past?

Fossils Preserved remains/traces of living things Clues to the past Usually found in sedimentary rocks Bones, shells, teeth, seeds, woody stems become fossilized It is rare for the soft part of an organism to become a fossil

Fossils provide information for: What the environment looked like Climate Shape of Earth’s surface

How do fossils form? Living things are buried by sediments Slowly harden into rock The shape of the organism is preserved

Who studies fossils? Paleontologists!

Kinds of Fossils: Petrified fossils Molds Casts Carbon Films Trace fossils Other

Petrified Fossils Petrified means “turning into stone” Minerals replace all/part of an organism

Molds & Casts Mold is a hollow area in sediment in the shape of an organism/part of an organism Cast is a copy of the shape of an organism when a mold is filled with mud, ash, lava Most common types of fossils

Carbon Films Extremely thin coating of carbon on rock Carbon comes from living things buried underground Carbon can preserve delicate parts of plants & insects

Trace Fossils Provide evidence of the activities of ancient organisms Ex: footprint, burrows, trails

Other Some processes preserve the remains of organisms with little/no change Ex: tar, amber resin, freezing

How are fossils classified? Paleontologists classify fossils in order in which they lived from earliest to latest into a fossil record Fossil records show how organisms have changed over time

How do geologists use fossils to decide how old the rocks are? Relative age Absolute age Law of Superposition Index fossils

Relative Age The rock’s age compared to the ages of other rocks

Absolute Age The number of years since the rock formed Geologists use radioactive dating to determine this Carbon-14 dating Potassium-Argon dating

Law of Superposition In horizontal sedimentary rock layers, the oldest layer is at the bottom and each layer above it is younger

Index fossils A fossil found in many different areas Represents a type of organism that existed only briefly Geologists look for index fossils in rock layers Index fossils are useful because they tell the relative age of the rock layers they are in

Index Fossil Example Trilobite: hard-shelled animal with 3 distinct body sections Many types of trilobites existed over 500 million years ago Geologists must find one trilobite that is different in some way than the others to date the rock layer in which it was found Ex: Large eyes

What if a small piece of time is missing? Unconformity is a gap in the geologic record Some rock records are lost due to erosion

What is the geologic time scale? A record of the life forms and geologic events in Earth’s history First developed by studying rock layers & index fossils Later radioactive dating helped to determine the absolute age of each divisions of the time scale

Different units of the time scale: Eras Periods Epochs

Eras Long units of time Precambrian Paleozoic Mesozoic Cenozoic

Periods Sub-divided units of an era Range in length from 2-10million years Names came from where geologists first discovered the rocks/fossils of that period Ex: Jurassic

Epochs Sub-divided units of a period

Precambrian Era 4.6billion-544million years Earliest life forms similar to bacteria of today called cyanobacteria Made food through the use of photosynthesis

Paleozoic Era 544million-245million years Explosion of life forms Animals began coming onto land Evolving: Hard-shelled organisms Invertebrates (jellyfish) Vertebrates (jawless fish) Amphibians Small reptiles Mass extinction ended this era

Mesozoic Era 245million-65million years ago Called the “Age of Reptiles” Evolving: Dinosaurs Mammals Birds Flowering plants Jurassic Period-dinosaurs ruled Earth for about 150million years Mass extinction occurred when an object from space hit Earth’s surface causing a dust cloud which blocked the Sun No dinosaurs survived!

Cenozoic Era 65million-Present Evolving: Marine mammals (whales) Grass Mammals, birds, & insects continue to flourish Fossil records show that humans appeared about 3.5million years ago