Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T. Mass extinctions are periods in Earth's history when abnormally large numbers of species die out simultaneously or within a limited time frame. The most severe occurred at the end of the Permian period when 96% of all species perished. This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species.
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440 Ma, two bursts of extinction, separated by one million years, appear to have happened. This was the second biggest extinction of marine life, ranking only below the Permian extinction. At the time, all known life was confined to the seas and oceans. More than 60% of marine invertebrates died including two-thirds of all brachiopod and bryozoan families.Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. The immediate cause of extinction appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially-driven cooling are likely driving agents for the Ordovician mass extinction.
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 374 million years ago. Overall, 19% of all families and 50% of all genera went extinct. A second, distinct mass extinction, the Hangenberg Event, closed the Devonian period
Although it is clear that there was a massive loss of biodiversity in the Later Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.
By the late Devonian, the land had been colonized by plants and insects. In the oceans, there were massive reefs built by corals and stromatoporoids on land. Euramerica and Gondwana were beginning to converge into what would become Pangea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the Mesozoic. The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than any single event. Surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs or other aspects of the Kellwasser event, while agnathans were in decline long before the end of the Frasnian.
The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera were killed. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions."
Researchers have variously suggested that there were from one to three distinct pulses, or phases, of extinction. There are several proposed mechanisms for the extinctions; the earlier phase was likely due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event. Suggested mechanisms for the latter include large or multiple bolide impact events, increased volcanism, and sudden release of methane clathrate from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 199.6 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class (conodonts) and twenty percent of all marine families disappeared. On land, all large crurotarsans (non-dinosaurian archosaurs) other than crocodilians, some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangaea started to break apart. In the area of Tübingen (Germany), a Triassic-Jurassic bonebed can be found, which is characteristic for this boundary.
Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
Several explanations for this event have been suggested, but all have unanswered challenges:
The isotopic composition of fossil soils of end Triassic and Early Jurassic has been tied to a large negative carbon isotope excursion (Whiteside et al. 2010). Carbon isotopes of lipids (n-alkanes) derived from leaf wax and lignin, and total organic carbon from two sections of lake sediments interbedded with the CAMP in eastern North America have shown carbon isotope excursions similar to those found in the mostly marine St. Audrie’s Bay section in England; the correlation suggests that the end-Triassic extinction event began at the same time in marine and terrestrial environments, slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco (Also suggested by Deenen et al., 2010), with both a critical CO2 greenhouse and a marine biocalcification crisis. Contemporaneous CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same places, making the case for a volcanic cause of a mass extinction. The catastrophic dissociation of gas hydrates (suggested as one possible cause of the largest mass extinction of all time, the so-called "Great Dying" at the end of the Permian Period) may have exacerbated greenhouse conditions.
The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago (Ma) at the end of the Maastrichtian, was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers. Non-avian dinosaur fossils are found only below the K–T boundary, indicating that non-avian dinosaurs became extinct during the boundary event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian clades passed through the boundary with few extinctions, and evolutionary radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.
Global palaeogeographic reconstruction of the Earth at the time of the Cretaceous-Tertiary mass extinction. Credit: Dr Ron Blakey, NAU Geology.