Modified by Shannon Peters from Sepkoski (2002)

Mass extinctions are brief moments in Earth’s history where there is evidence of significant loss of biodiversity. During these times we see dramatic die-offs in species, genera, and even families of taxa, as shown above in the famous Sepkoski curve at each of the red arrows. Although we see the termination of many taxa, there are some taxa that thrive during these ecologically apocalyptic times. These are known as disaster taxa and are useful indicators for marking large perturbations in one or more of the Earth’s spheres, specifically affecting the biosphere. Currently there are five recognized mass extinctions, all of which we will explore below. There are two additional extinction events that we will discuss which may one day be recognized as mass extinction events as well.

The important characteristic of mass extinctions for the purpose of our class is that they are consequences of Earth systems interactions. There are multiple mechanisms available throughout Earth history that have the ability to cause massive and rapid reductions in biotic diversity. Remember that in this class we are focused on how different spheres interact to cause major changes. Go through the events below, and analyze the proposed mechanisms for each extinction event. Make sure to pay attention to what data are available for each event, and why the different data are important. At the end of this module, you will have to submit a 3-5 page paper that uses the information you are learning on this page. 

 

The Big Five (in chronological order)

End-Ordovician (Ordovician-Silurian Extinction/Hirnantian Extinction)
The Late Ordovician is a dynamic time of greenhouse and icehouse conditions. Within this geologically short amount of time, there are two-phased extinction and two-phased cooling events. The combined two-phases of extinction represent the third largest mass extinction of ‘the Big Five’. Many hypotheses have been proposed for this extinction period, but most have now converged on the idea that some sort of climate perturbation must have occurred. This perturbation has been attributed to changes the configuration of paleocontinents, a cosmic gamma ray burst, the presence of a large igneous province, and the radiation of land plants. Below we will explore a few of these ideas. Once you have finished, there is a short quiz before the next mass extinction.

 

 

 

 

Late Devonian Extinction

The Late Devonian extinction represents one of the scariest times in life in all of Earth’s history. During this time, at least 50% of genera died out, and scientists are still trying to explain what caused this rapid die-off. Several hypotheses have been proposed for why this large event occurred which are reviewed below. Of interesting note, is that even though large amounts of biodiversity were lost, some groups of organisms were left almost unaffected. The jawed marine vertebrates, such as sharks and fish, were relatively unaffected during this time. Even more surprising is that agnathans, like lampreys and hagfish, did not fare so well and were dying off before the extinction really took off. Read the paper below for a deeper understanding of this event.

 

For more reading on this interesting time, you can read Late Devonian Times.

Permian Mass Extinction

The Permian Extinction, or Permian-Triassic Extinction, was one of the biggest mysteries in paleontology since we first recognized the the huge drop in species diversity in the paleontological record. Large amounts of research have been conducted in attempts to explain why this event took place, and for the longest time, most mechanisms that were hypothesized still could not explain all of the evidence that we saw. Through various proxies we knew that the climate changed drastically, and we knew that the biosphere crashed. What we didn’t know is why all of this occurred. There were no impact craters, volcanic outgassing alone was not enough, and just a general lack of evidence to explain the magnitude of these events. It wasn’t until closer re-examination of paleogeography and isotopes occurred that a working hypothesis could fully explain the data that were provided. Watch the following video for one perspective on what might have caused this large event. Give yourself time because it’s a little long, but you don’t have to watch the first part. The link below should start you off in the correct place.

credit: Space and Intelligence

The video above outlines a very plausible series of events that could have led to the demise of so many taxa. Note how intricate the interactions between each of the spheres must be for such a hypothesis to work. Although similar, another hypothesis has been proposed that relies even more on paleogeography and the ancient environments associated with it. To reach the additional carbon concentrations in the atmosphere, Greg Retallack and A. Hope Jahren propose an alternative method that doesn’t rely on methane clathrate release. They propose that the additional carbon flux to the atmosphere is more directly related to the mass volcanism outlined in the previous hypothesis. Take a look at the figure below, and see if you can make sense of their hypothesis. For optional reading if you’re interested, the paper has been attached below as well.

 

Retallack and Jahren

 

Triassic-Jurassic Mass Extinction

By now, you should be recognizing that many hypotheses are repeated for similar events. Note the similarities between some aspects of the hypotheses between the Ordovician (Hirnantian) extinction and the Permian extinction. It should be no surprise, then, that the Triassic-Jurassic extinction has also been hypothesized to have been a result of volcanic eruptions. You may already know that the K/T extinction was probably largely the result of a bolide impact, a hypothesis that has also been thrown at the Triassic-Jurassic extinction. Another hypothesis, one that is also being suggested for the loss of biodiversity witnessed during our time, is climate change. This blanket term must have a trigger, but it makes sense that changes in climate could have implications for what lifeforms might survive. Of interesting note during this time is that not all clades or groups of organisms were equally affected. Land plants experienced relatively little loss, while iconic cephalopods and marine reptiles from this time experienced excessive loss. Because of the similarity of this extinction with other extinctions, we will focus on this one less. If you are interested in this extinction specifically, Bristol University developed an online resource that outlines the various hypotheses in more depth, as well as the different taxa that were significantly affected. Their website can be found here.

 

Cretaceous-Paleogene (Cretaceous-Tertiary, K/T, K/P) Mass Extinction

The most recent of the ‘Big 5’ Mass Extinctions, is possibly the most famous one. The K/T extinction has received much attention from the general public and scientific community for a variety of reasons, mostly due to the iconic organisms that were lost and the nature of their demise. Many hypotheses had been proposed, but eventually most scientists agreed that the trigger was a bolide impact associated with the Yucatán Peninsula of Mexico. Although not immediately accepted, once the ‘smoking gun’ was agreed upon, there was a short amount of time where bolide impact was a plausible hypothesis for almost all of the mass extinctions. Although we now know better, after the K/T bolide crater was found, scientists scoured the Earth looking for other impact structures to explain each of the other ‘Big 5’.  To better understand the K/T event specifically, take a look at NASA’s quick synopsis here. Make sure that you understand what evidence there is, and think about what multi-sphere interactions would have occurred during this event. Once you’re done with this, you’ll be finished with each of the Big 5 Mass Extinctions, and almost done with this assignment!

 

The First Mass Extinction and the 6th Mass Extinction (maybe and maybe??)

Up until now, we have focused entirely on Phanerozoic extinction events. Just prior to the Cambrian, there was a group of enigmatic organisms collectively referred to as the Ediacaran biota. These organisms are characterized by a variety of unique symmetries and weird body plans entirely absent from the subsequent Phanerozoic. Although collectively referred to as the Ediacaran biota, there have been arguments that these organisms may not represent one single clade (or even life at all!). Instead, they may represent a few early lineages of life, including many that would not make it into the Phanerozoic. Although not currently recognized as one of the ‘big’ mass extinctions, during this time almost all of these organisms die out before or just after the Cambrian boundary. Take a look at the University of California Museum of Paleontology‘s website describing the Ediacaran biota. Make sure to take a quick look at the images, and note how morphologically unique they are.

It has also been proposed by multiple scientists that we are currently in the Sixth Mass Extinction, possible representing the largest the Earth has ever seen! During our time on Earth, we have seen the disappearance of a number of species, as well as large populations of some species still currently living. Although there has been some disagreement among the general public, most scientists agree that the signal reduction in biodiversity is an true signal. Given what you have read in this activity, and given what you have learned in class, are we in a mass extinction event? If so, how do we know?  What kinds of evidence should we look for? During our lifetimes, we haven’t experienced many of the same triggers as above, but those sphere connections are still present.

Write a summary piece that briefly compares and contrasts each of the Big 5, and go ahead and include if you think the end of the Ediacaran biota should be considered one. To answer this question, review what the Museum’s webpage indicates about the enigmatic life during this time. Lastly, devote a large portion of your paper to discussing if we should be considered in a mass extinction from a scientific standpoint. Is there enough quantitative evidence to suggest that we are? What forms of evidence support or refute being in one? Your ANALYSIS should be written in the following format:

• 12 point font size
• double space
• 1″ margins
• Times New Roman font
• 3 to 5 pages

You may submit this under the submission page found on the menu at the top of this page.