Main Points
- There were three major groups of animals that have changed through time: the Cambrian fauna, Paleozoic Fauna, and Modern Fauna.
- Background extinction is natural and often good for ecosystem dynamics.
- Mass extinctions are devastating and often cause total ecosystem restructuring events.
- The organisms on Earth have undergone many extinctions through time but five are most notable.
- There are many causes of extinction (e.g., sea level change, anoxia, plate tectonics, etc.) but often, these large events are a combination of many unfortunate changes on Earth.
- Humans are accelerating the extinction of many species.
The death of all individuals within a species or larger group so that the lineage is no longer in existence is known as extinction. Marine organisms are easier to preserve as they are underwater and they simply need to be buried, whereas animals that die on land are oftentimes hard to preserve (think of a deer dead in the woods; it’s not being covered by sediment that would preserve it in the rock record). That means that studying extinction in the fossil record is often easier in marine communities, and thus many interpretations about how long an extinction lasted, what types of species were impacted, etc. come from studying marine fossils. Large amounts of species-level data, which is necessary to study extinction patterns in the fossil record, is available, but often hard to study all at once because the information is stored in museums and collections, some of which do not have their fossil information available online. Therefore, data used to make interpretations of extinction events in the rock record are often collected at the genus and family level (Raup and Sepkoski, 1982). What exactly is a family? It is an arbitrary grouping of taxa that does not particularly hold meaning and is based on similarities of the organisms. Always remember that species are the unit upon which evolution acts.
Identification of rates at which extinction proceeds (how long an extinction event lasted) is an interpretation of the data that are gathered and used in an analysis. As more scientists work together to fix the issue of easier access to museum collection data at the species level, the data will become increasingly better! All of this is important to keep in mind while assessing results.
So, if these data are so difficult to get at… how do we actually try to address these questions about extinctions, such as how long they lasted, who went extinct, and what caused the extinction event.? Well, one way that has been used for many decades is to record the occurrences of families or genera and plot them against time. This is the famous (in paleontology) Sepkoski Curve. It documents the changes in marine invertebrate animals through the entire Phanerozoic. For the most part, the very severe extinction events have remained relatively unchanged in timing and magnitude (how many families went extinct at a given extinction event) Since Sepkoski first published in 1981. There are three main faunas that paleontologists often separate out. By faunas, we simply mean the grouping of animals that share some characteristics (such as they were alive during the same amount of time or lived in the same environment). These groups change through time as seen in the rock record. The Sepkoski Curve figured here, shows the three major faunas that have dominated the geologic record. These three faunas, all based on marine organisms, are discussed in further detail below:
The Cambrian Fauna consists of many trilobites, brachiopods, monoplacophora, echinoderms, and other small shelled animals. Generally speaking, this group of animals was not very diverse (not many different kinds of animals). Most of these animals were low-level suspension feeders (passively feeding from the water on or near the sea floor). There were quite a few animals, like trilobites, that were also scavengers.
The Paleozoic Fauna includes animals like corals, sponges, bryozoans, cephalopods, brachiopods, crinoids, and even reefs made of clams. This group of animals developed new ways to feed as active filter feeders (this means the animal creates a current to bring food into its mouth).
The Modern Fauna contains many animals we can see today! This includes corals, mollusks, and lots of foraminifera. This group of animals has been around for most of the Phanerozoic but in the background (they weren’t as abundant in earlier time of Earth’s history). They never really expanded until the other faunas really begin to decrease. Go exploring outside to find modern animals!!!
Types of extinction
There are two main types of extinction: (1) background extinction and (2) mass extinction. Background extinction refers to extinction that is normal and ongoing occurring at a relatively stable rate throughout geologic time. This type of extinction occurs from environmental or ecological factors including changes in climate, disease, competition with other animals, or loss of their habitat.
Mass extinctions are events with substantial losses of life. There have been five documented mass extinctions in the rock record, some being more severe than others. It has been suggested that we are currently in the midst of another mass extinction with Anthropogenic Climate Change as the driving mechanism. When scientists talk about extinction, are referring to birth and death rates. During times of mass extinction we have increased death, or extinction, rates. It is also, however, possible to have suppressed speciation (birth) rates.
Some of these events are called extinctions while others are just called crises. This is very intentional and the details here are important. We can calculate various statistics by understanding the amount of organisms (abundance of the same organism or the amount of different organisms) in an ecosystem and how they increase or decrease over time. Extinction occurs when animals are dying more quickly than they are able to reproduce, meaning the death rate outpaces the birth rate. If there are large ecosystem restructuring events (e.g., changing sea level, migration, or extinction causing large open space available) the birth rate can be modified to accommodate those severe changes.
Here is an example: a new clam species entered an environment and, rather then giving up and dying out the old clam population simply shared their space with the new clams. This probably means fewer of the old clams but not a mass dying of the old clams. In some cases the new clams eat all the food and the old clams do die but it is localized to one area rather than being world wide. Large scale movements of animals into new environments can leave a confusing fossil record and it appears that these large ecosystem restructuring events look very similar to extinction events.
The Big Five
The “Big Five” extinction events were first interpreted from Sepkoski’s family level curve. The identification of these events has remained stable as the data precision has increased. The generic level curves looks very similar with more peaks but the Big Five always stand apart from the rest. Each of these events are outlined below:
End Ordovician Crisis
Considered to be the second largest ‘extinction’ event, after the Permian extinction. This event occurred approximately 450-440 million years ago. This event, as with the Devonian crisis interval appears to not be a time of increased extinction but rather decreased speciation (or origination). Because there are fewer new species appearing, it leaves a record in the rock that implies loss or devastation. Another factor that has hindered the understanding of this event is the lack of rock in the Upper Ordovician and Early Silurian. The rock record is very sparse, and when there is little or limited data it becomes difficult to address and answer these questions.
End Devonian Crisis
This crisis was restricted to the marine realm and was not one, but several pulses of extinction over quite a long interval of time rather than one single event. The reef-builders (corals and stromatoporoids) were greatly affected by this event. Ammonites, trilobites, and jawed vertebrates (including our tetrapod ancestors) were also hit hard during this event. T The Devonian is a very important time in Earth’s history as this time marks the large migration from water to land. The earliest forests are found in rocks from the Devonian!
End Permian Mass Extinction
This event happened approximately 251 million years ago and is generally called “The Great Dying”. This event marked the end of the Paleozoic Era and the beginning of the Mesozoic. It was Earth’s most severe extinction event, and the only extinction event to drastically affect insects. Because the loss was so great, the rebound of life took significantly longer than the other events. It is estimate that as much as 96% of all marine species went extinct – but remember: attempting to quantify this based on available data that are sparse. But it is agreed upon that this extinction event was incredibly severe and set the stage for the Mesozoic communities.
Specifically, this extinction affected organisms that make their skeletons from CaCO3 (calcite and aragonite). Many of these organisms use carbon dioxide in the water to produce their skeletons and drastic changes in these levels have dramatic results on the animals. The animals that were decimated in this event were primarily sessile (stationary, attached to the sea floor) organisms and after the event there is a rise of mobile and more complexly active animals.
End Triassic Mass Extinction
Although this extinction is generally considered one of the Big Five, it has received less attention than the others, primarily due to the limited fossil bearing rocks – both in marine and terrestrial realms (Hallam, 2002). Many of the large marine groups, such as brachiopods, corals, and cephalopods, suffered large losses but did not go extinct as a result of this event. This extinction event is often considered the time that dinosaurs were able to rise as the dominant predators in ecosystems. Prior to this extinction synapsids were the top predator and after this event the mammals were much smaller in size, resulting in an open role to be filled: apex predator.
End Cretaceous Mass Extinction
This is most well known for the extinction of the non-avian dinosaurs at approximately 65.5 million years ago. This marks the transition between the Mesozoic (‘middle life’) to the Cenozoic (‘new life’). In addition to the loss of non-avian dinosaurs the marine realm was greatly affected. Many groups of mollusks (e.g., clams, snails, octopods) suffered heavy losses. The group known as Cephalopoda (‘head-foot’) lost many major groups – these are extinct relatives of squids, octopods, and cuttlefish.
What causes extinction?
There are a variety of natural causes of extinction that have happened once or many times in the history of life. Below we briefly outline some of the most common causes. Although each of these natural phenomenon can occur at separate times, often the extinction events are caused by multiple events.
(1) Rapid changes in sea level can cause extreme stresses on marine life. Imagine you are a clam, living just off of a beach environment in a wonderful shallow shelf environment. If sea level drops quickly you are plunged into a beach environment, the energy of a beach is more intense than that of your previous environment. You have difficulty adjusting and die. Conversely, imagine that you are back in your shallow shelf environment. If sea level rises you will again be transported into another environment. What if there is less food because you are deeper now? How do you eat? Again, you die.
Something that is closely tied to sea level is glaciation events, which can lead to global cooling. When the planet begins to cool, water is trapped on land as glaciers. This removal of water from the oceans leads to a drop in sea level. The cooling of the planet can also lead to the migration or movement of cooler water animals to more tropical regions, restricting the space available for the other animals.
(2) The way our oceans circulate today allows for a lot of oxygen to be transported around the water. But this has not always been the case. Ocean anoxia occurs when the cycling of water masses slows and is no longer bringing lots of oxygen to different parts of the ocean. Anoxia simply means little to no oxygen and we know that all animals need oxygen to perform normal daily tasks.
(3) Volcanic activity can be easily traced through time by ash beds and large amounts of basalt. When volcanoes erupt some of what they spew out are really awful chemicals into the atmosphere. These chemicals contribute to the warming of the atmosphere. In addition these harsh chemicals, ash is also thrown into the atmosphere. When the eruptions are massive (unlike anything seen in human history) the amount of ash can block out the sun. This can cause cooling and a lot of death. If there is no sun plants cannot undergo photosynthesis and since plants are at the very base of our food chain there would be extreme effects all across life.
(4) Something that happens more slowly is tectonic activity. The Earth’s crust is divided into many plates that move toward, away from, or slide past each other. This movement is terribly slow but over long amounts of time can drastically change the position of the continents. This movement or activity can connect of disconnect landmasses causing various exchanges with animals. When animals begin to move about there can be issues with competition for resources and not all of life can win so usually some animals go extinct.
Watch this video on how the Earth’s plates have moved through time:
(5) Extraterrestrial impacts are often included in extinction hypotheses. This means that objects from space hit the planet causing the extinction of species. If the space rock is large enough or the impact was great enough, scientists can trace the evidence in the rock record. There are certain minerals that do not occur on Earth but can be brought by these impact events. It is often difficult to assess any global impact of these effects but there would definitely have been regional or local impacts.
Are we in another mass extinction?
These events devastated the biotic and abiotic realms of the Earth. By carefully examining the ramifications of various perturbations to our global system we may better understand the nature of how these events happen and how life rebounds. If you noticed, these events usually mark the end of one time period and the beginning of another. Think back to relative age dating – this boundary was clearly identifiable by the types of animals that existed in the lower and upper units of rock. These easy to identify boundaries were later dated using more analytical techniques.
There is much discussion about if we are currently undergoing the sixth mass extinction. This can be confusing if you are not used to thinking in terms of deep time. A quick Google search results in a variety of results suggesting that we are currently undergoing a mass extinction event. But as scientists we must go back to the data, how do we figure out a way to compare the modern data to the ancient data? Is it even comparable? These are very difficult questions to wrap our minds around let alone attempt to solve!
This is a difficult topic to address – even if we are in the sixth great biotic crisis… will it be recorded in the fossil record? Remember what goes in to making a fossil: very unique circumstances provide the best opportunities to become a fossil. As humans, we are constantly modifying the Earth’s landscape and are likely overprinting and in some cases removing future fossil evidence (Plotnick et al., 2016).
For now, let us agree to understand that the way we, as humans, are manipulating the Earth is a detriment to the biosphere. Plants and animals (including ourselves) will continue to suffer from extreme changes that have been accelerated by human activity. Be aware of your impact and together we can make a difference!
Click here to learn more about the extinction crisis and what animals are at risk. Click here to learn how biodiversity loss directly affects you. Click here to read more about how many species we are actually losing.
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