Mesozoic mammals differ from Cenozoic mammals, and so forth.

whole groups of organisms that were once abundant and diverse, such as trilobites, can become extinct. 

The boundaries between the great blocks of geologic time called  Eras are defined by major changes in the types of fossils found in the rocks deposited in those Eras:

ome species can be found on both sides of a time boundary; however, the overall assemblage of organisms found in the rocks of a given age is recognizably different from the assemblages found in the rocks above and below. 

biogeography, taxonomy, anatomy, embryology and, most recently, genetics

clearly shows changes in life through almost any sequence of sedimentary rock layers.

remains first and foremost among the databases that document changes in past life on Earth.

observations about fossils and the rock record were made long before Darwin formulated his theory

particularly DNA analysis.

contain different groups or assemblages of fossil species.

Sedimentary rocks are, by far, the most common rocks at Earth’s surface.

As the sediments accumulate they bury shells, bones, leaves, pollen, and other bits and pieces of living things.

Examples include anything that is Pelagic/Vagrant, Benthic, or any other organism able to move around.

broken up several ways

supereon

are eons

Not much is know about the Precambrian, because all of the life forms lacked hard shells or skeletons, making preservation very unlikely

split into Eras

divided based on the dominant life forms at that time

We are living in the Cenozoic Era now.

e periods.

Epochs starting after the beginning of the Phanerozoic Eon

rarely are

divided into Ages

Chron

Cambrian:

Ordovician

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Mississippian and Pennsylvanian can be considered parts of the Carboniferous Period.

Triassic

Jurassic

Cretaceous

Apes begin to appear

aleocene (

further development of mammals.

Eocene:

Oligocene

Miocene

Pliocene

First modern animals

They developed near the beginning of the period, and became extinct before the end

Index fossils are fossils of organisms that lived only in one period

Note that this refers to genera or species, not entire classes or families.

xtremely useful for dating rock

relative dating

he organism must have had a wide geographic range, because if a fossil is found only on some barren outcrop in the desert, it can't be used to date rocks from many miles away.

Most multicellular organisms display some form of symmetry

main types are

fossils often have charecteristics that make them symmetrical

than the layers will be younger as they near the top.

Rocks are originally layered horizontally. If you have layers that are higher on one side than on the other, it is due to the tilting of rocks caused by a geological event.

his principle states that a fracture or cut in a rock caused by another rock (igneous intrusion) is always younger than the rock it cuts

Fragments of one rock in another rock must be older than the rock it is contained in.

Carbon 14

Potassium 40

Uranium 235

Uranium 238

Thorium 232

Rubidium 87

Samarium 147

evolutionary relationships between taxa (phylogeny)

there is no minimum age for a fossil.

Fossils vary in size

fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life

exquisitely rare

he footprint or feces

may also consist of the marks left behind by the organism while it was alive

ichnofossils

trace fossils

these are known as chemical fossils or biomarkers.

Fossils are the mineralized or otherwise preserved remains or traces

of animals, plants, and other organisms.

These time intervals are not equal in length

argest intervals of geologic time and are hundreds of millions of years in duration

Eons are divided into smaller time intervals known as eras

but are not as significant as those which bound the eras.

Eras are subdivided into periods.

Very significant events in Earth's history are used to determine the boundaries of the eras

he periods of the Cenozoic are frequently subdivided into epochs

Subdivision of periods into epochs can be done only for the most recent portion of the geologic time scale.

As a result, the history contained within these rocks can not be as clearly interpreted.

Often a small detail is all that seperates one species from another

observing the body structures and functions

hierarchical, with increasingly detailed separations among members of the groups. Today, scientists use more or less the same system.

Carolus Linnaeus

Living organisms are grouped according to their similar characteristics

the phylum level attempts to put animals into broad categories of similar body types.

similarity of features, even when the features are minute or on a chemical level.

Order

fossils belong to the same groups as modern-day animals.

Paleontologists look at minute details to determine further fossil identification and classification.

even the internal organs