Mass Extinctions and the Tree of Life over Geologic Time

Learning Objectives

1. Know the six major mass extinction events of the Phanerozoic, and recognize that extinctions define major boundaries between time periods
2. Describe the causes of the End Permian (P/T) and End Cretaceous (K/T) mass extinctions
3. Describe the effects of the P/T and K/T extinctions on global biodiversity, including which major groups of organisms died and which groups radiated into the vacated niches
4. Explain the causes and consequences of the sixth mass extinction

The Five Major Phanerozoic Mass Extinctions and their Effects on Biodiversity

The information below was adapted from Openstax Biology 47.1.

Changes in the environment can create new niches (living spaces) that contribute to rapid speciation and increased diversity events called adaptive radiations. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes that obliterate life, can result in devastating losses of diversity. Such periods of mass extinction have occurred repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve into the empty niches left behind. You are already familiar with the extinctions that occurred during the Great Oxygenation event, when cyanobacteria altered the gaseous composition so dramatically that anaerobic species, and even species that produced oxygen!, went extinct from the presence of the very reactive free oxygen. While that extinction was probably massive, it is not included in the list of five mass extinctions below, which focus on the mass extinctions of the Phanerozoic.

Before we discuss mass extinctions of the Phanerozoic, first let’s recall the Eras and Periods of this Eon:

Mass extinctions in the fossil record define boundaries of the geological periods of the history of life on Earth. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. At five points in the Phanerozoic, a large number of taxa were lost in a short geologic time span:

• The end-Ordovician extinction event was the second largest recorded extinction event, when about 85 percent of marine species (plus some land plants and a few groups of animals that lived outside the oceans) became extinct.
  • The leading hypothesis is that the end-Ordovician extinction was caused by a period of glaciation and then warming in a rapid (1 million year) timespan, affecting both climate and sea levels, and with the cooling and warming each causing a round of extinctions.

• The end-Devonian extinction appears to have affected primarily marine species and not terrestrial plants and animals. The causes of this extinction are poorly understood.

• The end-Permian extinction (also called P/T or Permian/Triassic) was the largest recorded extinction event so far in the history of life, with an estimated 96 percent of all marine species and 70 percent of all terrestrial species lost.
  • Causes: The causes for this mass extinction are not clear, and multiple hypotheses exist for climatic conditions that contributed to the extinctions. The leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. Evidence for this lies in the massive layers of basaltic rock of the Siberian Traps, which indicate extreme volcanic eruptions that lasted for approximately one-two million years. High global temperatures would have resulted from the increased carbon dioxide and methane released into the atmosphere from the volcanic activity. Consequent warming of the oceans, combined with nutrient runoff from dead and decomposing terrestrial species, would have caused the oceans to became largely anoxic (devoid of oxygen), ultimately suffocating much of the oxygen-dependent marine life. It is also suspected that the ENSO oscillations (El Nino-Southern Oscillation) were intensified by these global events, leading to more severe droughts, temperature fluctuations, and wildfires.
  • Impact on biodiversity: It was at this time that the trilobites became extinct, a group that survived the end-Ordovician and end-Devonian extinctions. On land, many seedless plant lineages disappeared, and the disappearance of some dominant species of Permian reptiles made it possible for the dinosaurs to emerge. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The warm and stable climatic conditions of the following Mesozoic Era promoted an explosive diversification of dinosaurs into every conceivable niche in land, air, and water. Among land plants, lycophytes and later gymnosperms radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became very large on the abundant food available.

• The causes of the end-Triassic extinction event are not clear, and hypotheses of climate change, asteroid impact, and volcanic eruptions have been presented. The extinction event occurred just before the breakup of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.

• The end-Cretaceous extinction event 65 million years ago was not the biggest extinction event, but it is the most famous because it involves the loss of all dinosaurs except a theropod clade that gave rise to birds. (The end Cretaceous is also called the Cretaceous/Tertiary or K/T; “K” is the geologic abbreviation for “Cretaceous,” and “Tertiary” is a now-obsolete term that describes the Paleogene).
  • Causes: The main cause of the end-Cretaceous extinction was the cataclysmic impact of a large meteorite, or bolide, off the coast of what is now the Yucatan Peninsula in Mexico. Evidence in support of this hypothesis includes an appropriately aged and sized impact crater; and a sharp spike in the levels of iridium (which occurs in meteors but is otherwise absent on Earth’s surface) at the rock stratum that marks the boundary between the Cretaceous and Paleogene periods. The bolide impact would have triggered wildfires, acid rain, ash-filled darkening of the skies, and a drop in sea levels. Additional evidence suggests a large area of volcanic activity in the Indian subcontinent, the Deccan traps. Similar to the consequences in the end-Permian extinction, the volcanism would have resulted in ocean acidification and hypoxia.
  • Impact on biodiversity: With drastic decreases in solar radiation, plants died, followed by starving herbivores and carnivores.In fact, every land animal that weighed more than 25 kg became extinct during this event. Recovery times for biodiversity after the end-Cretaceous extinction are shorter than for the end-Permian extinction, on the order of 10 million years. As plant biodiversity began to recover in the Cenozoic Era, the surviving mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs. Birds, descended from the only surviving group of dinosaurs, became aerial specialists. The rapid dominance of flowering plants on land created new niches for insects, birds, and mammals. In addition, changes in biodiversity were also promoted by a dramatic shift in Earth’s geography with the final break up of Pangaea. As continental plates slid over the crust into their current positions, this left some animal groups isolated on islands and continents, or separated by mountain ranges or inland seas from other competitors. Early in the Cenozoic, the evolution of grasses and coral reefs created new ecosystems. Late in the Cenozoic, further cycles of extinction-speciation events occurred during the ice ages. As ice covered high latitudes and then retreated, it carved new open habitats for colonization.

The Sixth Major Mass Extinction of the Phanerozoic

The information below was adapted from Openstax Biology 47.1 and Wikipedia “Holocene extinction”

We are currently living in the midst of a sixth mass extinction, caused primarily by the activities of Homo sapiens. The current mass extinction has been called the Anthropocene mass extinction. The Holocene Epoch is the name of the most recent ~12,000 years of Earth’s history, or the time since the last major ice age. (Epochs are subdivisions of Periods within geologic time.) Some ecologists have argued that we have transitioned from the Holocene into the Anthropocene Epoch, named for the outsized influence humans are having on the Earth’s climate and ecology; however, the Anthropocene is an unofficial epoch, used to describe Earth’s most recent history beginning in the 1950s.

Since the beginning of the Holocene period, there are numerous extinctions of individual species that are recorded in human writings. Most of these extinctions coincide with the expansion of European colonialism beginning in the 1500s. The Anthropocene mass extinction has accelerated since that time and includes multiple families of plants and animals, including mammals, birds, reptiles, amphibians, fish, and invertebrates, across both terrestrial and marine habitats. These extinctions are a result of a complex array of factors driven by human activity, which include:

• the unprecedented role of humans as a “global super-predator” by hunting apex predators,
• expansive land use changes for urban development and agriculture that displace species from their habitats
• large-scale impacts on the environment from widespread pollution, ocean acidification and global warming

Estimates of the Anthropocene extinction rates are hampered by the fact that many extinctions are happening without our ability to know it. Many species, especially invertebrates, may be extinct or on the brink of extinction, without ever having been described by science.  The best estimates of the current species extinction rate are 100 to 1000 times higher than the typical background extinction rate. Some researchers estimate that the current extinction rate is 10 to 100 times higher than the extinction rate of any earlier mass extinction experienced in the history of life on Earth.