Story Lines:
What was the largest
mass extinction event?
What happened to trilobites?
What happened to Stegasaurs?
What happened when Gondwana passed over the north pole?
Several mass extinctions are recorded in the fossil record and paleontologists have been able to recognize patterns within and between extinction events. There have been many major and minor extinction events. Some of the major events are listed below:
34 MYA - Tiny, shell-bearing sea creatures
65 MYA - Dinosaurs extinction event along with perhaps 50% of other species.
210 MYA - 35% of all animal families die out. Most early dinosaur families went
extinct along with many marine invertebrates, and mammal-like reptiles.
250 MYA - Perhaps over 90% of the marine species and 70%
of the terrestial species became extinct. Sea-floor protozoans, marine invertebrates, reef
builders, mammal-like reptiles. This was the largest extinction event yet recorded.
BBC News
Online article discussing this extinction event
380 MYA - Plankton, marine invertebrates, reef builders,
primitive fish
440 MYA - The second most devastating extinction to marine communities in earth
history. Trilobites, marine invertebrates and primary reef-building
organisms. Over 100 families of marine invertebrates perished.
650 MTA - The first great extinction event, stromatolites and 70% of the dominant Precambrian flora and fauna take a hit.
Some of the minor events include:
At the end of the Jurassic, a minor mass-extinction could have contributed to the demise of the stegasaurids and enormous sauropods (like the brachiosaurus). Many of the seas' bivalves and marine reptiles died out as well. The cause of this extinction is unknown.
These similarities between distinct extinction occurrences aid paleontologists in determining the agents the agents that perpetuated the disappearances of species in each extinction event. Such agents are currently divided into two types:
Although these agents can explain mass extinction, the causes of mass extinction events remains relatively unknown.
Several mass extinctions have occurred throughout earth history. Cycles found to be associate with these events has resulted in paleontologists proposing mechanisms to explain these events. It seems that mass extinctions occur at regular time intervals with a periodicity of approximately 26 million years. This concept was first suggested by Alfred Fischer and Michael Arthur, but has been further expanded upon by various researchers utilizing data collected by David Raup and John Sepkoski. Raup and Sepkoski constructed graphs utilizing incidences of extinction of marine families through time. The rate of extinction was represented on a graph as a series of rising and falling peaks. Surprisingly, these peaks coincided with most major events of extinction throughout earth history. However, a significant number of paleontologists believe Raup and Sepkoski's research to be flawed, arguing that the dates used in the study are incorrect, and that the spacing pattern is not always 26 million years. Until new evidence can be brought forth, the question of periodicity will clearly remain a source of debate within the paleontological community.
Some 34 million years ago, almost 90 percent of the tiny, shell-bearing sea creatures living along the U.S. Gulf Coast were wiped out and replaced by completely new species of shellfish or mollusks. The same pattern was occurring worldwide, marking the largest global mass extinction since the dinosaurs disappeared.
34 MYA - tiny, shell-bearing sea creatures
65MYA - Dinosaurs
210 MYA - Marine invertebrates, mammal-like reptiles
250 MYA - Sea-floor protozoans, marine invertebrates, reef builders,
mammal-like reptiles
380 MYA - Plankton, marine invertebrates, reef
builders, primitive fish
440 MYA - Marine invertebrates, reef builders
Miscellaneous Notes
Major Events -
The Precambrian and Vendian Mass Extinctions - (Two main extinctions) - Extinctions are proposed to have affected even life's earliest organisms. About 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction. This extinction strongly affected stromatolites and acritarchs, and was also the predetermining factor that encouraged the diversification of the following Vendian fauna. However, this distinct fauna, resembling modern-day soft-bodied organisms such as sea pens, jellyfish, and segmented worms also perished in a second extinction event at the close of the Vendian. This event, responsible for the demise of the Vendian organisms, may have been reponsible for the ensuing diversification of the Cambrian shelly fauna. The first extinction of the Precambrian, which largely affected stromatolites and acritarchs, has been correlated with a large glaciation event that occurred about 600 million years ago. This event was of such severity that almost all micro-organisms were completely wiped out.
The Cambrian Mass Extinctions - (4 main extinction events) - During the
Cambrian period the world was largely covered by epeiric seas, and existing organisms were
entirely marine. At the beginning of the period, only small skeletonized sponges and molluscs were present, but
by about the middle of the Cambrian, diversification of the shelly fauna occurred. The
most important phyla present in Cambrian communities included trilobites, archaeocyathids, brachiopods, molluscs, and echinoderms.
The first extinction occurred at the Early Cambrian epoch boundary. During this event, the
oldest group of trilobites,
the olnellids, perished as well as the primary reef-building organisms, the archaeocyathids. The
remaining three extinctions were irregularly distributed around the Late Cambrian epoch
boundary, and as a whole, severly affected trilobites, brachiopods, and conodonts. Both
glaciation and oxygen depletion theories have been proposed to explain these events.
The Ordovician Mass Extinction (440-450 MYA) - This
extinction, cited as the second most devastating extinction to marine communities in earth
history, caused the disappearance of one third of all brachiopod and bryozoan families, as
well as numerous groups of conodonts, trilobites, and graptolites. Much of the
reef-building fauna was also decimated. In total, more than one hundred families of marine
invertebrates perished in this extinction. The Ordovician mass extinction has been
theorized by paleontologists to be the result of a single event; the glaciation of the
continent Gondwana at the end of the period. Evidence for this glaciation event is
provided by glacial deposits discovered by geologists in the Saharan Desert. By
integrating rock magnetism evidence and the glacial deposit data, paleontologists have
proposed a cause for this glaciation. When Gondwana passed over the north pole in the
Ordovician, global climatic cooling occured to such a degree that there was global
large-scale continental resulting in widespread glaciation. This glaciation event also
caused a lowering of sea level worldwide as large amounts of water became tied up in ice
sheets. A combination of this lowering of sea-level, reducing ecospace on continental
shelves, in conjunction with the cooling caused by the glaciation itself are likely
driving agents for the Ordovician mass extinction.
The Devonian Mass Extinction - The Devonian saw the first appearance of sharks, bony fish, and ammonoids. During the Devonian the world's oceans were dominated by reef-builders such as the stromatoporoids, and corals, and some of the world's largest reef complexes were built. Terrestrial newcomers in the Devonian included amphibians, insects, and the first true land plants, giving rise to the first forests. The Devonian mass extinction occurred during the latter part of the Devonian at the Frasnian - Famennian boundary. The crisis primarily affected the marine community, having little impact on the terrestrial flora. This same extinction pattern has been recognized in most mass extinctions throughout earth history. The most important group to be affected by this extinction event were the major reef-builders including the stromatoporoids, and the rugose, and tabulate corals. This late Devonian crisis affected these organisms so severely that reef-building was relatively uncommon until the evolution of the scleractinian (modern) corals in the Mesozoic era. Among other marine invertebrates, seventy percent of the taxa did not survive into the Carboniferous. Amongst the severely affected groups were the brachiopods, trilobites, conodonts, and acritarchs, as well as all jawless fish, and placoderms. Many paleontologists to attribute the Devonian extinction to an episode of global cooling, similar to the event which is thought to have cause the late Ordovician mass extinction. According to this theory,the extinction of the Devonian was triggered by another glaciation event on Gondwana, as evidenced by glacial deposits of this age in northern Brazil. Similarly to the late Ordovician crisis, agents such as global cooling and widespread lowering of sea-level may have triggered the late Devonian crisis. Meteorite impacts at the Frasnian-Famennian boundary have also been suggested as possible agents for the Devonian mass extinction.
The Permian Mass Extinction - The Permian mass extinction occurred about 248
million years ago and was the greatest mass extinction ever recorded in earth history;
even larger than the previously discussed Ordovician and Devonian crises and the better
known End Cretaceous extinction that felled the dinosaurs. Ninety to ninety-five percent
of marine species were eliminated as a result of this Permian event. The primary marine
and terrestrial victims included the fusulinid foraminifera, trilobites,rugose and tabulate corals,
blastoids, acanthodians, placoderms, and pelycosaurs, which did not
survive beyond the Permian boundary. Other groups that were substantially reduced included
the bryozoans, brachiopods, ammonoids, sharks, bony
fish, crinoids, eurypterids, ostracodes,
and echinoderms.
With the formation of the super-continent Pangea in the Permian, continental area exceeded
that of oceanic area for the first time in geological history. The result of this new
global configuration was the extensive development and diversification of Permian
terrestrial vertebrate fauna and accompanying reduction of Permian marine communities.
There are many theories proposed including glaciation, the formation of Pangea and
the disappearance of continental self area, rapid warming and drying in the middle of the
earth with glaciation at the poles, volcanism and huge basaltic lava eruptions in Siberia.
Cretaceous (K-T) Extinction -
Minor extinction events
Triassic Extinction - Labyrinthodont amphibians, conodonts, and all marine reptiles(excluding
ichthyosaurs) were eliminated and mammal-like reptiles, thecodonts, brachiopods, gastropods, and molluscs were severly
affected by this event. The causes of the Triassic extinction are not well known, but
popular explanations for its occurrence include global climatic cooling, extra-terrestrial
impact, or comet showers. This extinction event is particularly important because it
allowed the dinosaurs to radiate into terrestrial niches that were previously unavailable.
Jurassic Events - Two extinction events are speculated to have occurred in the Jurassic. The first of these events is recognized in Pleinsbachian age strat from Europe. This extinction eliminated more than eighty percent of marine bivalve species, along with various other shallow water species. The second crisis occurred near the end of the Jurassic, by an event that severly affected ammonoids, marine reptiles, and bivalves. Dinosaurs were also severly affected as stegosaurs and most types of sauropods did not survive into the Cretaceous period. This event is not well understood so few hypothese have yet been proposed for its occurrence.
The Oligocene Extinction - The early Oligocene extinction event was triggered by
severe climatic and vegetational changes, and drastically affected land mammals. At this
time, the world experienced a global cooling that shuffled many of the existing biomes.
Tropical areas, such as jungles and rainforests, were replaced by more temperate savannahs
and grasslands. This change in biomass forced dramatic changes in the distribution of
Oligocene flora and fauna. Typically, forest dwellers declined as forested habitat became
less abundant, and in their place, hoofed animals flourished due to the growing number of
temperate grasslands. A number of predators also became extinct at this time, due
primarily to the faunal changes. Major mammal groups that perished included mesonychids
and creodonts.
The Neogene Extinctions - Six major pulses of extinction have occurred since the
beginning of late Miocene time. The first occurred about nine million years ago, and the
most recent occurred only about eleven thousand years ago. This last crisis was restricted
exclusively to large mammals, eliminating thirty-nine genera. Among the species eliminated
were saber-toothed cats, mastodons, wooly mammoths, huge ground sloths, short-faced bears,
and dire wolves. Causes for this extinction are also widely debated. Hypotheses for this
extinction include global climatic cooling, and human over-hunting.
Sources
Hooper Virtual Paleontological Museum http://park.org/Canada/Museum/extinction/tablecont.html