Fossils A Trip Through Geologic Time

Fossils  Fossils are preserved remains or traces of living things.  Most fossils form from animals and plants that lived near quiet water, such as swamps, lakes, or shallow seas, where sediments build.  The organism dies and the remains are buried by sediments.  The sediments slowly harden into rock and preserve the shape of the organisms.

Fossils  Fossils are usually found in sedimentary rocks.  When an organism dies, its soft parts often decay quickly or are eaten by animals leaving only the hard parts to fossilize.  Ex. bones, shells, teeth, seeds, or woody stems.  For a fossil to form, the remains or traces of an organism must be protected from decay.

Kinds of Fossils  Petrified Fossils: fossils in which minerals replace all or part of the organism. Ex: petrified wood  When the object is buried by sediment, water rich in minerals seeps into the cells. After the water evaporates, hardened minerals are left behind.

 Process in which minerals replace an organism’s tissues.  Permineralization: pore space (bone or wood) is filled with minerals.  Replacement: the organism’s tissues are completely replaced by minerals. Petrifaction

Kinds of Fossils  Molds and Casts are the most common type of fossil. A mold and cast of a trilobite. mold cast  A mold is a hollow area in sediment in the shape of an organism or part of an organism.  It forms when the hard part of the organism is buried in sediment.  Water carrying dissolved minerals and sediment may seep into the empty space.  If the water deposits the minerals and sediment there, the result is a cast.  A cast is a copy of the shape of an organism.  Molds and Casts are opposites. Molds and Casts are opposites.

Kinds of Fossils  Carbon Films: an extremely thin coating of carbon on rock that forms when materials that make up an organism become gases and escape leaving only carbon behind.  Fossilized Footprints: mud and sand that the animal stepped on was buried by layers of sediment. The sediment became rock, preserving the footprints.  A thin layer of carbon remains preserving the delicate parts of leaves and insects.  Trace Fossils provide evidence of the activities of ancient organisms. Ex: footprints, animal trails, or animal burrows.  Trace Fossils provide evidence of animals size and behavior.

Trace Fossils  Traces that reflect animal behavior. A = crawling traces B = resting traces C = dwelling traces D = grazing traces E = feeding traces

 Coprolite is fossilized feces (animal waste). Coprolite  Coprolites record the diet, feeding behavior, and habitat of prehistoric animals.  If the coprolite consists of partially digested plant material (leaves, seeds, bark, roots), the animal was probably an herbivore.  If the coprolite contains bits of animal material (crushed bits of bones, sinew, claws), the animal was a carnivore.

Kinds of Fossils  Preserved Remains are formed when an organism is preserved with little or no change.  For example when organisms become preserved in tar, amber (tree sap), or are frozen. Beetle in Tar Sand Insect in Amber Frozen Baby Mammoth

Amber  Insects get caught in tree sap and die.  More sap is added on.  The sap hardens, preserving the insect inside.

Asphalt  Pools of thick, sticky asphalt trap and preserve organisms. Freezing  Cold temperatures slow decay.  Organisms that freeze are preserved for long periods of time.  La Brea Tar Pits – Pit 91 La Brea Tar Pits – Pit 91

Fossil, or Not? Fossil Video

Why Study Fossils?  Scientists study fossils to learn what past life forms were like.  Paleontologists classify organisms in the order in which they lived.  All the information scientists have gathered is called the fossil record.

Fossil Record  The fossil record provides evidence about the history of life on Earth.  The fossil record also shows how different groups of organisms have changed over time.  It also provides evidence to support the theory of evolution.

Remember!  A scientific theory is a well-tested concept that explains a wide range of observations.  The fossil record shows that millions of types of organisms have evolved.  However, many others became extinct.

Using Fossils to Date Rocks!  Scientists use index fossils to match rock layers.  An index fossil must be widely distributed and represent a type of organism that existed only briefly.  They are useful because they tell the relative ages of the rock layers they are found in.

The Trilobite  One example of an index fossil is a trilobite.trilobite  Trilobites were a group of hard- shelled animals whose bodies had three distinct parts.  They evolved in shallow seas more than 500 million years ago.

Geologic Time Scale Since the history of the Earth is so long (4.6 billion years), geologists break the time period up into smaller pieces. The largest divisions are eons:  Hadean Eon  Archean Eon  Proterozoic Eon  Phanerozoic Eon

 Eons are divided into Eras.  Eras are divided into Periods.  Periods are divided into Epochs.  The boundaries between geologic time intervals represent when visible changes took place on the Earth.

 The Paleozoic Era (meaning "ancient life") lasted from 600 million years ago until 248 million years ago.  During the Paleozoic Era, many life forms still present today first appeared, such as jellyfish, coral, insects, spiders, fish, amphibians, reptiles and ferns.  There were few land organisms.  It was the first era well represented by fossils.  By the end of the era, amphibians and reptiles lived on the land and insects were abundant.  It came to an end with the largest mass extinction in Earth’s history.  Some scientists believe it was causes by ocean changes.

 The Mesozoic Era (meaning "middle life") lasted from 248 million years ago until 66 million years ago.  During the Mesozoic Era, many of the more complex life forms first evolved, including mammals, birds, and flowering plants.  However, the Mesozoic is best known for the presence of its largest and most famous animals: the dinosaurs!  It is known as the Age of Reptiles.  It’s believed that birds evolved from a type of dinosaur.  Along with the dinosaurs, about 20% of all species on Earth became extinct at the end of the Mesozoic Era.  The cause may have been global climate change.

 The Cenozoic Era started 66 million years ago and continues today.  This era is known as the Age of Mammals.  In the Mesozoic era, mammals had to compete with dinosaurs and other animals for food and shelter.  After the mass extinction at the end of the Mesozoic, mammals flourished.  Their unique characteristics may have helped them adapt and survive the environmental changes that caused the extinction of the dinosaurs.

 The Cenozoic is divided into several epochs: Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. In particular, the Miocene Epoch lasted from 24 million years ago until 5 million years ago. It is sometime during this epoch that the Gray Fossils were formed.

 We know that the Earth is very old. Scientists currently understand that it is about 4.6 billion years old. (That's 4,600,000,000 years!) This huge amount of time is called geologic time. The evidence of Earth’s age comes from its rocks. The rocks that are exposed at Earth’s surface are all different ages. Some are quite young, made in the past few million years. Others are quite old – many millions or even billions of years old. These old rocks are usually quite deep within the Earth’s crust but are often exposed as plate tectonics pushes ancient rocks to the surface. Scientists who study rock layers developed a timeline of Earth history called the geologic time scale to describe the ages of various rock layers.Earthrocks  The geologic time scale expresses the 4.6 billion years of geologic time along a timeline that is divided into sections. Broad sections of time are called Eras and smaller sections of time are called Periods. Having names for the different sections of time helps people communicate about when events happened long ago such as when a dinosaur lived, when a volcano erupted, or when an asteroid hit.volcano