Fossil Records

A fossil is evidence of life existing in the past, and can include body fossils and trace fossils. Most organisms are not fossilized because fossilization requires specific conditions to occur. Body fossils are typically hard tissues (teeth, bones, shells, or exoskeletons, plant parts) that were not consumed by other organisms or subject to decay. The odds of becoming a fossil are better for hard-shelled marine species than for small, land-dwelling organisms or organisms that only have soft body tissues. However, fossilization rarely occurs, and few fossil remains are found. Trace fossils include burrows, tracks, footprints, tooth marks, or fecal matter (coprolites), which also require specific, rare conditions for preservation.

The fossilization process is as follows: First, the organism died in or near a body of water; a river or shallow sea was ideal. The body was covered quickly by sediment, and layers of sediment accumulated over time. The sediment came under pressure and formed sedimentary rock, and the remains were mineralized as more time passed. It is possible for fossils to be preserved in igneous rock (rock formed by the cooling and solidification of volcanic ash. or metamorphic rock (rock formed from other rocks by heat and pressure), but the majority of fossil formations are found in sedimentary rock (rock formed through the sediment being deposited and solidifying). Additional forms of fossilization include petrification, in which minerals replace organic matter, thus turning that matter to stone. In some instances, freezing preserves fossil remains in glacial ice, and desiccation fossilizes remains by drying them out in desert conditions. The fossil record (all the fossilized artifacts taken in the context of their placement within Earth's geological strata) represents a biased view of past living organisms because aquatic species are more heavily represented than land species. Species with hard shells or bones are more likely to become fossils than other organisms, and fossils held in exposed rock are more likely to be found than fossils concealed by many deep rock layers. The fossil record has similar gaps, as there are significant numbers of species not represented. The fossil record contains about 250,000 species, but that number represents somewhere between 1% and 5% of all organisms that have lived on Earth. The bulk of fossils comes from more recent periods, mainly because Earth is geologically active and constantly changing. Fossilized remains from earlier periods may have undergone geological upheaval, with new metamorphic rock formed from this activity. In that new rock, most if not all previously fossilized remains would be destroyed. According to the law of fossil succession, fossils fall into a hierarchy in which younger fossils succeed older fossils vertically in rock strata. Fossil remains of ammonites, an extinct group of marine mollusks, found in multiple layers of rock show evolutionary changes from one period to the next. Ammonites became extinct during a mass extinction event (the loss of many species due to global conditions) that took place 65 million years ago. The extinction of many species leaves a gap for new species to evolve and thrive, a macroevolutionary event.

There are two ways of dating fossil remains: relative dating and absolute dating. Relative dating involves establishing whether a fossil is older or younger than a reference, which might be a rock layer or an easily dated fossil. For example, ammonites are considered index fossils because they lived only between 245 million years ago and 65 million years ago. Rock layers found with ammonite fossils must be within that same age range. Other fossils found in the same rock layer also lived within that time frame. Further, if rock in other places contains ammonites, that rock is also the same relative age. In general, if layers of rock are undisturbed, rock in lower layers is older than rock in upper layers, so the layer in which a fossil is found also helps determine the fossil's relative age. Relative dating can help form a chronological record of when species first appeared. For example, vertebrate fish appear in a lower rock layer. Amphibians appear in a higher rock layer. If the rock has remained intact, the first vertebrate fish lived longer ago than the first amphibians.

Absolute dating is more exact, but dates in geologic time are spans of time, not specific dates. Determining absolute age is done with radiometric dating, the scientific process of using the half-life of radioactive elements to establish the absolute age of a fossil. Rocks contain trace amounts of radioactive elements that decay at specific rates, starting from when the rock was formed. Radiometric dating is like a clock that measures thousands or millions of years, depending on the type of isotope being used for analysis. Carbon isotopes allow for accurate dating of remains up to 40,000 years old. Older fossils require the use of isotopes with long half-life periods, such as uranium-253 and potassium-40.

Radioactive material decays from the parent isotope to the daughter isotope. Its rate of decay is measured by the amount of time it takes for a parent isotope to decay so that the isotopes measured are 50% parent and 50% daughter, called a half-life. Decay continues until, after six half-life periods, the parent isotope is completely replaced by the daughter isotope. As an example: A bone is dated using carbon-14. The amount of carbon-14 (parent) present compared to the amount of nitrogen-14 (daughter) present is 25% carbon-14 and 75% nitrogen-14. The half-life of carbon-14 is 5,730 years. The carbon isotope has decayed through two half-life periods, or 11,460 years.

Using isotopes for dating is not always successful. Mollusks get the carbon in their shells from aged carbonate. Their shells form with decayed carbon-14, so dating the half-life of carbon-14 in such a shell gives a false result. Plants get carbon from carbon dioxide while they are living; animals that eat plants get carbon that has already started the decay process. Sediments that surround a fossil during formation may be from rock formed at different ages, so determining the fossil's absolute age is more complex and less accurate.

The five great mass extinctions—Ordovician, Devonian, Permian, Triassic, and Cretaceous—resulted from different causes, but the results were the same: the loss of the majority of existing living species.

The Ordovician mass extinction, which occurred 439 million years ago, may have been caused by rapid plate tectonic shifting combined with an ice age that reduced sea levels. Up to 60% of marine genera died in this extinction, and up to 86% of all species became extinct.

The Devonian extinction may have been a single catastrophic event, such as an asteroid striking Earth, or it may have been the result of another ice age. In this extinction, which occurred 367 million years ago, ocean and sea life suffered the greatest losses, but up to 80% of species disappeared.

The Permian episode, which occurred 245 million years ago, had the greatest impact on living organisms, destroying 95% of all marine species as well as trilobites, large land insects, and seed ferns; 96% of all species became extinct during this event. It was most likely caused by several smaller events that resulted in a complete collapse of the environment.

The Triassic has been a frustrating mystery to scientists because there appears to be no single cause and extinctions cascaded over a period of 10,000 years. This extinction occurred 208 million years ago, and 76% of species became extinct.

The most recent, the Cretaceous extinction, brought about the end of the dinosaurs as well as many reptiles, birds, and mammals. It is possible that the Cretaceous extinction was caused by an asteroid striking near Mexico's Yucatan peninsula and a simultaneous climate cooling period, possibly due to mass volcanic activity in Siberia. This extinction, which occurred 65 million years ago, resulted in 76% of species becoming extinct.

Current extinction rates are such that scientists are concerned that Earth may soon experience a sixth mass extinction. The combination of human factors and global climate change may produce a cascading event in which extinctions continue to occur exponentially. How this will, ultimately, impact humans and the Earth is unclear.