4.6 Fossils Part 1

What’s on this rock?

This rock contains a fossilized stalked crinoid. Scientists study fossils of plants, animals, and other organisms in order to better understand what life was like on Earth many years ago and how it has changed over time. Fossils are important evidence for the theory of evolution.

The Fossil Record

Fossils are preserved remains of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed. Paleontologists are scientists who study fossils to learn about life in the past. Paleontologists compare the features of species from different periods in history. With this information, they try to understand how species have evolved over millions of years (Figure below).

Evolution of the horse. Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones in this process. Notice the 57 million year evolution of the horse leg bones and teeth. Especially obvious is the transformation of the leg bones from having four distinct digits to that of today’s horse.

Until recently, fossils were the main source of evidence for evolution (Figurebelow). Through studying fossils, we now know that today’s organisms look much different in many cases than those that were alive in the past. Scientists have also shown that organisms were spread out differently across the planet. Earthquakes, volcanoes, shifting seas, and other movements of the continents have all affected where organisms live and how they adapted to their changing environments.

About 40 to 60 million years ago these insects were trapped in a gooey substance, called resin, that comes from trees. The fossils in the movie Jurassic Park were trapped in resin.

Rock Layers and the Age of Fossils

There are many layers of rock in the Earth’s surface. Newer layers form on top of the older layers. Therefore, you can tell how old a fossil is by observing in which layer of rock it was found. The fossils and the order in which fossils appear is called the fossil record. The fossil record provides evidence for when organisms lived on Earth, how species evolved, and how some species have gone extinct. Geologists use a method calledradiometric dating to determine the exact age of rocks and fossils in each layer of rock. This technique measures how much of the radioactive materials in each rock layer have broken down (Figure below).

This device, called a spectrophotometer, can be used to measure the level of radioactive decay of certain elements in rocks and fossils to determine their age.

Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion years old. The oldest fossils are between 3 and 4 billion years old. Remember that during Darwin’s time, people believed the earth was just about 6,000 years old. The fossil record proves that Earth is much older than people once thought.


Did you know that plant and animal fossils have been found at Valley Forge? It’s true and this is one of the most significant deposits in the National Park System.

Fossils from Giant Sloths found at Port Kennedy in Valley Forge NHP

Port Kennedy Cave, located in what is now Valley Forge NHP, produced one of the most significant assemblages of Pleistocene fossils in North America. 14 plants and 48 animals are represented, including wolverine, Wheatley’s ground sloth, long-nosed peccary, Hay’s tapir, and lesser short-faced bear. More…


On the trail of an important Ice Age fossil deposit

Rediscovering the Port Kennedy Cave (Middle Peistocene), Valley Forge National Historical Park, Montgomery County, PennsylvaniaBy Edward B. Daeschler, Matthew C. Lamanna, and Margaret Carfioli

During the late 1800s a treasure trove of Ice Age fossils was uncovered by limestone quarrying operations at Valley Forge, Pennsylvania. The fossil site, known as Bone Cave or Port Kennedy Cave, was formed in the Irvingtonian stage of the Pleistocene epoch (approximately 750,000 years ago). In a series of important early scientific papers, C. M. Wheatley (1871), E. D. Cope (1871, 1895, 1896, 1899), H. C. Mercer (1895, 1899), and others described plant, insect, turtle, snake, bird, and numerous mammal fossils from the deposit, including several that were new to science. The Port Kennedy Cave is now recognized as one of the most significant Middle Pleistocene vertebrate fossil localities in North America (Kurten and Anderson 1980, Daeschler et al. 1993).

The Port Kennedy Cave was actually a vertical solution cavity in Paleozoic limestone that was briefly opened to the surface, forming a sinkhole in the forested Pleistocene landscape. Plant and rock debris accumulated in the shaft along with the remains of hundreds of animals that were trapped or carried to the edges of this sinkhole by predators (Daeschler 1996). Fauna from the site include giant ground sloth, mastodon, tapir, peccary, skunk, short-faced bear, saber-toothed cat, and many other taxa (fig. 1). Site excavations byWheatley, Cope, and Mercer in the early 1870s and mid-1890s resulted in the collection of more than 1,200 fossils, which today are curated in the vertebrate paleontology and paleobotany collections at the Academy of Natural Sciences in Philadelphia.

Although the fossiliferous deposit was never exhausted (see fig. 2), excavation of the Port Kennedy Cave ceased in 1896 because of groundwater inrush that prevented further work (Mercer 1899, Witte 1957). Then in the early 20th century, the limestone quarry containing the fossil site was filled with asbestos-containing waste materials from the nearby Ehret Magnesia Manufacturing Company. The filling of the quarry and the passage of time have combined to obscure the exact location of this important fossil site, including which of at least three possible quarries holds the site.

At the behest of the Valley Forge National Historical Park, which now encompasses the land containing the fossil deposit, we conducted a study to determine, with as much precision as possible, the location of the Port Kennedy Cave. Rediscovery of this lost site is of great interest to Valley Forge National Historical Park and the scientific community.

SOURCE: http://www.nature.nps.gov/parkscience/index.cfm?ArticleID=52


  • fossil: Preserved part of animals, plants, and other organisms from the distant past.
  • fossil record: Complete set of fossils that has been discovered, and the order in which the fossils appear.
  • paleontologist: Scientist who studies fossils to learn about life in the past.
  • radiometric dating: Procedure used to determine the age of rocks or fossils by measuring how much of the radioactive materials in each sample were broken down.


  • Fossils, or preserved parts of organisms from the distant past, have shown that species change over time.
  • Radiometric dating can be used to determine the age of fossils by measuring the how much of the radioactive materials in each rock layer have broken down.


Use the resources below to answer the questions that follow.

  1. Why is it believed the first animals left no fossilized bones?
  2. What do paleontologists look for when they search for evidence of early organisms?
  3. How old are the geologic deposits Dr. Hagadorn is searching? How old is the evidence he has found for the first mobile organisms? Looking at the fossils can you think of another way these patterns could be formed?
  4. How does the diversity of organisms in the fossil record change after the first evidence for mobility appears?
  1. How old are the fossils from the Burgess Shale? What geologic period are these fossils from?
  2. What is a “tetrapod”? What question do paleontologists hope they can answer by studying them?
  3. In what geologic period are tetrapods first found in the fossil record?
  4. What is special about “Boris”?
  5. How old are the fossil tracks from Ireland? To what geologic period does this correspond?
  1. What evidence is there that modern whales once had hindlimbs?
  2. Pakicetus and Rodhocetus are considered to be ancestors of modern whales. Scientists still argue about how aquatic Pakicetus was, butRodhocetusis considered to be a largely aquatic animal.
    1. Where is the nostril located on Pakicetus?
    2. Where is the nostril located on Rodhocetus?
    3. What is the relationship in time between these two species?
  3. What modern animal is most closely related to modern whales? What is the evidence? In what kind of environment does this modern relation live?


  1. What is a fossil? Give examples.
  2. What has the fossil record revealed about life on Earth?
  3. How does radioactive dating work?

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