Age inference

8.E.2.1Students know:

·        that fossils provide important evidence of how life and environmental conditions have changed. The earth processes we see today, including erosion, movement of lithospheric plates, and changes in atmospheric composition, are similar to those that occurred in the past. Earth’s history is also influenced by occasional catastrophes, such as the impact of an asteroid or comet.

·       that a fossil is the preserved remains or traces of an organism that lived in the past. Fossils give clues to the diversity of living things over the history of Earth, give clues to past climate and surface changes on Earth, and give clues to changes that have occurred with organisms over time.

  ·        the formation process of mold, cast, petrified, preserved, carbonized, and trace fossils.

  ·        the different types of fossils based on how they were formed. The formation process of fossils varies depending on where and under what environmental conditions they formed.

o   Mold fossil – forms when sediments bury an organism and the sediments change into rock; the organism decays leaving a cavity in the shape of the organism.

o   Cast fossil – forms when a mold is filled with sand or mud that hardens into the shape of the organism.

o   Petrified fossil (permineralized fossil) – forms when minerals soak into the buried remains, replacing the remains, and changing them into rock.

o   Preserved fossil – forms when entire organisms or parts of organisms are prevented from decaying by being trapped in rock, ice, tar, or amber.

o   Carbonized fossil – forms when organisms or parts, like leaves, stems, flowers, fish, are pressed between layers of soft mud or clay that hardens squeezing almost all the decaying organism away leaving the carbon imprint in the rock.

o   Trace fossil – forms when the mud or sand hardens to stone where a footprint, trail, or burrow of an organism was left behind.

·        that the geologic time scale is a record of the major events and diversity of life forms present in Earth’s history. The geologic time scale began when Earth was formed and goes on until the present. At the end of each era a mass extinction occurred, many kinds of organisms died out, although there were other extinctions going on during each period of geologic time. Using the fossil record, paleontologists have created an idea of the different types of common organisms in each geologic period.

o Precambrian Era

§  The Precambrian Era is Earth's first era of time. It began with the creation of the Earth around 4.6 billion years ago.

§  5 major events occurred during this era: (1) the formation of the Sun and light, (2) the creation of the Earth, (3) the creation of the atmosphere through volcanic out-gassing, (4) the creation of the oceans, and (5) the creation of life.

§  Began with simple life forms such as bacteria and simple algae.

§  There was a rise of simple organisms such as jellyfish and sea worms by the end of the era.

§  Few fossils because the life forms were soft-bodied and had no hard skeleton.

o Paleozoic Era

§  Began with the early invertebrates, such as trilobites and brachiopods; continued to develop early vertebrate fish, then

arachnids and insects;; later came the first amphibians, and near the era’s end the reptiles became dominant.

§  Early land plants included simple mosses, ferns, and then cone-bearing plants.

§  By the end of the era, seed plants were common.

§  The mass extinction that ended the era caused most marine invertebrates as well as amphibians to disappear.

o Mesozoic Era

§  Reptiles were the dominant animals of this era, including the various dinosaurs.

§  Small mammals and birds also appeared.

§  Toward the end of the era, flowering plants appeared and the kinds of mammals increased.

§  The mass extinction that ended the era caused the dinosaurs to become extinct.

o Cenozoic Era

§  New mammals appeared while others became extinct.

§  The diversity of life forms increased.

§  Flowering plants became most common.

§  Humans are also part of the most recent period of this era.

 ·   that various models, diagrams, and pictures can be used to illustrate the vastness of time involved in geologic time and to show the diversity of life evident across geologic time. Through the illustrations, not only does the diversity of life-forms increase, but the complexity of those life-forms also increases.

·   that millions of fossils have been collected and studied. The fossil record gives important information about past life and environments on Earth. Certain fossilized organisms could only live in specific environments or under particular climate conditions. Extinction of life- forms as well as how and when new life-forms appeared is part of the fossil record.

·   that the relative age means the age of one object compared to the age of another object. Relative age does not tell the exact age of an object. The relative age of rocks and fossils can be determined using two basic methods: ordering of rock layers and index fossils:

o Ordering of Rock Layers Scientists read the rock layers knowing that each layer is deposited on top of other layers. The law of superposition states that each rock layer is older than the one above it. This law is used to read rock layers.  Using this understanding of layering, scientists infer that the relative age of the rock or fossil in the rock is older if farther down in the rock layers. Relative dating is best used when the rock layers have been preserved in their original sequence. Over millions of years, tectonic plate motion can distort these layers. As a result of this, the youngest layers of rock are not always found on top, because of folding, breaking, and uplift of layers.

o Index Fossils Certain fossils, called index fossils, can be used to help find the relative age of rock layers. To be an index fossil –an organism must have lived only during a short part of Earth’s history;; many fossils of the organism must be found in rock layers; the fossil must be found over a wide area of Earth; the organism must be unique. The shorter time period a species lived, the better an index it is. A key example of an organism used as an index fossil are trilobites, a group of hard-shelled animals whose body had three sections, lived in shallow seas, and became extinct about 245 million years ago. Therefore, if a trilobite is found in a particular rock layer, it can be compared with trilobites from other layers to estimate the age of the layer in which it was found.

 ·   that geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive “parent atoms” decay into stable “daughter atoms.” When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed. Absolute geologic dating and relative geologic dating are two methods by which scientists try to determine the age of geologic evidence. Carbon-14 dating is an example of absolute dating, and the law of superposition is an example of relative dating.