Michael Novacek
Provost of Science
Six (Mass) Extinctions in 440 Million Years
All things must pass. But the idea that a species could go extinct
is a relatively new one, first proposed by anatomist Georges Cuvier in a
presentation in Paris in 1796 in a lecture on the extinction of the
mastodon, then thought by some to still be roaming the ill-explored
western reaches of North America.
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Discovered in 1845 in Newburgh, New York, the Warren mastodon now resides in the Museum's Hall of Advanced Mammals.
©AMNH/D.Finnin
©AMNH/D.Finnin
Cuvier’s suggestion that life on Earth was not static, and that
species could disappear, was groundbreaking. Studying the collections of
the National Museum of Natural History in Paris and records from other
collections around the world, he soon identified several species whose
like we would never see again, including the mosasaur, the cave bear,
and the Irish elk.
Buoyed by the research of scientists like Charles Lyell and Charles
Darwin, the idea that species developed gradually, over time, gained
acceptance in the scientific community. For generations, it was dogma
that extinctions happened slowly, too. The idea that species could be
wiped out in a fell swoop, even one with catastrophic consequences,
wasn’t given much credence.
That changed in the late 1980s and early 90s, when the Alvarez
hypothesis, which stated that a huge comet or asteroid impact was
responsible for the sudden disappearance of non-avian dinosaurs and many
other forms of life 66 million years ago. Proposed by physicist Luis
Alvarez and his son, geologist Walter Alvarez, the hypothesis took time
to gain acceptance, but buoyed by evidence like the crater pictured
below, an impact is now the most widely accepted explanation for the
Cretaceous-Paleogene (K-Pg) extinction.
Credit: Shane Torgerson
That acceptance also opened the door for further study of geological
and fossil records, which led researchers to a surprising conclusion:
While the K-Pg extinction event was a very bad day for life on Earth, it
was by no means the only one on record. Researchers now think that the
K-Pg was just the latest of five major extinction events—and that we’re
currently in the middle of a sixth mass extinction, one caused not by a
volcano or asteroid impact, but by humans.
Each event had a different impetus. Some took place over the span of
millions of years while others were extremely sudden. What they have in
common, though, is that they reshaped the face of life on Earth by
wiping out a significant portion of it.
About 445 Million Years Ago: Ordovician Extinction
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Trilobites were once one of the most common lifeforms on the planet,
but many species were wiped out during the Ordovician extinction.
©AMNH
©AMNH
The earliest known mass extinction, the Ordovician Extinction, took
place at a time when most of the life on Earth lived in its seas. Its
major casualties were marine invertebrates including brachiopods,
trilobites, bivalves and corals; many species from each of these groups
went extinct during this time. The cause of this extinction? It’s
thought that the main catalyst was the movement of the supercontinent
Gondwana into Earth’s southern hemisphere, which caused sea levels to
rise and fall repeatedly over a period of millions of years, eliminating
habitats and species. The onset of a late Ordovician ice age and
changes in water chemistry may also have been factors in this
extinction.
About 370 Million Years Ago: Late Devonian Extinction
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Tiktaalik, considered a transitional species between fishes and the first legged animals, developed during the Devonian Period.
© National Science Foundation
© National Science Foundation
Towards the end of the Devonian period around 370 million years ago,
a pair of major events known as the Kellwasser Event and the Hangenberg
Event combined to cause an enormous loss in biodiversity.
Given that it took place over a huge span of time—estimates range
from 500,000 to 25 million years—it isn’t possible to point to a single
cause for the Devonian extinction, though some suggest that the amazing
spread of plant life on land during this time may have changed the
environment in ways that made life harder, and eventually impossible,
for the species that died out.
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Dunkleosteus terrelli is one of the species of armored fish called placoderms that went extinct at the end of the Devonian Period.
©AMNH/D.Finnin
©AMNH/D.Finnin
The brunt of this extinction was borne by marine invertebrates. As
in the Ordovician Extinction, many species of corals, trilobites, and
brachiopods vanished. Corals in particular were so hard hit that they
were nearly wiped out, and didn’t recover until the Mesozoic Era, nearly
120 million years later. Not all vertebrate species were spared,
however; the early bony fishes known as placoderms met their end in this
extinction.
252 Million Years Ago: Permian-Triassic Extinction
The Permian-Triassic extinction killed off so much of life on Earth
that it is also known as the Great Dying. Marine invertebrates were
particularly hard hit by this extinction, especially trilobites, which
were finally killed off entirely. But you don’t get a nickname like the
Great Dying for playing favorites; almost no form of life was spared by
this extinction, which caused the disappearance of more than 95 percent
of marine species and upward of 70 percent of land-dwelling vertebrates.
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The Permian Period, depicted in this diorama at the Museum, ended in an extinction known as The Great Dying.
©AMNH/D.Finnin
©AMNH/D.Finnin
So many species were wiped out by this mass extinction it took more
than 10 million years to recover from the huge blow to global
biodiversity. This extinction is thought to be the result of a gradual
change in climate, followed by a sudden catastrophe. Causes including
volcanic eruptions, asteroid impacts, and a sudden release of greenhouse
gasses from the seafloor have been proposed, but the mechanism behind
the Great Dying remains a mystery.
201 Million Years Ago: Triassic-Jurassic Extinction
This extinction occurred just a few millennia before the breakup of
the supercontinent of Pangaea. While its causes are not definitively
understood—researchers have suggested climate change, an asteroid
impact, or a spate of enormous volcanic eruptions as possible
culprits—its effects are indisputable.
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The Triassic-Jurassic extinction event took place just a few thousand years prior to the breakup of the supercontinent Pangea.
Wikimedia Commons/Massimo Pietrobon
Wikimedia Commons/Massimo Pietrobon
More than a third of marine species vanished, as did most large
amphibians of the time, as well as many species related to crocodiles
and dinosaurs.
66 Million Years Ago: Cretaceous-Paleogene Extinction
The most recent mass extinction event is also likely the best understood of the Big Five.
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Tyrannosaurus rex was among the many species of dinosaurs that went extinct as a result of the Cretaceous-Paleogene extinction event.
©AMNH/C.Chesek
©AMNH/C.Chesek
In addition to its most famous victims, the non-avian dinosaurs, the
K-Pg event caused the extinction of pterosaurs and extinguished many
species of early mammals and a host of amphibians, birds, reptiles, and
insects. Life in the seas was also badly disrupted, with damage to the
oceans causing the extinction of marine reptiles like mosasaurs and
plesiosaurs, as well as of ammonites, then one of the most diverse families of animals on the planet.
In all, scientists estimate that 75 percent of species living at the time of the K-Pg extinction were wiped out.
Now: The Holocene Extinction
The Holocene Extinction hasn’t been defined by a dramatic event like
a meteor impact. Instead, it is made up of the nearly constant string
of extinctions that have shaped the last 10,000 years or so as a single
species—modern humans—came to dominate the Earth. Some have even
suggested that the Holocene Extinction would be more aptly named the
Anthropocene Extinction, after the role humans have played in this
ongoing loss of biodiversity around the world.
The now extinct Tasmanian tiger, as depicted in John Gould’s The Mammals of Australia in 1863, was the world's largest meat-eating marsupial.
©AMNH/D.Finnin
©AMNH/D.Finnin
Once a staple of America's Great Lakes, the blue walleye fell victim to overfishing and has been extinct since the 1980s.
Courtesy of NOAA/GLERL
Courtesy of NOAA/GLERL
Plate from Zoological Illustrations (1829) by William Swainson. Accepted as Urania sloanus
Courtesy of Wikimedia Commons
Courtesy of Wikimedia Commons
The now extinct Tasmanian tiger, as depicted in John Gould’s The Mammals of Australia in 1863, was the world's largest meat-eating marsupial.
©AMNH/D.Finnin
©AMNH/D.Finnin
“Many of the past mass extinction events are mysterious in some ways
because we really don’t know the cause,” says Michael Novacek, the
Museum’s provost of science and a curator in the Division of Paleontology.
"But we have a good idea of what the cause of the current changes are,
this century and the centuries before: it’s human activity.”
Humans have contributed to factors like climate change and the
introduction of invasive species, which are leading to even more
extinctions as animal habitats disappear or are disrupted by new
species. “Some biologists think that the current rate of species loss
is probably a thousand times what the normal rate is,” says Novacek.
Many of the species going extinct are doing so before they are even
identified. In light of this, researching new species for a fuller
understanding of the world’s biodiversity grows ever more urgent for
institutions like the American Museum of Natural History. These records,
Novacek says, are vital to our knowledge of the world around us.
“The collections in the Museum here and other museums are really a
record of life," Dr. Novacek says. “They’re very important for not only
telling us what went extinct, but what survived.”
Siberian tigers are muscular, packed with large heads and powerful
forelimbs. Their colors vary from orange to brown that are splashed with
white areas and black stripes. Their faces have long whiskers that are
longer in male tigers, featuring eyes that don rounded pupils that have
been accentuated by yellow irises. The ears, however, are small and
rounded with black markings that surround white areas called ocelli,
which are not decorative but actually contribute to communication within
the species.
The tiger has nine subspecies. Three of these are already extinct.
The historical range of tigers all across Asia is now significantly
smaller. The surviving subspecies of the Pathera tigris trigris, which
is popularly known by its common name, Bengal tiger can be found in
Bangladesh, Bhutan, India and Nepal. It is still the subspecies with the
most living individuals left, even with its mere 2,500 adult tiger
population. Bengal tigers can be found in alluvial grasslands,
rainforests of both the tropical and subtropical kinds, mangroves,
deciduous forests and scrub forests. Male Bengal tigers can reach up to
270 to 310 centimeters (110 to 120 inches) while the females reach up to
240 to 265 centimeters (94 to 104 inches). The males are also heavier,
at 175 to 260 kilograms (390 to 570 pounds), while the females weigh
around 100 to 181 kilograms (220 to 400 pounds). Places also have an
effect on the weight and length of the Bengal tiger, with those living
in Nepal and India growing to become the largest bunch.
The Panthera tigris corbetti, more commonly known as the Indochinese
tiger, can be found in several Asian countries such as Burma, Cambodia,
Laos, China, Vietnam and Thailand. They are darker in color and smaller
in size than Bengal tigers but they are not lightweights either, with
their capacity to reach the maximum weight of 420 pounds for males and
310 pounds for females. Indochinese tigers prefer to live in forests in
areas that are either hilly or mountainous. There aren't a lot of these
tigers anymore. The government estimates the subspecies population to be
at a mere 350. Even those that have been left behind are still in
danger of being poached or even of starving due to the fact that their
primary choice of prey, such as wild pigs and deer, are decreasing. In
Vietnam, about 75% of the tigers have been poached to serve as Chinese
pharmacy stock.
The Panthera tigris jacksoni, otherwise known as the Malayan tiger,
can only be found in the south of the Malay Peninsula. The Malayan tiger
was only recognized as a subspecies in 2004. A research conducted by
Luo and others, from the Laboratory of Genomic Diversity Study (part of
the United States' National Cancer Institute), came up with the new
subspecies classification. There are about 500 still-living Malayan
tigers but they are endangered by poaching. These are the smallest
subspecies on the mainland and even the second smallest living tiger
subspecies. The males average at 120 kilograms while the females average
at about 100 kilograms. Note that the Malayan tiger has a cultural
significance in Malaysia as it has made it to the country's coat of
arms. It is also the logo of Maybank, a Malaysian bank. V- Sumatran
tigers
Panthera tigris amoyensis, also known as the South China tiger, is
actually the most endangered tiger subspecies. They are even more
endangered than the Sumatran tigers, which are already heavily watched
by conservationists. The South China tiger has even made the list of the
world's ten most endangered species. The South China tiger belongs to a
group of smaller tiger subspecies, with lengths spanning from 2.2 to
2.6 meters (87 to 100 inches). The range of the length of South China
tigers is true for both male and female tigers. Males have a weight
range of 127 to 177 kg (280 to 390 pounds); females, on the other hand,
have a weight range of 100 to 118 kilograms (220 to 260 pounds).
Some tiger subspecies may still be mentioned in books but they have
already been extinct as habitats continue to be destroyed. The Panthera
tigris balica, which is popularly known as the Bali tiger, used to be
limited to Bali, which is an Indonesian island. When it still existed,
the Bali tiger was the smallest tiger subspecies, with the males
weighing 90 to 100 kilograms (200 to 220 pounds) and the females
weighing 65 to 80 kilograms (140 to 180 pounds). It is unfortunate that
people can no longer appreciate the beauty of a Bali tiger in its
compact size. The tigers have become extinct because of hunting. The
last of its kind, which is an adult female, was believed to have been
hunted and killed in Sumbar Kima.
The Panthera tigris virgata is better known as the Caspian tiger as
well as under other names, such as the Hyrcanian or the Turan tiger.
This tiger subspecies used to be found in sparse forests. It was also
found south and west of the Caspian Sea. At least, it wast still sighted
in the wild up to the early 1970s. The closest living subspecies to the
Caspian tiger is the Amur tiger.
The Panthera tigris sondaica, otherwise known as the Javan tiger,
used to live only on the island that it was named after. Sightings have
been recorded up to the middle part of the 1970s. The tigers in these
subspecies are bigger than Bali tigers, with the males weighing 100 to
140 kilograms (220 to 310 pounds) and the females weighing 75 to 115
kilograms (170 to 250 pounds). Sadly, the Javan tiger has no longer been
sighted after 1979 when it was last seen around the Mount Betiri area.
The white tiger is produced with the help of the allele called
chinchilla albinistic and is rather a rare sighting in the wild. It has,
however, a very attractive color, which makes it a popular choice in
zoo breeding. The color trait is recessive. So, breeding white tigers
usually ends up in inbreeding. Inbreeding should be avoided because it
can cause several physical defects, which include scoliosis, strabismus
(crossed eyes) and cleft palates. White tigers, even the healthiest
ones, have shorter lives. There have been attempts to breed white tigers
to orange ones to try to fix the situation. When orange and white
tigers mix, there is a mixing of subspecies.
Another recessive gene can result into an interesting "golden" hue.
The color is sometimes called "golden tabby" and also sometimes called
"strawberry". Golden tigers actually have gold-colored fur, with pale
orange stripes and supported by strong but pale-colored legs. They have
thicker fur than what is usual for tigers. As of the last count, there
are only around 30 golden tigers that are held captive.
