57.2: Mass Extinctions of The Past and Possible Causes

Age (Ma)	Percentage of Genera Lost	Hypothesized Trigger or Cause	Affected Groups
Ordovician/Silurian - 439 Ma	86%	The shift in continents closer to poles caused glaciation, low sea levels; too many plants removed too much \(\ce{CO2}\) from the air which reduced temperature causing an ice age; falling sea levels due to huge ices sheets in the southern hemisphere; climate changes; ocean chemistries changed; the Appalachian Mountains forming	Marine species (most organisms were marine at this time) such as some bryozoans, reef-building brachiopods, trilobites, graptolites, and conodonts
Devonian - 364 Ma	75%	Flourishing of land plants released nutrients in the oceans causing algal blooms that depleted oceans of \(\ce{O2}\). Bacteria flourished, Volcanic ash cooled the Earth’s temperatures. Sea levels changes; asteroid impact; climate change;	Life is shallow seas such as corals, other shallow marine organisms including brachiopods, and single-celled foraminiferans; Terrestrial species such as spiders, scorpion-type creatures; Elipistostegalians; helped with vertebrate evolution.
Permian - 251 Ma	96%	“The Great Dying” Volcanic activity in Siberian Traps released lava flows the size of the United States and over 1,000 feet thick. Emitted large amounts of \(\ce{CH4}\). Global warming and atmospheric changes; oceans warmed and became acidic. Pangaea also taking shape, changing ocean currents; meteorite may have hit too. Climate change	96% of all marine species 70% of all land vertebrates The only mass extinction event that significantly impacted insects; Earth’s largest extinction event, decimating most marine species such as all trilobites, plus insects and terrestrial animals; life descended from the 4% of surviving species;
Triassic - 200 Ma	48%	Volcanism leading to flooding basalts and a tremendous amount of sulfur dioxide and carbon dioxide in the atmosphere, causing climate change; an asteroid impact	Many types of animals died, including marine reptiles, large amphibians, reef-building creatures, cephalopod mollusks; plants were not affected. This extinction allowed for the evolution of the reptiles
Cretaceous - 65 Ma	76%	Most famous event; a.k.a. “K-T Extinction”; Astroid imact hypothesis - asteroid impact in Yucatan Peninsula; Deccan Trap flood basalts were also erupting in India, but while they may have been a contributor, were likely not the ultimate cause	Extinction of many species in both marine and terrestrial habitats; ammonites, many flowering plants including pterosaurs, mosasaurs, many other dinosaurs, and other marine reptiles, many insects, and all non-Avian dinosaurs. This extinction allowed for the evolution of mammals.

The "Big Five" mass extinctions in the Earth's past and hypothesized causal factors. There are very likely more mass extinctions than listed here. And, we may also be in the midst of one today. How will it compare?

When extinction rates increase sharply above background rates for a geologically short period of time, the event is referred to as a mass extinction. Mass extinctions are rare, but when they happen they make huge adjustments to the patterns of life on the planet. Traditionally, there five really large mass extinctions that are set aside as special. In order, they mark the end of the Ordovician Period, the end of the Devonian Period, the end of the Permian Period, the end of the Triassic Period, and the end of the Cretaceous Period.

Figure \(\PageIndex{1}\): Biodiversity during the Phanerozoic Eon according to the fossil record. There are a wide variety of extinction events that have occurred between the Cambrian Period (Cm) and the Neogene Period (N). The “Big 5” are marked with the yellow triangles and include the end-Ordovician, the three-stage end-Devonian, the end-Permian, the end-Triassic, and the end-Cretaceous (K). The Permian end-Guadalupian Stage extinction is not marked, but can be seen in the rapid biodiversity drop just before the end-Permian event. Despite the extinctions, note the increase in biodiversity since the Trirassic (T)

Our understanding of these events changes as time and research themselves evolve. Compared to one another, the “Big Five” certainly vary in intensity, with the end-Permian being the most intense of them, often being referred to as “The Great Dying” because of the loss of an estimated 95% of all life on Earth. Many scientists feel that today we could be in the midst of a sixth mass extinction, caused by human activity. There are challenges in comparing today’s biodiversity loss to what we see in the fossil record. Whether or not today’s extinctions make the cut and join the other five is still an open question. Eventually, as things in the science progress, so will our conclusions about these events.

Mass extinctions are caused by powerful natural events. Every mass extinction contains similarities in their story lines. There is usually a significant element of climate change, whether as the main driver or a secondary result of the primary cause. There is massive loss of life, though how this plays out on land and in the sea can vary by event.

Ordovician Extinction

The end-Ordovician event is described as being caused by global cooling. More extreme cooling events during the Proterozoic Eon, referred to as “Snowball Earth” events, certainly occurred. But, there is no record of mass extinction. Most life was single-celled and it did not fossilize well. The end-Devonian, accompanied by massive black shale deposits, is unusual in that it actually happened over millions of years. The evolution of vascular plants on land may have been a factor, as the increase biomass to waterways could have caused mass eutrophication across large sections of the shallow epeiric (inland) seas that existed.

Cretaceous/Paleogene Extinction

The 3D model below illustrates the famous K/T boundary extinction event in the rock record. This event, caused by the Chicxulub asteroid impact, caused the extinction of the dinosaurs. Located near Gubbio, Italy, this is the very outcrop where Walter Alvarez collected samples to analyze for microfossils across the boundary. He noted that below the boundary, they were abundant, diverse, and some were very large. Above the boundary, they were sparse and tiny. Enlisting his Nobel-prize winning physicist father, Luis Alvarez, they determined that the boundary clay (the inset area in the model that looks like a gap) was unusually rich in iridium. Many such similar boundary clays rich in iridium have since been identified globally.

Are we currently in the midst of a sixth (or seventh!) mass extinction? In order to see the rate of species loss we see in the record over the last century, between 2,000 and 10,000 years should have been necessary. Given a conservative estimate of the background, pre-human extinction rate of about 0.1 extinction per million species years, the current observable rate is about 1,000 to as much as 10,000 times higher, at a bout 100 extinctions per million species years or more. These rates are quite alarming. Causes of current biodiversity loss include a wide variety of factors. Climate change, overhunting, habitat destruction, and habitat fragmentation are some of the largest. These are all directly related to human activity. At this point, this would be be the only mass extinction in geologic history caused by a single species.

Extinction as Opportunity

Extinction leads to origination. While it is certainly a way organisms fade into the memory of the rock record, these events also create opportunity. With extinction comes new niches, into which species can move. These new niches provide new environments, food sources, and habitats. They also provide new opportunities for evolutionary change. Organisms that survive extinctions and that move into environments that are changed or even marginal for their current life mode may adapt well enough to pass on their genes to another generation who may be better adapted to the new environment. While extinctions on various scales are certainly tragic for those groups or organisms that experience them, they can also provide the foundation for success stories. As humans, we have inherited the successes of our evolutionary ancestors.