Rock Strata-Index Fossils-Evolution: Evidence of Earth's Past

Rock Strata-Index Fossils-Evolution:
Evidence of Earth's Past

How does rock strata tell us about the past?

EXAMPLE:
How to determine the best index fossil?

After eliminating all the fossils that don't fit the requirements, we have only 1 left that appears in
all the outcrops, and in only one layer per outcrop. So the correct index fossil is:

Rock Strata - Law of Super Position
* The Law of Superposition holds that in any undisturbed sequence of strata, the oldest layer is at the bottom of the sequence, and the youngest layer is at the top of the sequence.
* The laws of stratigraphy are principles that geologists use to determine the relative age of rock layers.
*Index fossils, which are widespread fossils that existed over a short period of time, are used to help correlate rock strata found in different locations.
*Evidence for dinosaur extinction and other extinctions can be found in rock layers throughout the world. be found in rock layers throughout the world.
*The Cross-Cutting Law, which states that any geologic
features that cut across rock strata is younger than the rocks they cut through.

INDEX FOSSILS

What is an index fossil?
*the remains of an organism that lived and died during a particular time frame in Earth’s history.
*used to define periods of geological time, determine the age of rock layers, and help date other fossils found near and around them.

CHARACTERISTICS OF
INDEX FOSSILS

1. The fossil must be easily recognizable. This means it must be distinctive – clearly one particular type of organism that can easily be identified.

2. The organism had to be abundant, meaning there were a lot of them.

3. The organism needed to have been widespread, meaning it lived over a large geographical area. This means we are able to find fossils of the organism miles and miles away from each other, even on different continents.
(Most index fossils are marine organisms, because ocean currents allowed them to populate over a large area.)

4. The organism only lived for a short period of time. This means it will only be found in a few rock layers.

We can...

* eliminate “fossil 1” because it shows up in multiple layers in the same outcrop. (It lived for too long of a time period in Earth’s history to help establish dates of other rocks).

*eliminate “fossil 2” because it shows up in multiple layers in column 2. (It lived for too long a time period).

*eliminate “fossil 3” because it is only in one layer in one outcrop.

EVOLUTION:
*a change of over time. specifically, it is the change of traits in a species over successive generations.
*As environments change, species evolve. Some species may die off while new species are created.

Comparative Structures

Scientists often compare the structure, or anatomy, of different species when studying evolutionary relationships between organisms. This is called comparative anatomy.

Three Types of Structures When Comparing Anatomy:

1. homologous structures-structures within different species that look the same but have different functions.
Meaning, different species possess these structures because they descended from a common ancestor but they look different because they are adapted for different purposes.

2. analogous structures -structures within different species that are adapted to perform the same function.

3. vestigial structures- structures within an organism that are functionless. Sometimes these structures are the remnants of once functioning structures.

Example: The Human Tailbone
It is the remnant of tail bones found in mammals that have a tail. Other mammals have tails to help them balance. However, when humans learned to walk on two legs, their tail became useless.

Natural Selection

*the process whereby nature selects which members of a species survives based on certain traits.
Example: Suppose there is a population of mice that live in a field that is full of tan-colored dry grass. There are black mice and tan mice. The predators in the area can see the black mice easier and eat more of the black mice because they are easier to locate. Over time, the mice that carry the genes for a tan coat survive more often than the mice with black coats. This increases the population of tan mice and increases the likelihood that the majority of future generations will be tan.