Looking at Fossils Chapter 3 Section 4 p. 74 - 79 Vocabulary: 2. trace fossil 3. mold 4. cast 5. index fossil

Fossilized Organisms p. 74 The remains or physical evidence of an organism preserved by geologic processes is called a fossil. Fossils are most often preserved in sedimentary rock. Hard parts of organisms, such as shells and bones, are more resistant to decay than soft tissues are. So when sediments become rock, the hard parts of animals are much more commonly preserved than are soft tissues. Hardened tree sap is called amber. Some of our best insect fossils are found in amber, as shown in this picture.

Fossils can be found in other ways as well. Some organisms are preserved by petrification. This is process in which minerals replace an organism’s tissues. Permineralization is a process in which the pore space in an organism’s hard tissue (bone or wood) is filled up with mineral. Replacement is a process in which the organism’s tissues are completely replaced by minerals. In some places on the Earth’s surface, asphalt wells up and forms thick, sticky pools. The LaBrea asphalt deposits in Los Angeles, California, for example, are 38,000 years old and have preserved organisms for all of those years. Sometimes, cold temperatures slow down decay and fossils are preserved from the last ice age. Here is an example of a fish found in ice from Argentina.

Other Types of Fossils p. 76 Any naturally preserved evidence of animal activity is called a trace fossil. Animal footprints fill with sediment and become preserved in rock. Burrows are another trace fossil. Burrows are shelters made by animals such as clams, that bury in the sediment. A third type of trace fossil, called a coprolite, is preserved animal dung (yuck!)

Molds and Casts p. 76 Molds and casts are two more examples of fossils. A cavity in rock where a plant or animal was buried is called a mold. A cast is an object created when sediment fills a mold and becomes rock. A cast shows what the outside of the organism looked like. This photograph shows two molds from an ammonite. The image on the left is the internal mold of the ammonite, which formed when sediment filled the ammonite’s shell, which later dissolved away. The image on the right is the external mold of the ammonite, which preserves the external features of the shell.

Using Fossils to Interpret the Past p. 77 The fossil record only offers a rough sketch of the history of life on Earth. Scientists know more about organisms that had hard body parts than about organisms that had soft body parts. They also know more about organisms that lived in environments that favored fossilization. The fossil record is incomplete because most organisms never became fossils. The fossil record also reveals a history of environmental change. Marine fossils help scientists reconstruct ancient coastlines and the deepening and shallowing of ancient seas. Fossils of plants and land animals help scientists reconstruct past climates by telling whether the climate was cooler or wetter than it currently is.

Sometimes, scientists find fossils where they don’t expect to find them. For example, would you expect to find marine fossils on the top of a mountain? The presence of marine fossils means that the rocks of mountaintops formed in a totally different environment – the bottom of the ocean! This scientist found marine fossils on mountaintops in Canada. The fossil of Marrella, shown in the picture tells the scientist that these rocks were pushed up from below sea level millions of years ago.

History of Changing Organisms p. 78 Scientists know that older rock layers contain organisms that are often different from the organisms found in younger rock layers. The fossil record is incomplete and doesn’t provide paleontologists with a continuous record of change. They look for similarities between fossils and their closest living relatives and try to fill in the blanks on the fossil record.

Using Fossils to Date Rocks p. 78 Scientists have found that particular types of fossils appear only in certain layers of rock. By dating the rock layers above and below these fossils, scientists can determine the time span in which the organisms that formed the fossils lived. These types of fossils are called index fossils. Index fossils are fossils of organisms that lived during a relatively short, well-defined geologic time span. To be considered an index fossil, a fossil must be found in rock layers throughout the world. One example is an index fossil is the fossil of a group of ammonites called tropites. They were similar to a modern squid. They lived in a coiled shell about 230 – 208 million years ago.

Trilobites p. 79 Fossils of a group of trilobites called Phacops are another example of an index fossil. Trilobites are extinct. Their closest living relative is the horseshoe crab. Phacops lived approximately 400 million years ago. Scientists assume that any rock layer containing a fossil of the trilobite Phacops is about 400 million years old.

How Fossils Are Formed

Quiz Time! 1. How are a cast and a mold different? A mold is a cavity in rock where a plant or an animal was buried. A cast is an object created when sediment fills a mold and becomes rock. 2. Why do the frigid temperatures of Siberia and the sticky asphalt of the La Brea tar pits preserve fossils well? Both environments slow down the decay of an organism and help preserve it. 3. _________ can be used to establish the age of rock layers. Index fossils

4. Fossils are most often preserved in A. ice B. amber C. asphalt D. rock D 5. Name three types of trace fossils. tracks, burrows, and coprolites Science Humor… What do you call a fossil that never wants to work? Lazy bones!!