Fossil Record

Fossil Record Definition

A fossil record is a group of fossils which has been analyzed and arranged chronologically and in taxonomic order. Fossils are created when organisms die, are incased in dirt and rock, and are slowly replaced by minerals over time. What is left is a mineral impression of an animal which once existed. Many fields and specialties are utilized to categorize and arrange these fossils, including comparative anatomy, radiometric dating, and DNA analysis. Using the data from the fossil record, scientist try to recreate phylogenies, or trees describing the relationships between animals, both alive and extinct. The fossil record helps inform how different groups of animals are related through evolution.

Fossil Record Examples

Humans

The human fossil record is perhaps one of the best documented, due in part to the historical contention that has surrounded the debate of evolution. Nearly every “missing link” in the chain has been unearthed, revealing a solid chain of fossils from modern humans to our earliest ape-like ancestors. The fossils can be differentiated by their distinct features, and can be dated using radioactive isotopes for very accurate dating. The oldest fossils of members of the Homo genus were found to be around 1.5 million years old. These belonged to Homo ergaster. It is believed that Homo ergaster spread from Africa and diverged into the species seen below.

Human evolution chart

Homo erectus and Homo neanderthalensis both have a distinct fossil record, and it is likely that they competed with modern Homo sapiens. Genetic analysis have confirmed that modern human genomes contain traces of genes from the Neanderthals, suggesting that the two species interbred at some point in the past. The fossil record of humans can be traced back even further, all the way to very ape-like ancestors which still climbed trees.

Whales

Surprisingly enough, the fossil records of many animals have been assembled in near completion as well. The fossil record of whales, for instance, has a set of several well-defined members that lead inevitably to modern whales. Fossils have been found of coyote-sized semi-aquatic predators which were thought to live amphibious life-styles near the shore. Later fossils reveal a much more aquatic predatory animal, still resembling a dog, but with a much larger tail and a head adapted to hunting in the water. Around 35 million years ago, an animal existed which was almost fully aquatic and had lost its hind limbs. 5 million years later, the fossils of what appear to be modern whales start to appear.

The Oldest Fossils

When talking about the fossil record of life on Earth, the record goes back much further. The oldest known rocks that have been analyzed are around 3.8 billion years old. Tracing the minerals and hydrocarbons present in certain rocks has led to the conclusion that some form of single-celled life was present around 2.7 billion years ago. Fossil impression of single celled organisms, thought to be some early ancestor of plant and animal cells, can be found around 1.6 billion years ago in the fossil record.

However, the first multicellular life that left abundant fossils is usually dated to around 1.2 billion years ago, with a large expansion sometime around 600 million years ago. This period is dubbed the Precambrian period, and marks the start of highly complex life. Shortly after, the Cambrian period starts with an enormous radiation of marine life worldwide. The Cambrian period includes the rise of vertebrates with internal skeletons and of crustaceans and arthropods with external skeletons. This period hold the “Cambrian explosion”, one of the largest expansions of species found in the fossil record. Terrestrial animals did not appear until much later, around 415 million years ago in the Silurian period. By comparison, humans diverged from our ape relatives around 2 million years ago, during the Tertiary period. These divisions can be seen on the following chart.

Geologic time scale

Quiz

1. A new fossil is found, which appears to be some sort of human ancestor. How will scientists know how to place it into a human phylogeny?
A. DNA analysis
B. Radiometric Dating
C. Comparative Anatomy
D. All of the Above

Answer to Question #1
D is correct. To be thorough, scientists always use a number of methods to determine where and how a new specimen fits in with other fossils and living members of a taxonomic group. Together, the many facets of evidence can support the positioning of the fossil in relation to the other members of the group.

2. How do we know how old the oldest fossils are?
A. Radiometric Dating
B. Guessing
C. Historical References

Answer to Question #2
A is correct. By measuring the decay of radioactive isotopes, we can get an accurate measure of time on a large scale. Radioactive substance break down at a given rate, and if we know that rate we can determine when and how much of a substance was incorporated into a fossil, telling us the exact age of that fossil. Historical references only go back about 5,000 years, maximum. With radiometric dating, we can date fossils that were deposited over 2 billion years ago.

3. Why is the fossil record for vertebrates so much better than the fossil record for jellyfish?
A. Jellyfish have been around longer
B. Vertebrates preserve better
C. There have been more vertebrates

Answer to Question #3
B is correct. With no hard tissues, it is very unlikely that a jellyfish will remain intact long enough after death to become a fossil. The rise of vertebrates is much easier to track in the fossil record simply due to the fact that the bone of the vertebrate was much easier to fossilize than soft tissues.

References

  • Brusca, R. C., & Brusca, G. J. (2003). Invertebrates. Sunderland, MA: Sinauer Associates, Inc.
  • Pough, F. H., Janis, C. M., & Heiser, J. B. (2009). Vertebrate Life. Boston: Pearson Benjamin Cummings.
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