The Nature of Fossils: A Historical Perspective
Fossils are an integral part of our culture. We encounter
them in museums, schools, television, and movies. We know
so much about fossils; it is difficult to believe our ancestors
were so unsure about their nature.
recorded history there have been both naturalistic and
super-naturalistic explanations for fossils. The Greeks
believed mammoth fossils were the bones of human giants.
Xenophanes (ca. 570-500 BCE), a Greek philosopher, hypothesized
that there existed a cycle in which moisture eroded land
into mud followed by another beginning. Xenophanes cited
marine fossils found on land as evidence to support his ideas
(Kirk, Raven, and Schofield 1983, p. 177). Aristotle (384-322
BCE) speculated that ancient fish swam into cracks in the
rock and got stuck. Popular conceptions included the ideas
that crinoids with star-shaped centers were formed by falling
stars and that ammonites were decapitated snakes. The word
fossil is Latin for “dug up” and was coined during
the Renaissance. People wondered whether fossils are pranks
of nature, works of the Devil, or supernatural representations
of ideal life forms. Many believed that fossils formed during
Noah’s flood. Leonardo da Vinci (1452-1519) recognized
that fossil shells in the Apennine Mountains of Italy were
the remains of ancient sea life and argued they could not
have formed during Noah’s flood. He recognized that
different specimens had been formed at different times (Prothero,
2004, pp. 5 & 6).
Many people in Western cultures were taught to believe
in a literal interpretation of the Bible. It was natural
to believe every species in existence was made in a single
creation event. This idea also extended to rocks, believed
to have been formed as we see them during the first days
of creation. Thus in the absence of two key concepts, extinction
and sedimentary rock formation, a more accurate understanding
of fossils was not possible. Robert Hooke (1635-1703) and
Niels Stensen (1638-1686) would put science on a more productive
path to understanding the origins of fossils and the formation
of sedimentary rocks.
Niels Stensen (Latinized
to Nicholaus Steno) was an anatomist and naturalist. A chance
encounter between determined fishermen and a great
white shark off the Tuscan coast in 1666 sparked a chain
of events that would help change our views of fossils and
of Earth’s geologic past (Cutler 2003, pp. 5-8). Steno
dissected the head of this shark and realized that fossil
tongue stones previously believed to be petrified snake or
dragon tongues were actually fossil shark teeth (Prothero
1998, p. 3). One problem still existed: how do fossils become
embedded in solid rock? Steno recognized that fossils represent
organisms that became buried in sediment, which later turned
into rock. The realization that sediments turn into rock
was counter to the view that all rocks on Earth formed in
a single creation event. Once Steno recognized that the fossils
he was contemplating (sharks teeth and sea shells) were formed
in the sediments of oceans he was able to work out the basic
rules that would eventually be the foundations of a new branch
of geology called stratigraphy. Steno formalized the laws
of superposition, original horizontality, and original continuity
in his 1669 publication titled De solido intra solidum
naturaliter contento dissertationis prodromus (Prothero 2004, p. 6).
The law of superposition
is the foundation of Steno’s
work on stratigraphy. The principle of superposition states
that in an undisturbed sequence of strata each layer is older
than the one above and younger than the one below. The law
of original horizontality states that sedimentary strata
are deposited in horizontal sheets. If these layers are not
horizontal, subsequent movements have occurred. The law of
lateral continuity states that strata extend laterally in
all directions and pinch out at the edge of their deposition.
Modern forms of these laws include lava flows. Steno’s
description of how fossils become embedded in water born
sediments that later harden into rock also hints at the principle
of inclusion. Later, James Hutton, William Smith, Georges
Cuvier, and Alexandre Brongniart would add to these basic
rules making stratigraphy an effective way to study the distribution,
deposition, relative age, and fossil life of rock strata.
Robert Hooke, an English scientist and inventor, made some
of the first accurate illustrations of fossils. He suggested
that species may have a fixed “life span” and
might be used to order rocks chronologically (Prothero, 2004,
p. 7). Hooke argued that there was evidence
for three stages in fossil formation. The first stage could
be seen in the organic bones, shells and plant matter found
in mud, peat and moss. Plant and animal parts that were modified in layers
of lignites and brown coal represented the second stage. The third stage could be witnessed
by examining plant parts and shells that had turned to
coal embedded within layers of coal. The fossils had changed
to rock just as the peat had changed to lignite and the
lignite into coal (Winchester, 2001, p. 38). Not long after
Hooke’s discovery of plant cells he was asked at
a meeting of the Royal Society to examine petrified wood
under his microscope. Hooke discovered the fossil wood
had the same structure as living wood. At the time petrified
wood was thought to form as stone in rock layers and then
into clay that eventually turned to wood. This idea was
seemingly supported by a clay deposit in Italy that contained
fossil wood in various stages of petrifaction. After examining
the fossil wood Hooke came to the conclusion that the wood
had soaked in mineral solutions that filled the pores and
turned to stone (Cutler, 2003, p. 131). Thus, rock layers
did not generate petrified wood that transformed into wood;
rather, once living wood buried in sediments turned to
stone as minerals replaced the once living material of
Hooke, like Steno,
championed the idea that fossils represented ancient plant
and animal life. Hooke and Steno helped to
jump-start the science of stratigraphy, making it possible
to answer the question, “what came first?” Steno
recognized that his principles could be used to reconstruct
the geologic past (Cutler, 2003, p. 114). The ideas of Hooke
and Steno would not be widely accepted for another century;
although, Carolus Linnaeus (1707-1778) treated fossil organisms
as if they were living organisms in his 1735 publication
Systema Naturae, which was an attempt to classify
life on Earth (Prothero, 2004, p. 7).
Deep Time & Extinction
Steno and Hooke argued for a naturalistic interpretation
of fossils. Perplexing questions remained, such as the
age of rock layers and the proper interpretation of the
unfamiliar organisms that appear in the fossil record.
James Hutton (1726-1797),
often considered the father of geology, developed the theory
of uniformitarianism, which states that geologic events
are caused by natural processes, many of which are operating
in our own time. Put another
way, the natural laws that we know about in the present have
been constant over the geologic past. Hutton argued that
many of Earth’s surface features were formed through
a slow cycle of land erosion, seabed deposition, and uplift.
Hutton’s theory of uniformitarianism and the principles
of stratigraphy would be fully developed and made popular
by another Scottish geologist Charles Lyell (1797-1875) with
his classic three volume work, first published from 1830
to 1833, entitled Principles of Geology: being an attempt
to explain the former changes of the Earth’s surface
by reference to causes now in operation (Levin, 1999, p.
Hutton formulated three principles used in the science of
stratigraphy that must be considered when seeking relative
dates for rock layers. The principle of inclusion states
that any inclusion is older than the rock that contains it.
An inclusion may be a fossil or rock fragment contained in
another rock. Care must be taken with this law as crystals
and concretions may be deposited by groundwater within already
formed rock. The principle of cross-cutting states that any
feature that cuts across a rock or sediment must be younger
than the rock or sediment through which it cuts. Examples
include fractures, faults, and igneous intrusions. Igneous
intrusions are sometimes referred to as a separate principle,
the principle of intrusive relationships. Unconformities
represent gaps in geologic time when layers were not deposited
or when erosion removed layers. This principle includes three
types of unconformities. A disconformity is an unconformity
between parallel layers. An angular unconformity exists when
younger more parallel strata overlie tilted strata. A nonconformity
is formed when sedimentary layers are deposited on igneous
or metamorphic rock.
The concept of geologic
time or deep time was a logical consequence of Hutton’s theory. In 1788 John Playfair (1748-1819), a Scottish geologist and mathematician,
came to see Hutton’s Unconformity in Inchbonny. The
angular unconformity at Siccar Point in Eastern Scotland
consists of many vertical tilted layers of grey shale overlaid
by many layers of horizontal red sandstone. Playfair later
commented that, "the mind seemed to grow giddy by looking
so far into the abyss of time." McPhee (1998) points
out that Hutton removed humans from a specious place in time
just as Copernicus had removed humans from a specious position
in the universe (p. 74).
the permanent disappearance of a species, is an all too
familiar concept. In recent history we have
witnessed the extinction of many species due to human
activities of hunting and habitat destruction. As little
as 200 years ago many people thought extinction was impossible.
Theology fueled opposition to the idea of extinction. People
believed that the biosphere constituted a perfect creation.
The concept of extinction was contrary to several deeply
held Christian beliefs. First, was the concept of divine
providence. An all-powerful and all-loving God would never
allow any creature to become extinct. This is illustrated
in the story of Noah’s ark. God would not suffer any
species to become extinct and thus ordered Noah to populate
the ark with two of each creature. Second, was the idea of
the plenitude or fullness of nature. God’s creation
was perfect and an organism’s extinction would render
it incomplete. Finally, was the concept of a “Great
Chain of Being.” This chain linked animals to humans
to angels to God. Extinction would take away links in the
chain leading to its destruction (Prothero, 2004, p. 8).
Thomas Jefferson (1743-1826)
initially resisted the idea of the extinction of species.
In his “Notes on the
State of Virginia,” published in 1789, he denied that
the Mammoth is extinct on the grounds that the economy of
nature would not allow any break in nature’s “great
work” to be broken (Peterson 1975, p. 86). Jefferson
and many others argued that the strange organisms found in
the fossil record must still exist in unexplored parts of
the world. A large fossil claw prompted Jefferson to ask
Lewis and Clarke to look for a giant prairie lion on their
expedition. This claw was later found to be a part of the
extinct giant ground sloth. By the end of his life, however,
Jefferson had apparently become convinced of the fact of
extinction, for in an 1823 letter to John Adams, he clearly
accepts the idea that “certain races of animals go
extinct” (Adams 1983, p. 411).
The great French anatomist Georges Cuvier (1769-1832) established
extinction as a fact in a historic lecture given to the French
Institute in 1796. Cuvier talked about mammoths, woolly rhinos,
giant cave bears, and the sea reptile mosasaur in his lecture.
In his paleontological studies Cuvier came to recognize what
we now call mass extinctions at the end of the Permian and
Cretaceous periods. Cuvier developed the Theory of Catastrophism.
Cuvier believed supernatural cataclysms occurred before Noah's
flood (antideluvial) and were regional not global (Prothero,
2004, p. 82).
Stephen J. Gould (1941-2002),
American paleontologist and writer, believed that extinction
is needed to tell
is the motor of evolution. If all the species stayed the
same there would be no way to tell geologic time. Furthermore,
if species didn’t disappear there would be no room
for new ones to evolve. For example, during the reign of
the dinosaurs, mammals never got bigger than an ordinary
house cat. If the dinosaurs had not died out there is no
reason to believe that mammals would have increased in
size and, in that case, we would not be here (Infinite
It is ironic, that in the last two hundred years scientist
have gone from believing that extinction was impossible to
establishing that 99.9% of all plant and animal species that
have ever existed on Earth are now extinct.
The Fossil Record & A
History of Life on Earth
As already noted, Cuvier established the revolutionary idea
of extinction. It is said that Cuvier could identify the
remains of an organism from just a few bones. Several people
were key to ordering fossils chronologically and thus building
a history of life on Earth.
The Industrial Revolution
helped to enlarge our understanding of fossils and the
history of life on Earth. Large machines
used for digging coal, making railroad beds and canals removed
great volumes of earth exposing many rock layers and their
fossils. William Smith (1769-1839), a British engineer, was
in charge of building the Somerset Canal. Smith realized
that fossils exhibited a regular pattern in different strata.
Thus Smith could recognize a particular rock layer from the
combination of fossils present. This observation allowed
Smith to predict the locations of different rock layers making
him more efficient and successful when surveying for canal
construction. Smith was able to map out the succession of
fossils found in different rock formations. His geologic
maps showed that life forms appear and disappear through
time (Winchester, 2002, pp. 117-119).
At about the same
time, Cuvier and Alexandre Brongniart (1770-1847), a French
naturalist and geologist, were mapping
the Paris Basin. In reconstructing the changing sea levels
of the Atlantic Ocean, Brongnairt and Cuvier showed that
fossils had been laid down during alternating fresh and salt-water
conditions thus establishing the fact that there existed
a succession of fossils in different formations representing
different environments. Thus, Smith, Cuvier, and Brongniart
added another basic principle to the growing science of stratigraphy.
This is the principle of faunal succession which states that
fossil organisms succeed one another in a definite, irreversible,
and determinable order (Prothero, 2004, p. 8). Embedded within
the principle of faunal succession is the concept of an index
fossil. Good index fossils possess several characteristics
that make them excellent tools for determining the age of
the rock layer in which they are found. Index fossils are
abundant and have a wide geographic distribution, that
is, they are found in many locations. Index fossils are
easy to identify even when the specimens are incomplete.
index fossils existed for only a short geologic time. Thus,
index fossils help to pinpoint the age of a geologic formation
with precision. Index fossils are often used to correlate
the age of related formations.
Cuvier noticed that
the more ancient a fossil the less it resembled present
day organisms. In ordering fossils
Cuvier, like Smith, was constructing a history of life on
Earth using geologic strata. Thus began the science of biostratigraphy.
Smith did not know why each unit of rock had a particular
fauna. Cuvier was opposed to early theories of evolution
and viewed faunal succession as evidence for a cycle of creation
and extinction known as the Theory of Catastrophism. Cuvier's
contributions to our understanding of geologic time, extinction,
and fossil vertebrates were essential in developing the concepts
of deep time and evolution. And yet, as Michael Benton (2001),
an English paleontologist, points out Cuvier himself was
unable to ". . . make two vital connections: between
extinction and evolution, and between geological change and
time (p. 99)." Later, the work of Charles Darwin (1809-1882), the great English naturalist and geologist, would make
it possible to see that rock units of different ages contain
different assemblages of fossils because life has evolved
One of the chief legacies of these 19th century efforts
is the geological time scale. The present day scale or column
divides geologic time into intervals separated from each
other by changes in rock type and abrupt changes in fossil
groups. Gould believed the geologic time table to be one
of the greatest contributions to human understanding. According
The establishment of a time scale, and the working out of
a consistent and worldwide sequence of changes in fossils
through the stratigraphic record, represents the major triumph
of the developing science of geology during the first half
of the nineteenth century....By 1850, geology had developed
a coherent global chronology based on life's history. This
discovery and construction of history itself must rank as
the greatest contribution ever made--indeed, I would argue,
ever makeable--by geology to human understanding (Gould 2001,
The science of stratigraphy
allowed geologists to work out the spatial and temporal
relationships of rock layers
possible a relative time scale. The biogeographical distribution
of species through time and space revealed by this work would
be a critical influence on Charles Darwin’s Theory
of Evolution by Natural Selection published in 1859 (Darwin,
1859/2009, pp. 223-225). In the 20th century scientists would
develop and apply radiometric dating which places absolute
dates on the relative time scale. Whereas relative dating
only specifies the chronological sequence of events absolute
time is measured in units such as years. Radiometric dating
confirmed and reinforced the consistency between deep time, relative time and
Darwinism (Miller, 1999, pp. 68 & 69).
The science of stratigraphy changed our view of the world.
Where before the world was viewed as static, now it is
seen as dynamic and changing. Fossil deposits of different
ages reveal that different organisms have lived at different
times. The rock in which these fossils are embedded is
geologic truth, speaking to the fact that environments
change. The fossil record affords only pieces of the past.
Science has learned to use these pieces to work out the
evolution of life on Earth using a system of independent
empirical verification. Together, impressions of the past
explored by this most important human epistemology work
out to be a way for nature to remember itself.
Cuvier not only established extinction as fact, he was
also the first to recognize that mass extinctions have
occurred at the end of what we now call the Paleozoic and
Mesozoic eras (Stanley, 1987, p. 2). Extinction is the
total disappearance of a species and is represented by
the contraction of the geographical range of the species
and the reduction of the population to the number zero.
This contraction and reduction is governed by limiting
factors. Limiting factors include the physical environment,
competition, predation, and chance factors. Climate is
one of the most important environmental factors (Stanley,
1987, p. 10).
In 1973, Leigh Van
Valen (1935-2010), an American evolutionary biologist, published a study that compared the duration of certain
groups of organisms against the
number that survived. He found that species do not become
at avoiding extinction as they persist through time; old
species have the same probability of becoming extinct as
young ones. He inferred from this data that organisms can
never be perfectly adapted as environments are not static.
Thus, natural selection enables organisms to maintain not
improve adaptation. Van Valen called this the Red Queen
Hypothesis. The Red Queen in Lewis Carrol’s Alice
Through the Looking Glass told Alice that she must keep
running to stay in the
same place. Thus, species must constantly evolve to avoid
extinction (Milner, 1990, p. 387; Prothero, 2004, p. 86).
In 1982 David Raup (1933-2015) and John Sepkoski (1948-1999), American paleontologists,
plotted the number of extinctions in marine invertebrate
and vertebrate families during the last 560 million years.
They discovered a steady background rate of 2 to 4 family
extinctions per million years. However, five intervals stood
out in which 10 to 20 families became extinct per million
years. They also identified 10 mass extinctions of the second
order over the last 600 million years (Stanley, 1987, p.
What is the nature
of these five extinctions and how do they differ from background
extinctions? In 1986 David Jablonski (1953-), an American geophysical scientist,
published a study comparing the extinction of Cretaceous
aged molluscan species with different larval developments.
In one type of development larvae feed while floating on
the ocean for weeks, thus attaining a wide geographic distribution.
In the second type of development larvae do not feed and
float for hours, days or not at all. During background
extinctions the mollusks with a wider geographic range
were more extinction
resistant. However, both groups suffered equally during
the terminal Cretaceous extinction (Stanley, 1987, p. 17).
study suggests that during mass extinctions organisms with
extinction-resistance qualities, such as wide geographic
distributions, are just as likely to become extinct as
those without these properties. Thus, mass extinctions
be fundamentally different from normal background extinction.
Looking for the causes
of mass extinction has fired the imagination of the public
and scientists from various backgrounds.
Steven Stanley (1941-), an American paleontologist and evolutionary biologist, in his book Extinction identifies five themes in mass extinction (1987, pp. 17 & 18).
occurs on land and sea.
the land, animals suffer extinctions repeatedly while
plants seem to be more extinction resistant.
is preferential disappearance of tropical life forms
in mass extinction.
groups experience extinction repeatedly (trilobites &
themes identified by Stanley
may imply a common agent or agents
Theories of Extinction & Seeking
A multitude of factors that are associated with or might
contribute to mass extinction have been put forward. A
brief overview of proposed agents of biological catastrophe
is in order.
Glaciation occurs as a result of global cooling. Much of
the Earth’s water can be locked up in ice sheets
that expand over oceans and land. Evidence for glaciation
comes from deposits containing glacial sediments and the
disappearance of warmer climate species from the fossil
record. Global cooling and the drop in sea levels obviously
disrupt many ecological niches. Ocean salinity and oxygen
content may also change during periods of glaciation. A
quick cooling event may also result in an overturn of ocean
water. Cold, nutrient rich, but oxygen poor water may be
brought to the surface. This water may be toxic to benthic
life in shallow warmer waters. It is clear that glaciation
is associated with climate change.
In 1980 Luis Alvarez (1911-1988), an American physicist, hypothesized that a large extra terrestrial
impact had caused the great Cretaceous extinction. A large
asteroid could trigger global fires, earthquakes, tidal
waves, atmospheric dust, acid rain, and global warming.
Atmospheric dust could cause a nuclear winter in which
the Sun’s light is blocked out to such an extent
that plants have problems photosynthesizing. Evidence for
the Late Cretaceous impact comes from the presence of a
rare element called Iridium found in a layer at the K-T
boundary (Cretaceous-Tertiary boundary). Iridium is rare
in Earth’s crust, but can be common in asteroids
and volcanoes fed by the Earth’s mantle. Shock quartz
or quartz grains that are formed from high pressures are
also found in these layers as well as a form of carbon
formed under intense heat and pressure. Finally, in 1981
a large crater 65 million years old and of the correct
size to fit Alvarez’s theory was found in the Yucatan
Peninsula of Mexico. The name of this crater is Chicxulub.
Some have suggested that asteroids could cause a large
distribution of the element nickel which can prevent plants
Sea level changes known as marine regressions can cause major
disruptions in ecological niches. Sea level changes can
be caused by the movement of the Earth’s crustal
plates. As two plates come together seaways can slowly
drain away. Sea levels drop and rise as glacial periods
come and go. Sea level changes can also affect the salinity
and gas content of the water.
Volcanic activity can fill the air with large volumes of
dust and gases causing climate change.
Carbon dioxide and sulfur dioxide emissions from volcanic activity act as greenhouse gases. However, sulfur dioxide quicky reacts with moisture in the air forming sulfate aerosols that absorb and scatter sunlight, which can cause global cooling (Wignall, 2001, p. 2). As with asteroids, volcanic activity can produce iridium.
In the geologic past there have been extremely large accumulations of intrusive or extrusive igneous rocks within a short geologic time, a few million years or less. These large eruptions of mostly basaltic (mafic) magmas are known as Large Igneous Provinces (LIP) and are unrelated to normal sea-floor spreading and subduction. LIP's occur as continental flood basalts, oceanic flood basalts, ocean ocean plateaus, and volcanic rifted margins (Ernst, 2014).
Methane clathrate is a solid similar to ice in which large amounts of methane are trapped within the crystaline structure of water. Deposits of methane clathrates are found in sedimentary structures at shallow depths under cold or deep oceans and in continental polar permafrost regions. During global warming episodes the release of methane from these repositories could significantly contribute to the warming trend (Wignall, 2001, p. 14).
Cosmic radiation could increase to dangerous levels from
a nearby supernova. This cosmic radiation could cause mutation
and increased cancer rates among organisms.
The idea that mass extinction occurs at regular intervals
is heavily debated. There have been many attempts to explain
the proposed periodicity of mass extinctions, these include:
comet showers, the existence of a planet X, the existence
of a companion star to our Sun called Nemesis, sudden overturns
of the Earth’s mantle causing pulses of volcanism,
Earth’s oscillation through the Milky Way galactic
plane, meteor impacts, basalt floods, climatic cooling,
marine regressions and species-species interactions (Interactions
between species may occasionally lead to an instability
that cascades through an ecosystem). Some believe that
these periods between extinctions just represent the time
it takes for extinction sensitive species to evolve (Stanley,
1987, p. 215). Some of these ideas, such as planet X, a
companion to our Sun, and sudden overturns of the Earth’s
mantle have been discredited (Prothero, 2004, pp. 93 & 94).
However, as we look at the “Big Five” we will
see evidence pointing to some of these proposed causes.
The Five Major Mass Extinctions
Stanley defines mass extinction as “the extinction
of many taxa on a global scale during a brief interval of
geologic time” (Stanley 1987, p. 238). Five intervals
of extinction stand out as the most devastating. Scientists
look for common patterns within these events in the hope
of developing a general theory of extinction. We will give
a brief overview of the effects and possible causes for the “Big
The marine ecosystems experienced extinction on a global
scale towards the end of the Ordovician period. The Ordovician
extinction may be second only to the mass extinction that
would end the Paleozoic Era. Heavy extinction occurred
in the reef communities. Graptolites, bryozoans, brachiopods,
nautiloids, and trilobites were especially hard hit. Nearly
25 percent of all animal families were wiped out (Selden & Nudds,
2004, p.36). Over fifty percent of trilobite families went
extinct (Nudds & Selden, 2008, p. 69). There is evidence
of glaciation and with this a lowering of sea level, and
the expansion of cold water adapted species to lower latitudes
as the Ordovician
extinction event unfolded (Stanley, 1987, pp 71-75). An ocean
turn over may have accompanied the cooling event bringing
deep ocean water to the surface, which would have been toxic
to the sensitive shallow marine benthic community. The Ordovician
event took place over a span of 2 million years (Prothero,
2004, p. 90).
The mass extinction that occurred in the Late Devonian affected
mainly the marine environment with terrestrial plants escaping
the crises. It is estimated that up to 75% of marine species
and 50% of marine genera were lost (Prothero, 2004, p.
90). Brachiopods, trilobites, conodonts, ammonoid, corals,
and stromatoporoids were hit hard. Reef building communities
were decimated. Tabulate corals and stromatoporoids would
never again be major reef builders after the Devonian crises.
The rest of the Paleozoic would see very little reef building.
Reef building would recover in the Mesozoic with the appearance
of modern corals (Stanley, 1987, pp. 78-79). The Devonian
crisis seems to be correlated with cooling. Coral reefs
were in decline as cold water glass sponges expanded. Shallow,
warm water marine species declined. Freshwater fish that
were adapted to seasonal environments survived, while warm
water marine fish experienced heavy extinction. As these
shallow warm water species declined the stromatolites had
a small resurgence in reef building. There is evidence
of glaciation and lower sea levels. Both the Ordovician
and Devonian cooling events may be tied to the movement
of Gondwanaland over the South Pole (Stanley, 1987, pp.
86-89). The cooling event may also explain Late Devonian
carbon and oxygen isotope anomalies. A severe cooling would
trigger a massive overturn within the ocean. This overturn
would bring deep ocean water to the surface. The deep ocean
water is nutrient rich, but cold and oxygen poor. The Devonian
crises lasted for 4 million years (Prothero, 2004, p. 90).
The Permian period ended with the largest recorded mass extinction
that hit both aquatic and terrestrial environments. It
is estimated that 75 to 90 percent of all living species
became extinct over a period of 10 million years (Stanley,
1987, pp. 96-97). Sixty percent of marine families became
extinct (Palmer, 1999, p. 90). In the marine realm crinoids,
brachiopods, bryozoans, and ammonoids were hit hard. Fusulinids,
trilobites, graptolites, blastoids, rugose corals, tabulate
corals, and eurypterids met with extinction. Among the
fish Acanthodians and Placoderms became extinct. Rhipidistians,
lobe-finned fish (Osteichthyes) that are the ancestors
of land vertebrates also went extinct. Extinction in the marine realm marked a change from a Paleozoic dominated fauna composed of crinoid, coral, bryozoan, and brachiopods to a modern fauna dominated by bivalves, gastropods, and echinoids (Prothero, 2004, p. 86).
of the amphibian and reptile families met with extinction.
The larger terrestrial vertebrates did not fare as well. Thirty-three percent of amphibian
families went extinct at the end of the Permian (Palmer,
1999, p. 90). Among the amphibians some labyrinthodonts would
survive into the Triassic. Lepospondyls (Lepospondyli) amphibians
went extinct by the end of the Permian. All but one group
of anapsid type reptiles died out. The fossil evidence for
diapsid reptiles is sparse during the mid Permian, although
many new groups make their first appearance during the late
Permian. The most primitive groups of diapsids went extinct
at the end of the Permian (Dixon, 1988, p. 84). The first
synapsids were the pelycosaurs, which made up 70% of the
vertebrate terrestrial fauna in the early Permian. During
the middle Permian another group of synapsids, the therapsid,
would evolve and displace the pelycosaurs. Pelycosaurs died
out in the middle Permian. Therapsids would loose 21 families
at the end of the Permian (Palmer, 1999, p. 90).
the first time insects suffered a mass extinction. Many
of the primitive
orders of insects went extinct during the
Permian event. Among the fixed-winged insects (Paleoptera)
the following orders went extinct: Palaeodictyoptera, Megasecoptera,
Diaphanopterodea, and Protodonata. Among the folded-winded
insects with incomplete metamorphosis (exopterygota Neoptera)
the following orders went extinct: Protorthoptera, Caloneurodea,
Protelytroptera, and Miomoptera) (Carpenter & Burnham,
1985, p. 302). Insect fossils found after the Permian belong
mostly to modern insect groups.
plants experienced their greatest losses during the Permian
Only 9 out of 22 known families survived
into the Triassic (Cleal & Thomas, 2009, p. 209). As
noted earlier, the swamp forests of the Carboniferous
contracted during the Permian. As the clubmosses waned, ferns
and primitive conifers expanded to take their place. The
change from Paleophytic to Mesophytic flora occurred over
a period of 25 million years. Tropical plant ecosystems suffered
major disruptions with some extinction at the end of the
Permian period. Cordaites went extinct as well as the seed
fern Glossopteris. The dominant conifer families (Walchiaceae,
Ullmanniaceae, and Majonicaceae) of the time went extinct.
For a geologically short time, woody coniferous forests were
replaced by herbaceous species of clubmosses and quillworts
(4-5 million years). In the Triassic, woody coniferous forests
of a different type would be reestablished (Kenrick & Davis,
2004, p. 154).
Uranium-lead zircon geochronology has been used to date ash layers, associated with the Siberian Traps, at the Permian-Triassic boundary in Southern China. The results establish a date of 251 Ma (Wignall, 2001, p. 8). The extinction interval is thought to be very short on the order of 165,000 years or less (Prothero, 2004, p. 87). What caused the "mother of all extinctions?"
An increase in dune deposits, evaporite salts, and a lack of coal forming swamps may indicate arid conditions in some terrestrial environments. There is evidence of a marine regression, which would reduce habitat in shallow marine environments. A rapid warming trend occurred at the end of the Permian. A shift in oxygen isotopes may record this event. An increase in O-16 over O-18 in the calcite skeletons of marine organisms indicates global temperatures may have increased by as much as 6 degrees Celsius. An increase in C-12 found in terrestrial and marine sections could be an indication of increased volcanic activity and massive death in the marine and terrestrial realms (Benton, 2003, p. 38). Like the Ordovician and Devonian events a reduction in the
formation of marine limestone and reef building occurred
after the Permian extinction. Layers containing abundant pyrite above the limestone layers indicate a low oxygen environment.
Onset of flood basalts making up the Siberian Traps occur at the Permian-Triassic boundary. This LIP formed in northern Asia and may have been the source of carbon dioxide that started a global warming event. As the climate warmed methane may have been released from methane clathrates accelerating the warming event. The release of these gasses into the atmosphere
is called the "big belch" and may have increased
temperatures and lowered oxygen levels (Cleal & Thomas,
2009, p. 209). Climate change may have also altered oceanic
circulation in such a way as to bring stagnant
deep water rich in carbon dioxide and hydrogen sulfide to
the surface. The Permian crises would usher in a new era represented
by different flora and fauna evolved from the small percentage
of survivors who were, at first, cosmopolitan in their distribution.
The Triassic ended with mass extinctions in marine and terrestrial
environments. The terrestrial extinctions took place millions
of years before the marine crises. The Triassic crisis
is actually several extinctions that took place over a
17 million year time span (Prothero, 2004, p. 91). Labyrinthodont
amphibians and dicynodonts (a group of mammal-like reptiles)
went extinct. Land plants were hit hard, especially the
gymnosperms with 23 of their 48 known families going extinct
during the last third of the Triassic (Cleal & Thomas,
2009, p. 211). In the marine realm placodonts, nothosaurs,
and conodonts went extinct. Ammonoids, brachiopods, gastropods,
and bivalves took heavy losses. It is estimated that 20%
of marine families went extinct during the Triassic crises.
Reef growth was greatly reduced as well as marine limestone
and dolomite deposition.
What caused the end-Triassic extinction (ETE) 201 million years ago? There is some evidence for sea
level changes and some cooling. An abundance of black shales
and geochemical anomalies indicate massive oceanic changes.
Some believe the rifting of the North Atlantic may
have released large volumes of volcanic gasses contributing
to global climate change (Prothero, 2004, p. 91). The Central Atlantic Magmatic Province (CAMP) is a LIP that formed from the rifting of Pangea and spans the Triassic-Jurassic boundary. CAMP is associated with the breakup of the supercontinent Pangea and the formation of the Atlantic ocean basin. At an estimated 11 million square kilometers CAMP covers the largest area of any known LIP. It is also one of the most voluminous at an estimated 2 to 3 million cubic kilometers.
Remnants of CAMP are found on four continents including North America, South America, Europe, and Africa. Using samples from these remnants Blackburn et al. (2013) demonstrated that zircon uranium-lead geochronology provides a temporal link between the ETE and CAMP. The release of magma and associated atmospheric flux occurred in four pulses over 600,000 years. The earliest known eruptions took place at the same time as the extinction events. Further pulses of CAMP occurred as life was recovering from the extinction event. Although a temporal link between early pulses of CAMP and ETE has been established, we still do not understand the details of how these massive eruptions induced a global biological crises (Blackburn, 2013, p. 943). As a group, dinosaurs benefited
from this extinction event, as they would undergo a great
adaptive radiation during the Jurassic period.
At the end of the Cretaceous, 65 million years ago, 85% of
all species would go extinct, making this event second
only to the Permian mass extinction (Hooper Museum, 1996).
Sixteen percent of marine families went extinct. Ammonoids,
belemnoids, rudist bivalves, inoceramid bivalves and many
brachiopod groups went extinct. Most of the large marine
reptiles (ichthyosaurs, plesiosaurs, and mosasaurs) were
lost. Some families of sharks and teleost fishes went extinct.
Eighteen percent of terrestrial vertebrate families would
go extinct (Siegel, 2000). Dinosaurs, pterosaurs, many
lineages of early birds, and some mammals went extinct.
In fact most terrestrial animals more than 1 meter in length
would go extinct (Nudds & Selden, 2008 p. 169). One
third of higher level plant taxa went extinct and for a
short time ferns became dominant over the angiosperms and
conifers in North America (Stanley, 1987, p. 157). Some
of these organisms mentioned went extinct before the K-T
(Cretaceous-Tertiary) boundary, while others were on the
decline. Some groups disappeared catastrophically right
at the KT boundary. Some interesting ecological patterns
can be observed.
The hardest hit marine organisms were free-swimming or surface
forms (plankton, ammonites and belemintes). On the sea floor
filter feeders (corals, bryozoans, and crinoids) were hit
hard while organisms that fed on detritus were little affected.
Open water fish fared well. Mollusks with wide geographic
ranges had a higher survival rate than those with a small
geographic distribution. Tropical species were affected more
than those who were cold tolerant. In the terrestrial realm,
as we have already mentioned, being large was a disadvantage.
The only large land animals to survive were crocodilians
(Benton, 2005, pp. 248-251). Amphibians seem not to have been
affected by the extinction event. At the family level, 70
to 75% of taxa surived the event (Benton, 2005, p. 255).
What contributed to this mass extinction?
Scientists at the University of California at Berkeley including
Luis and Walter Alvarez, Frank Asaro, and Helen Michel discovered
an iridium anomaly in a fine-grained clay layer in several
K-T (Cretaceous/Tertiary) boundary sites around the world
(now the Cretacous/Paleogene boundary or K-Pg). These K-T
boundaries are found in both marine and terrestrial deposits
and show the same succession, an ejecta layer followed by
the clay enriched iridium layer (Benton, 2005, p. 250). The
group recognized that iridium is abundant in stony meteorites
and proposed that the fallout from a meteorite on the order
of 10 kilometers could explain the anomaly and possibly the
extinction event. Subsequently, a crater was found beneath
the Gulf of Mexico off the Yucatan Peninsula during exploration
for oil. The Chicxulub crater is of the right size and age.
Volcanic activity may also act as a source of iridium. The
Deccan Traps in India represent a large terrestrial flood
basalt. Ironically, the Deccan Traps would have been positioned
on the opposite side of the Earth at the time of the Chicxulub
There is also evidence for climatic changes as well as floral
and fauna changes leading up to these events. Many organisms
were already on the decline during the Late Cretaceous. Planktonic
foraminiferans experienced major losses before the end of
the Cretaceous. Calcareous nonoplankton were also on the
decline. Ammonoids, inoceramid bivalves, and the reef building
rudists experienced attrition. Multiple lines of evidence,
including preferential survival of cold water tolerant organisms
and isotopic ratios, suggest the climate was cooling. There
is also evidence to support a decline in abundance and diversity
of dinosaurs (Stanley, 1987, pp. 133-171).
However, the iridium anomaly, which in some areas is also
associated with shocked quartz grains (quartz grains that
bear criss-crossing lines produced by meteorite impacts),
glassy spherules close to the impact site (produced from
melted material under the crater and then ejected into the
air), carbon particles associated with massive fires, the
spike in ferns (associated with ash falls), and the Chicxulub
crater support that a meteor impact may have caused a final
pulse of extinction that occurred on a global scale. Whether
this mass extinction was the result of multiple factors or
primarily one, its effects on the evolution of life had great
The largest mass extinction at the end of the Permian period
provided reptiles with the opportunity to become the dominant
vertebrate life forms on Earth. Roughly, one hundred and
eighty-six million years later the second largest mass extinction
would take away Mesozoic reptilian dominance and usher in
the Cenozoic, an age for mammals. Mass extinctions of the
past have severely reduced biodiversity, but ironically have
also provided opportunities for survivors to evolve and diversify.
of extinctions can be seen within each mass extinction
event. The timing
and duration of these pulses appears to
be different for each of the “Big Five.” This
may indicate that there is not a common cause for mass extinction
(Prothero, 2004, p. 93). It is clear that climatic cooling,
marine regressions, large igneous provinces, and at least one of the
many meteor impacts recorded in the geologic record can be
implicated in mass extinction events. The study of extinction
is in its infancy, but has major implications for our own
A Sixth Mass Extinction?
The study of mass extinctions and their causes is important
because it allows humans to look to the past to anticipate
future possibilities. Programs designed to monitor extraterrestrial
objects that may impact Earth, such as NASA’s Near
Earth Object Program are steps to possibly diverting such
objects (NASA, 2013). USGS monitors volcanic activity around
the world (USGS, 2008). At this point the monitoring of
volcanic activity can save lives in local areas. The monitoring
of both local and global threats is in its infancy; however,
these efforts are informed
by knowledge of past events deciphered from the geologic
Many scientists argue that we are in the midst of a sixth
mass extinction as revealed by contemporary extinction rates
that are on the order of 100 to 10,000 times greater than
background rates calculated from the fossil record (Holsinger,
2011, p. 8; Center for Biological Diversity, 2013). The sixth
mass extinction event may have started in the Pleistocene
and continued in the Holocene with the loss of mammals known
as megafauna roughly 50,000 to 10,000 years ago. Both climate
change and the proliferation of humans are thought to be
factors in the extinction of megafauna.
the Holocene extinction event continues with humans playing
an ever larger role. The global human population
is now at 7 billion (Worldometers, 2013). Humans need resources
and space for their growing populations. The quest for these
needed resources and space play a role in this possible
sixth mass extinction. Habitat destruction,
invasive species, pollution, burning fossil fuels, and commercial
hunting place incredible stress on ecosystems worldwide.
Alterations to the environment in North America since 1600
illustrate how humans can affect entire ecosystems. In the
lower 48 states since the year 1600 over 90% of old growth
forests have been cut down, over 50% of wetlands have been
drained, and over 98% of grasslands have been plowed under
(EPA: Wetlands, 2013; EPA-Smart Growth, 2013; Global Deforestation,
2010; Pieper, R.D. (2005). Similar patterns of habitat destruction
can be found worldwide (Holsinger, 2011, p. 5).
As humans have spread over the globe a massive biotic exchange
has taken place. Humans have directly and indirectly introduced
non-native species to many ecosystems. Non-indigenous species
that have an adverse affect on the ecosystem to which they
are introduced are referred to as invasive species. The introduction
of invasive plants, animals and disease has had devastating
effects on both human and non-human populations across the
globe (Crosby, 2003; National Invasive Species Information
Center, 2013; Natural History Museum 2013).
use of fossil fuels is having a global impact on the
atmosphere and climate.
The burning of fossil fuels has released
large amounts of carbon dioxide thereby increasing the amount
of greenhouse gases in the atmosphere. The rise in greenhouse
gasses can be correlated with an increase in Earth’s
surface temperature (EPA-Causes of Climate Change, 2013).
Coal burning power plants and the combustion of gasoline
in cars contribute to the formation of acid rain. Our massive
use of fossil fuels leads to other kinds of pollution, such
as oil spills.
fact, the industrial revolution has created human societies
that consume Earth’s resources at increasingly
alarming rates. Waste products resulting
from this massive consumption often
pollute the environment. Humans create government agencies
in an attempt to manage the production and disposal of waste
products that pollute the environment (EPA, Home 2013).
cover 71% of the Earth’s surface and are not
immune to the effects of human activity. Many marine ecologists
rank commercial overfishing as the greatest threat to our
oceans (DUJS, 2012). Humans even try to keep track of the
threats posed to Earth’s biological diversity and attempt
to organize solutions with governments worldwide (IUCN, 2009).
In this fact, there is hope.
A naturalistic interpretation of rock and fossil genesis
allowed us to
decode principles that govern the formation of the geologic
record. Applying these principles through
a system of independent
empirical verification has afforded us a glimpse of the
3.8 billion year history of life on
Earth, a history
that includes our own origins.
Modern humans (Homo sapiens) range from 160,000 years ago
to the present (Benton, 2005, p. 385). The Cro-Magnon
people of Europe (40,000-30,000 years ago) are associated
Paleolithic tools, carved art objects, and the
cave paintings of France. Archaic H.
sapiens increasingly used
and wood to make more sophisticated tools. Benton
(2005, p 387) recognizes major benchmarks in human evolution:
Brain (3-2 Ma)
Tools (2.6 Ma)
Geographic Distribution (2-1.5 Ma)
of Fire (1.5 Ma)
(35,000 years ago)
(10,000 years ago)
can include two more benchmarks, the awareness of the
evolutionary history of life on Earth as revealed
(200 years ago) and the awareness of our own species
role in a mass extinction event (50 years ago).
or not evolution
Is there a trend
in detail the
of each group
exhibits an evolutionary
is very bushy.
The branch that
ends with humans
is only one of
however, it is
special in that
a way for the
universe to know
humans are the
the history of
life on Earth.
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