Q2 W7 – LESSON Fossils
Fossils Provide Evidence of Earth”s Geological History
Students, The main point of this chapter is that the many layers of earth directly below our feet have remains of ancient organisms that tell us much regarding what lived in the past and what the earth was like millions of years ago.
Students, scientists use the fossil record to reconstruct events in Earth’s history. For example,when the same fossils were found on the western coast of Africa and the eastern coast of South America scientists theorized that the two land masses where once joined. Or scientists have discovered whale bones in the Andes mountains in South America which indicates that those mountains were once under the ocean. Or fossils of ancient trees in the Mojave Desert indicate that the land once was a forest.
Fossils indicate the past events in the Earth’s geology. To learn about this let’s first examine what are fossils, how are they made in nature, and how scientists determine their age.
The Dynamic Changing Crust Always throughout the earth there is weathering and wearing down mountains and rocks. The process of erosion carries and deposits rock and sediment to resting places like rivers, lakes and oceans. Hence, there is always places on the earth where new layer of sedimentary rock is formed. Likewise there are volcanoes that spew out new layers of igneous rock. There is magma rising toward the surface to create new layers of granite and other igneous rock that at first lies underground but later through erosion at some point becomes surface rock. On the ocean floors sediment is building and new layers of the earth are formed and millions of years later these layers of the ocean floor rise in elevation and become the surface ground we live and walk on. The earth”s crust is in a constant change that is hard to comprehend as it ocurrs over millions of years. Geologist study these earth changes and the study of the layers of rock tells scientists about the earth’s ancient past.In this sense, ancient geological structure and activity and the remains of biological records (fossils) can beused to reconstruct Earth”s history.One of the primary laws of nature used in geology and paleontology (the study of fossils) is the Law of Superposition. Let”s now turn to that law.The Law of Superposition States:Although not in every case, the deeper a layer of rock exists underground, the older that layer is compared to the layers above it.
The Earth’s Crust as Layers of RocksThe earths crust below our feet consists of many layers of different kinds of rock. All these layers at one time were on the surface of the earth and organisms, animals and plants, once lived on it. Some layers of rock formed on the ocean floor but later became land. In every case as scientists investigate the fossils remaining in the rock layers they learn what life was like millions of years ago. The layers are stacked one upon the other and in general the lower layers are more ancient in time and the upper layers are more recent in geological time. A fossil of an organism found at a lower layer should be of an animal that existed further in the past than a fossil found in a layer above it.This is called the Law of Superposition in geology.
Dinosaurs were NOT the first fossils and therefore were not the first animals. Dinosaurs came later and have left and new species of animails and plants emerge and then they become exstinct, and so forth.
Students in the above images, you see that older layers are on the bottom and therefore the animal fossils found in those older layers are therefore older animals. More recent organisms are found on the top layers. The word “recent” in geological time may mean 20 millions years ago. The word “older” in geological time may mean 300 million years ago. The dinosaurs became exstinct only 65 million years ago.
Organisms Leave Fossils Behind Animals die and their bodies rest in sediment which protects it. Hard body parts like bones and teeth remain preserved for thousands and millions of years. The soft body organs deteriorate quickly or are eaten. The soft tissue of animals is seldom preserved. We have dinosaur bones but we don’t have a record of their internal organs. The study of fossils of ancient remains of animals and plants tell us about the past. Paleontologists study fossils to reveal that animals and civilizations were like in ancient times.
Notice from the image on the left, the many organisms that lived long before dinosaurs. Fossils are a record of all the biological life on the planets history.
Geologists study rock formations and Palentologists study the fossil records found in rock formations. Paleontologists study fossils to reveal what animals and civilizations were like in ancient times.
Paleontologists discover ancient artifacts and serve museums. The picture in the middle is of discoverying dinosaur foot prints in stone.
Check For Understanding
1. The layer of rock at the bottom of a mountain face that has several rock layers above it shouldbe a) the oldest or b) youngest rock
2. Dinosaur fossils are the oldest fossils on Earth. True or False?
3. What is the difference between a geologist and a paleontologist?
The 7 Ways Fossils are Preserved Mineral replacementIt may sound strange but organisms that die (plants and animals) can be turned into mineral stones. When a tree falls and starts to decay water can saturate the entire tree. This water has minerals in it and those minerals start to precipitate, that is harden, into stone in the very cells of the tree. The minerals harden in the exact likeness of the tree. The stone looks just like the tree did. All the tree fiber and cells are gone but the minerals that replaced thsoe cells and fiber have an appearance just like the tree it replaced. When this happens to a tree it is called petrified wood. The tree decayed away and minerals replaced it.
Here you see trees that have been minierlized and are now stones, veryheavy petrified stones that have the identical look of the tree they mineralized.Well animals and plants can have this done to them too. This is called mineral replacement. That is a good name for it. Any organism when dead if it is covered with sediment and saturated with water can have its bones and teeth and even in rare occations its interal organs mineralized by the minerals in the water precipitating, hardening, right in its cells and taking the very exact shape of the organism it is replacing.Hence, many fossilized bones are minerals in the identical shape of the bone. But more recent fossils that have not mineralized are the actual bone itself, providing the bone is well preserved. PermineralizationPermineralization is very similar to mineral replacement. Mineral replacement seems to be more thorough and can even mineralize the soft tissue of animals. But this does not happen too often. In permineralization minerals in water enter the bone and encases the bone material to preserve it instead of replace it. For our purposes we can think of permineralization and mineral replacement as pretty much the same thing.
These bones have been mineralized as stones butstill in the orignal shape of the organism. Carbon Remains – Carbon Films Carbon film is another way ancient remains are preserved. Dead organisms that are at the very bottom of a sediment layer will have intense heat and pressure on it. The carbon body of the organism looses all liquids and gasses and the remaining carbon material is turned into a film, which is thin layer of carbon in the form of a silhouette of the organism. Literally the organism was carbonized.
Here the leaf is only a thin film of carbon from the orignal leaf. Coal is a fossil that is carbonized but it is classified more as a fossil fuel and not as organism remains. Plants are carbonized in the heat and pressure and their atomic structure changes into coal.
Molds and Casts Molds and casts are a means to record ancient organisms. A shell falls into sediments, is dissolved by acid and leaves a mold. A mold is an open space that represents the exact empty shape of the organism that occupied it.
You have probably used molds in plaster art projects. Water will fill up this mold space and the minerals in the water will precipitate (harden) out of the water filling the mold space. Then the hardened mineral is in the shape of the shelled organism that once existed there. The hardened minerals taking the shape/form of the shell is called a cast. The empty space or impression is the mold. The filling the empty space with minerals is a cast.
Here you see a mold of an acient organism. It did not fill up with minerals. The illustration on the right shows a shell dissolving, leaving a mold space and minerals precipitating in the space and lastly a mineralized shell of the identical organism. The minerals that filled in the shape of the shell is the cast.
|Here you see a cast of an an ancient shell. The shell deteriorated long ago, leaving and empty space, a mold. The mold filled in with minerals that formin the very same shape as the original shell.|
Original Remains – Body Fossils On rare occasions an ancient organism can be preserved entirely even its soft tissue. In other words the entire body is preserved. Mosquitos and insects in amber (tree) sap is preserved with soft tissue in whole for millions of years. Tar pits and frozen ground can preserve the whole body of an organism, like a tiger or mammoth.
Here you see a mosquito in amber. The other picture is a lizard in amber.Both organisms are of their original tissue well preserved in tree sap.
Ice also will preserve an entire organism. In 1991 a human body was found on a glacier near the Austria and Italian border. The body was 5,000 years old and was preserved in the ice to maintain good condition to study. Do a search for “Ice Man” to learn more about it. Also do a search for whole mammoth fossils. Their pictures are unsightly to some so I did not post them here. The first video you will watch for this chapter has a mammoth find in it. Trace Fossils Trace fossils are footprints in the mud that later hardens into rock and remain for millions of years. There are many examples of this from the era of dinosaurs.
Trace fossils of dinosaurs. Index Fossils Index fossils are a special species that lived for a short period of geological time but were abundant on the planet and did not change (evolve) during its time. Index fossils are used by scientists are markers by which they can gauge the age layer of rock is compared to other layers without the index fossil in it. This is called releative age. Remember the age of a layer of rock is also the age of the fossils found in it. Scientists have the earth history designated into certain Periods or Eras as we will learn next chapter. Index fossils help determine these eras.
A trilobite is an index fossil as they were common everywhere at certain eras in history .