Lesson 1 Fossil Evidence

Fossils When organisms die, their soft tissues are either eaten by other animals or they break down (decay). Hard remains such as bones, teeth, shells, etc…can be preserved as fossils. FOSSILS are preserved remains or evidence of once-living organisms.

Fossil Record All of the fossils ever discovered on Earth make up the FOSSIL RECORD. The fossil record can help scientists figure out what species that no longer exist looked like when the organisms were alive.

Fossil Formation MINERALIZATION is when the remains of an organism are buried in mud or sand under a body of water. Over time, the molecules that once formed the remains are replaced by minerals in the water. Most mineralized fossils are formed of shell or bone, but wood can also become a mineralized fossil.

Fossil Formation CARBONIZATION occurs when a dead organism is compressed over time and pressure drives off the organism’s liquids and gases.

Fossil Formation Sometimes organisms can make an impression in sand or mud. MOLD fossils form an impression in rock. They can only show external features. CAST fossils form when impressions are filled with sediments that harden to become rock.

Fossil Formation Sometimes, organisms can leave evidence of their movement or behavior, such as tracks in the mud. These types of fossils are called TRACE FOSSILS.

Fossil Formation In rare cases, the original tissues of an organism can be preserved. Mammoths frozen in ice Insects trapped in tree sap

Determining a Fossil’s Age Scientists cannot date most fossils directly. Instead they usually find the age of the rocks around the fossils.

Determining a Fossil’s Age The LAW OF SUPERPOSITION - in an undisturbed rock formation, older layers of rock are below younger layers of rock. RELATIVE-AGE DATING uses the law of superposition to help scientists determine the relative order species have appeared on Earth over time.

Determining a Fossil’s Age ABSOLUTE-AGE DATING is more precise than relative-age dating and involves unstable isotopes that decay to become stable isotopes over time.

Fossils Over Time The GEOLOGIC TIME SCALE is a chart that divides Earth’s history into different time units. Four eons: Hadean, Archean, Proterozoic, Phanerozoic Phanerozoic sub-divided into three eras: Paleozoic, Mesozoic, Cenozoic Neither eons nor eras are equal in length (they vary). When scientists began developing the geologic time scale, they did not have absolute dating, so they marked time boundaries using fossils.

Extinctions Extinctions occur when the last individual organism of a species dies. When many species die off within a few million years of each other, it is called a MASS EXTINCTION. The fossil record shows evidence of five mass extinctions during the Phanerozoic eon.

Extinctions Extinctions can occur if the environment changes quickly; for example, as a result of a meteorite impact. Extinctions can also occur if the environment changes gradually; for example, as a result of the formation of mountain ranges.

Extinctions The fossil record contains clear evidence of the extinction of species over time as well as evidence of the appearance of many new species.

Change Over Time The fossil record also contains evidence that species change over time in what appears to be a sequence For example, horses descended from organisms from which only fossils remain today. BIOLOGICAL EVOLUTION is the change in populations of related organisms over large amounts of time.