Fossil succession
Faunal succession refers to the order in which different groups of organisms appear and disappear in the fossil record. Fossils preserved in distinct geological layers reveal a consistent pattern of species distribution over time, helping scientists to reconstruct the evolutionary timeline of life on Earth. For example, trilobites first appear in early Cambrian strata, marking one of the earliest major evolutionary radiations of life. In contrast, dinosaur fossils are found in Mesozoic rock layers, while mammal fossils become more prominent rocks of the Cenozoic era.
Use this page to revise the following concepts within fossil succession:
Fossils
Fossils are the preserved remains, traces, or imprints of organisms that lived in the past. They occur in various forms, each providing unique insights into ancient life. Body fossils (as shown in the image below left), are the physical remains of organisms like bones, shells, or teeth. Trace fossils (below right) are indirect evidence of an organism’s existence, such as footprints, burrows, or nests. Petrified fossils are created when organic material is gradually replaced by minerals, turning the organism remains into stone. Amber fossils result from organisms becoming trapped in tree resin that eventually hardens into amber over time and preserves fine details, often including insects or small plants.
The figure below details the slow process of fossil creation.
Relative Dating and Index Fossils
Index fossils are fossils of species that existed for a relatively short period of time but were widely distributed across different geographic regions. These attributes allow the species to serve as markers that assist geologists and paleontologists to date the age of rock layers and other fossils within them. This method is an example of relative dating, a technique that estimates the age of fossils based on their position within sedimentary strata. In general, the deeper a fossil is found within the layers, the older it is considered to be. However, geological processes such as erosion and the movement of tectonic plates can affect the positioning of rock layers. Erosion can remove or distort layers, while tectonic forces may cause layers to shift or fold, complicating the accurate interpretation of strata age. Despite these challenges, relative dating remains a powerful tool, especially when combined with other dating techniques.
Transitional Fossils
Transitional fossils are those that exhibit traits common to both ancestral and derived groups. These fossils provide evidence for evolutionary change over time as they illustrate the intermediate stages in the evolution of major biological groups. Two famous examples are the Tiktaalik, a fish-tetrapod transitional fossil that displays features of both aquatic fish and early land-dwelling tetrapods, and Archaeopteryx, a feathered dinosaur that shows characteristics of both older reptiles and birds.
Tiktaalik is a well-known transitional fossil that lived approximately 375 million years ago and marks a pivotal stage in the evolution from aquatic to terrestrial life.It possessed a combination of fish-like traits, such as scales and fins, and tetrapod-like characteristics, including a flat skull, a mobile neck, and limb-like fins capable of supporting its body in shallow water or on land. These adaptations illustrate a key evolutionary step toward land-dwelling vertebrates.
Image source: Eduard Solà, CC BY-SA 3.0, via Wikimedia Commons
Archaeopteryx is an often cited transitional fossil, as it exhibits traits of both the evolutionarily older theropod dinosaurs and modern birds. It had feathers and wings like birds but also retained characteristics like teeth, a long bony tail, and claws on its wings, typical of theropod dinosaurs. Archaeopteryx provides key evidence for the evolution of birds from theropod dinosaurs.
Source: H. Raab (User: Vesta), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Absolute Dating of Fossils
Sometimes referred to as ‘numerical dating’, absolute dating determines the actual age of a fossil in years by measuring the decay of radioactive isotopes. This method provides more precise numerical estimates, unlike relative dating, which only establishes the order of events.
One of the most commonly used techniques is radiometric dating, which relies on the known rates of radioactive decay of certain isotopes. For example, Carbon-14 dating is effective for dating relatively recent fossils (up to about 50,000 years old) by measuring the decay of carbon-14, an isotope absorbed by living organisms. Potassium-Argon dating is used for much older fossils (millions of years old) and works by measuring the ratio of potassium-40 to argon-40 in volcanic rocks surrounding fossils. Another radiometric dating method is Uranium-lead dating, which measures the decay of uranium isotopes into stable lead isotopes in minerals. This method is particularly useful for dating ancient rocks and fossils that are over a million years old.
In radiometric dating, scientists correlate the relative proportion of the radioactive isotope in the sample with the number of half lives of the isotope required to achieve that proportion. The different time periods that each method is able to date varies with the half-life of the different isotope used.
| Radiometric dating method | Half Life |
|---|---|
| Carbon-14 dating | 5730 years |
| Potassium-Argon dating | 1.25 billion years |
| Uranium-Lead dating | 4.5 billion years |
When taken together, relative and absolute dating methods allow scientists to construct a more accurate and comprehensive timeline of Earth's history and the evolution of life.
Image source: UC Museum of Paleontology, Understanding Evolution, retrieved Dec 2024.
