History of Life Exam 2

stratigraphy

the study of sequences of layers of rock

• only gives us the relative ages of layers of rock

biostratigraphy

dating rocks by the successive types of fossils appearing in layers of rock

• only gives us the relative ages of layers of rock

radioactive dating

• C14 half life is only 5,730 years, only good for relatively recent dates (prehistoric campfires ex.)

• Uranium is most useful for dating the “big picture”, slowly decays to lead (Pb206) with a half life of 4.5 billion years

• (uranium is the isotope that’s used to date rocks because C14 doesn’t go back that far)

chronostratigraphy

absolute ages of rocks, as agreed upon by an international team of geologists

Geological time is divided into

• eons

  • Hadean (origin of sun to oldest surviving rocks)

  • Archean (first life)

  • Proterozoic (early life)

  • Phanerozoic (abundant life)

• Eras (Phanerozoic = Paleozoic, Mesozoic, Cenozoic)

• Periods (Mesozoic = Triassic, Jurassic, Cretaceous)

• Epochs (Cenozoic = Eocene, Oligocene, Pleistocene, etc.)

Boundaries between eons, eras, periods, are usually determined by an unconformity. Define unconformity.

visible break or transition in an orderly sequence of rocks

Precambrian

This is not a single period but a "supereon" that covers the vast majority of Earth's history, from its formation about 4.6 billion years ago until the beginning of the Cambrian period around 538.8 million years ago. The Precambrian is further divided into the Hadean, Archean, and Proterozoic eons

• Hadean – beginning of Earth to end of bombardment

• Archaean Eon - first life - 3. 9 bya to 2.5 bya

  • Prebiotic evolution

  • First prokaryotes (anaerobic)

  • First photosynthetic bacteria

• Proterozoic - early life - 2.5 bya to 600 mya

  • First aerobic prokaryotes

  • First eukaryotic cells

• Phanerozoic - abundant life - 600 mya - date

  • First multicellular animals

Directly beneath the Cambrian rocks

• a very thick layer with no visible fossils

Ediacaran

This is the last period of the Precambrian's Proterozoic Eon, lasting from about 635 to 538.8 million years ago. It is known for producing some of the earliest evidence of complex, multicellular life.

Cambrian

This is the first period of the Phanerozoic Eon and the Paleozoic Era, beginning approximately 538.8 million years ago. It is defined by the "Cambrian Explosion," a time of significant diversification and proliferation of new animal life.

• First bigger organisms

• First predators

• Most modern phyla

• All aquatic

• these rocks are filled with an abundance of fossils- swarms of trilobites, mollusks, arthropods…

• Suddenly find most major phyla of animals in place -  annelids, molluscs, arthropods, ancestors of all modern animals, including chordates

Cambrian Explosion

This spectacular divergence of body plans only took ~ 30-50 million years. Not one major new phylum has evolved since then, though several have gone extinct. This appearance of diverse and complex animal life, in a relatively short period of time. 

The Cambrian Explosion is in the ___ Eon

Phanerozoic (abundant life)

What marks the boundary between the Cambrian and the Precambrian?

The farther back in time we go, the fuzzier these boundaries become. The line between no animal life and an abundance of animal life, however, is sharply drawn in the rocks

Burgess Shale

fossil bed exposed in the Canadian Rockies of British Columbia, Canada discovered by Charles Doolittle Walcott that contained

• arthropods, mollusks, worms, 

• contained Soft parts of organisms are incredibly well preserved

• contained Many creatures are unlike anything seen on Earth before or since, many seem unrelated to any living creature

how did all those animals pile up there?

• Animals lived in mud banks at the foot of a steep primitive reef, formed by thick mats of algae

• Mud banks were unstable, mudslides were probably common

• Entire community was suddenly swept away by an ancient mudslide (organisms were probably already fossilized then swept away)

• Found tumbled into a heap, lying every which way, one atop another

• Little or no oxygen or scavengers, fine-grained sediments

• Perfect conditions for exquisite preservation, even soft parts

anomalocaris

• One of the most unusual critters in the Burgess Shale

• the “terror of the trilobites”

• Mighty 3 feet long, the world’s first ferocious predator, great white shark of the Cambrian oceans

Were the animals of the Burgess Shale an anomaly and the decimation of those animals a major turning point in the history of life?

That was S.J. Gould’s thesis in his classic book on the Burgess Shale “Wonderful Life” - our ancestors survived by blind luck alone. Since then, we have discovered that the Burgess Shale is not unique. Now have about 30 sites around the world, including Greenland and mainland China with same bizarre fossils. These were the normal organisms of the Cambrian oceans. Similar Chinese and Polish Cambrian fauna are even older- 535 to 540 mya. All these diverse body plans appear to have evolved in a mere 5 -10 million years! Survivors of the Cambrian seas are the ancestors of all life on Earth.

The Cambrian Explosion—What was Earth like? what kind of organisms evolved?

• Thin air, hard to breathe

• The surface of the Earth was bare rock and dirt, no large plants

• A few colonies of algae and bacteria in shallow fresh water habitats

• World of green slime…

  • These are the first land plants!

• And then Non-vascular land plants appeared!

  • Maybe as early as 700 mya!

  • At least by 500 mya

  • Mosses

• In shallow ocean waters, algae and bacteria may have formed small reefs

• Deeper ocean was a dark and murky abyss with only anaerobic bacteria, high concentrations of hydrogen sulfide

Gondwana and Laurentia

• At the end of the Precambrian, the Earth was undergoing major changes.

• Breakup of the supercontinent Pannotia ~ 600mya and the formation of super continents ____ and ____

  • oceans covered 85% of planet

Precambrian life (Ediacaran world)

• Before ~ 650 mya, there are no definite traces of multicellular life in the fossil record

• occurred during the Proterozoic

• Small soft-bodied multicellular organisms

• No large fossils of any kind

• No evidence of passage of organisms through stromatolites - no trails of grazers, no tunnels of burrowers

  • several episodes of glaciation, at one point reaching almost the equator; decline of stromatolite communities world wide; stromatolites were almost gone, eaten away by armies of small grazing organisms

• Ediacaran fauna dates to ~585-550mya (Cambrian border is 543mya) (need to understand some of these animal groups)

• The extreme position is that these animals left no descendants

• Ediacarans seem to thin out and disappear before the start of the Cambrian Early mass extinction event?? Imperfections in the fossil record??

• Dickinsonia (oldest known animal fossil)

• At several sites, the fauna fades out well before the start of the Cambrian. This apparent gap between Ediacaran and Cambrian faunas was thought to represent the total extinction of Ediacarans

• Some sites (Namibia) the Ediacaran fauna ends much closer to the start of the Cambrian…

• Ediacaran fossil beds include numerous surface trails. They are the very first animal footprints on the Earth!

• a peaceful, tranquil world filled with slow moving grazers, or immobile filter-feeding animals

• The Ediacaran may have seen the rise of the world’s first visible ecosystem, with slow moving grazers, stalked filter -feeders, and a handful of tiny predators

Ediacaran fauna — Precambrian life

• dates to ~585-550mya (Cambrian border is 543mya) (need to understand some of these animal groups)

  • consists of a wide variety of curious creatures

  • mostly animals

  • hard to interpret these strange organisms, because they don’t really resemble modern animals

  • many leaf-shaped, “quilted” forms, appear to be assembled from long tubes

    • may be early colonial animals, like sea fans

  • Many disk-shaped forms, may be similar to modern Cnidarians (sea anemones, corals, jellyfish)

  • Not many worm-like fossils, or vertical burrows - worms were common in the Cambrian We do, however, find long grooves, tracks and traces of worms whose bodies were not preserved

  • most likely filter feeders, sessile, or sedentary (relatively inactive)

  • not clear ancestors of Cambrian fauna (controversial topic)

  • Ediacaran organisms turned out to be common, globally distributed

Dickinsonia

• a precursor of modern life

  • has been interpreted as a primitive type of flatworm, or an early segmented annelid worm

Small Shelly Fauna (SSF)

• About 540 mya, in the earliest Cambrian, a new type of fossil appears abruptly, in large numbers: small pieces of shell, mostly smaller than one-half inch

• marks the beginning of a new phase of animal life, the acquisition of hard parts

• Many of the fossils are complete animals, esp. primitive types of mollusks

  • Others are parts of larger animals, such as skeletal plates

punctuated equilibrium

• a pattern of fossils appearing abruptly and disappearing abruptly; New types of fossils often appear abruptly, and last through long periods of relative stability until they abruptly disappear

• SSF serves to demonstrate the pattern of how fossils appear as we trace the rocks through time

  • Sudden appearance may be an illusion, an artifact of the way the fossil record is formed

• Species would be most likely to evolve in small, isolated populations (need to know the background of these things)

  • Because of the small size of these “founder” populations, extremely few of these early individuals would end up preserved as fossils

  • Over time, as the population grew large enough, a few individuals would be preserved as fossils

  • It would appear as though they had instantaneously emerged into the world

  • Once they began to spread, they might drive less competitive species into extinction

  • One species seems to abruptly disappear (old), replaced by a very similar species (new)

  • So the fossil record shows a pattern of long periods of relative equilibrium (slow or no change), interrupted by bursts of speciation

What is punctuated equilibrium and what does it mean about evolution? Does it mean Darwin was wrong about slow, gradual change?

it is the abrupt appearance of organisms/fossils and abrupt disappearance of organisms/fossils. maybe evolution is not linear but it’s a slow, gradual process. Darwin was not wrong about slow, gradual change.

eggs and embryos

• discovery made by Shuhai Xiao

• Small balls discovered in fossil beds in Doushantuo, China, were thought to be colonial green algae

• Xiao determined that they were the eggs and embryos of animals

• Animals must have existed 570 mya, though we can’t determine the types of animals from these embryos. Suggests that complex animal life had a very ancient history

trilobites

• one of the earliest arthropod groups

• First appeared 521 mya and flourished for over 270 my!

  • Lived from Cambrian until Permian extinction 

• One of the most successful groups of all early animals

• Highly diversified and geographically dispersed.

  • Predators, scavengers, filter feeders (many different species)

  • came in all shapes and sizes and had many different species throughout the 270 million years that they existed.

  • They also exhibited interesting behavior

• Why so successful?

  • One of the first organisms to develop eyes, and really good eyes at that – independent from vertebrate eyes

    • Constructed of calcite, unlike modern arthropod eyes

    • Great color vision underwater

eyes

• evolution of the nervous system

• neurons developed as specialized electrical signaling cells in colonial and multicellular animals

• eventually simple nerve nets evolved in organisms like Cnidaria

  • followed by nerve chords in bilaterally symmetrical animals

cephalization

• evolution of head-like regions

• organisms evolved a control center for the nervous system— THE BRAIN

• allowed for more complex animals

list the theories of the Cambrian Explosion (have to be able to describe some— “was this idea a driving force for the Cambrian Explosion?”)

• Artifact of preservation - no real explosion

• Evolution of skeletons/hard parts (shells etc.)

• Critical oxygen threshold reached

• Evolution of better organs and systems

• Evolution of developmental (Hox) genes

• Evolution of sex

• Nutrient availability from deep sea upwelling

• Nutrient availability from continental shelves

• Snowball Earth

• Evolution of food webs, ecosystems

• Evolution of predators

• Light Switch hypothesis

Evolution of Hard Parts/shells (theory of Cambrian Explosion)

• certainly a major force driving the diversification of early animal life

• Shells fill many important functions

  • Protect soft bodied organisms from predators

  • Provide support for larger body sizes, more diverse functions

• Shells in the form of tubes buried in ocean sediments were also an important new niche

  • Vertical tubes suddenly appear in large numbers in Cambrian rocks

• Shell material also used for spines, rudimentary jaws (sea urchins ex.)

Oxygen Threshold Reached (theory of Cambrian Explosion)

• Increasing levels of atmospheric oxygen may have spurred the Cambrian Explosion

• Once hard parts were invented, many new evolutionary pathways opened up - skeletal systems, jaws, legs etc…

• The evolution of shells could not have occurred without an ample supply of oxygen

• Shelled organisms would have also required higher levels of oxygen to survive simply because of their shells

• Early animal bodies had to absorb oxygen through their skin (no respiratory organs)

• Shells would have covered large parts of the skin, reducing the diffusion of oxygen

• Trade off between reducing naked skin in favor of a protective shell would only make sense in high oxygen environments

• the rise in atmospheric oxygen is most widely accepted

  • Many of the other hypotheses proposed may have contributed to the incredible explosion of animal life in the early Cambrian

Evolution of organs and organ systems (theory of Cambrian Explosion)

Perhaps it was the evolution of multicellularity itself . More efficient communication, integration of cells into organs and organ systems would certainly have been a critical stage in early animal evolution

Evolution of Hox genes (theory of Cambrian Explosion)

• maybe it was a change in how many cells are organized into a developing organism

• Large-scale patterns of development are controlled by many interacting genes

• Most important are these genes

  • they determine how each section of the body develops, front to back

• are a group of genes that control the body plan of an embryo, and after the embryo forms, they determine the formation of appendages.

• when these genes are reordered or duplicated, you get things like new body plans

• Studies on arthropods have demonstrated that mutations in these genes are behind the incredible diversity of arthropod bodies

Evolution of Sex (theory of Cambrian Explosion)

• The evolution of sexual reproduction would have provided a tremendous boost to the overall rate of evolution

• Cell division that forms sperm and eggs is a special type of cell division called meiosis

  • Meiosis shuffles the parental chromosomes in several ways

  • Dramatically increases the amount of variation that can arise from a single mating

  • More variation, more raw material for natural selection to work on

• Can’t really say when eukaryotic cells evolved the ability to reproduce sexually…

• The first step to sex was evolving diploidy – having two sets of chromosomes

the evolution of sex (a theory of that caused the Cambrian explosion) would have involved…

• Diploidy

• Then evolution of meiosis

• Evolution of sperm and egg

• Evolution of reproductive structures/system

• Evolution of mating types and eventually separate sexes – not necessary for sexual reproduction

  • Hermaphrodites can still have sexual reproduction

Nutrient availability from deep sea upwelling (theory of Cambrian Explosion)

• Some theories rely on changes in environmental nutrients to explain the Cambrian Explosion

• Nutrients like phosphorous and calcium often limit the growth of organisms

• Evolution of shelled organisms would have drained the available supply

• We think this period was marked by intense tectonic activity (movement of the vast plates on which the continents sit) which caused ocean upwelling, releasing vast amounts of nutrients that had been buried in deep ocean sediments

Nutrient availability from continental shelves (theory of Cambrian Explosion)

• Some theories rely on changes in environmental nutrients to explain the Cambrian Explosion

• Nutrients like phosphorous and calcium often limit the growth of organisms

• Evolution of shelled organisms would have drained the available supply

• Another possibility is that changing sea levels flooded low-lying areas which created vast new continental shelves over “new” soils that had not yet been depleted of nutrients

climate cycles (in relation to snowball earth hypothesis)

• Several long-term planetary cycles contribute to a periodic cooling that leads to an Ice Age

• consider shining a beam of light on a surface

  • As you change the angle of the flashlight, you don’t change the total amount of energy that hits the top of the table

  • But you reduce the amount of energy that hits any one part of the table top - the beam becomes “smeared out” over a wider area

  • if more at an angle, light covers more area than wen straight on

• Now think about the tilt of our planet’s axis

  • The angle at which the suns rays strike part of the earth determines how much solar energy that part of the earth receives…

• the tilt of earth’s axis is not constant. it changes over a range of degrees

• the spin of the Earth’s axis is not perfectly circular

  • axis wobbles back and forth 

• Superimposed on these two long-term cycles (axial tilt and precession) is a third cycle - changes in the shape of the Earth’s orbit around the sun

  • Sometimes the orbit is nearly circular, sometimes elliptical - eccentricity of orbit

  • Changes in orbital shape also affect the amplitude of the effects of precession (reduce or enhance)

• basically: earth tilts at an angle but tilt changes; the way it’s spinning also wobbles and doesn’t stay the same either

• tilt, spin/wobble (precession), and shape of orbit (eccentricity) all effect the climate on earth and effects how much sun a portion of the earth is getting

• So the Milankovitch cycles explain the long-term climate pattern of our planet, and the timing of the glacial periods

  • Ice Ages are a normal part of Earth’s history

  • But in the Precambrian, we almost froze solid forever

Milankovitch Cycles (rebound from snowball earth)

• The three cycles that affect Earth’s climate.

  • earth’s tilt, wobble (precession), and shape of orbit (eccentricity—round or oval)

  • effect the climate on earth and how much sun a portion of the earth is getting

• Predicted that these cycles could cause variation in sunlight in the Northern Hemisphere by up to 20% - couldn’t prove it, no precision data

Snowball Earth Hypothesis (theory of Cambrian Explosion)

• the idea that the Cambrian Explosion was a rebound effect from a catastrophe which nearly ended life on Earth before it had a chance to begin -not once, but twice

• Milankovitch cycles (the three cycles that affect Earth’s climate and how much sunlight a portion of the earth is getting)

• proposes that Earth's history included periods when the entire planet was covered in ice. This was caused by a significant drop in greenhouse gases, leading to runaway glaciation. The theory suggests that volcanic activity eventually released enough carbon dioxide (CO2) to break the ice, leading to a rapid warming period that may have spurred the evolution of complex life

Rebound from Snowball Earth

• There is extensive evidence for glaciation in the Cambrian and later periods

  • This is completely normal…

• Our recent “Ice Age” was only the latest in a long series of similar ice ages, stretching back to the beginning of the planet

• the Milankovitch cycles

• what saved the planet?

  • greenhouse effect saved the planet from an untimely end

    • glass lets the heat and light through, but some of the heat (infrared) is reflected back inside by the glass

  • Our atmosphere acts like a gigantic pane of greenhouse glass

  • Some atmospheric gases like carbon dioxide (CO2 ) and methane (CH4 ) absorb and emit some of the infrared radiation released from the heated surface of the Earth

  • Gases like carbon dioxide and methane are called greenhouse gases

  • Increasing the atmospheric content of these gases raises the average global temperature

• FIRST SNOWBALL EARTH

  • CO2 buildup would have eventually leaked out, warming the atmosphere from the greenhouse effect

  • Rapid increase in temperature, CO2 would have been a banquet for photosynthetic organisms

  • Rapid increase in photosynthesis would have sharply increased the level of oxygen

  • Hence the banded iron deposits!

• The second glaciation (~ 600-800 mya) was equally severe

• This time, the rebound effect was the rapid evolution of multicellular animals

• what tamed the Ice Ages

  • The evolution of the lowly worm… tamed the 2nd snowball earth

  • Marine worms tunnel through ocean sediments, causing CO2 to escape from organic matter buried in the sediment

  • This liberated CO2 warms the atmosphere just enough to take the edge off the severity of the Ice Ages!

What caused the snowball earth?

• When glaciers advance and retreat, they leave several tell-tale traces:

  • Large parallel grooves and scratches in the rock

  • Stray boulders (erratics)

  • Terminal moraines

  • Sedimentary deposits called tillites

• Tillite deposits are made up of small pieces of sharply angled rock, resemble a rock version of particleboard

  • Tillite deposits are found across the globe at 2.4 bya and 600-800 mya

  • In both cases, the deposits extend almost to the equator

• Typical Ice Age, glaciers don’t extend past the temperate zones

  • Glaciers are generally confined to land, oceans remain open

  • In these “Snowball Earth” events, the planet almost froze completely

• Oceans may have been covered in pack ice 500-1500 meters thick!!

  • Average temperatures were -20 to -50oC (-4 to -58oF)

  • Life would have been restricted to a narrow belt along the equator, perhaps deep sea vent communities

• High Obliquity Hypothesis

• Zipper-Rift Hypothesis

  • Proposes two pulses of continental "unzipping“

    • Breakup of Rodinia

    • Splitting of Baltica from Laurentia 

  • The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography; this high ground could then host glaciers.

    • Created lots of basalt which soaked up all the CO2, dropping temperatures

• Extreme amount of ice and snow could have triggered a potentially catastrophic feedback

  • Would have reflected more and more light/heat back to space, making even more ice and snow…

High Obliquity Hypothesis

A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's axial tilt was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce. (might have caused snowball earth)

Zipper-Rift Hypothesis

• (might have caused snowball earth)

• Proposes two pulses of continental "unzipping“

  • Breakup of Rodinia

  • Splitting of Baltica from Laurentia 

• The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography; this high ground could then host glaciers.

  • Created lots of basalt which soaked up all the CO2, dropping temperatures

Snowball Earth summary

Caused by:

• Milankovitch cycles

• Reduction in greenhouse gases – buildup of photosynthetic organisms releasing O2?

• Breakup of Rodinia and basalt absorbing CO2.

• The more ice there is, the more heat gets reflected back

end:

• Greenhouse gases slowly get released from deep down

  • Worms, tectonic plates

Evolution of Ecosystems (theory of Cambrian Explosion)

• trophic structure and the flow of energy through ecosystems

  • Maybe the evolution of herbivores and carnivores created an entirely new level of biodiversity, the first true ecosystems

  • Ecosystems consist of producers (like cyanobacteria, plants etc.) and consumers (herbivores, carnivores)

• The evolution of Cambrian ecosystems was driven by an ecological cascade, primarily the rise of predation, which interacted with environmental and genetic factors. The emergence of predators led to an evolutionary arms race, forcing prey to develop defenses like shells and improved mobility, which in turn spurred predators to evolve more sophisticated hunting methods like complex vision and speed. This co-evolutionary feedback loop, supported by increasing oxygen levels and new genetic tools, created a complex, highly competitive ecosystem that accelerated the diversification of life

Light Switch Hypothesis (theory of Cambrian Explosion)

• related to Predation Hypothesis

• The Cambrian Explosion might have been caused by the evolution of the eye

• Early Cambrian fossils mark the appearance of the first true eyes, eyes with lenses to focus the image on light-sensitive cells

  • This was a very big event in a world formerly dominated by chemical, tactile, mechanical senses

• Evolution of the compound eye forever changed the way animals interacted with one another

• No longer easy for prey to stay hidden

• Both predator and prey could see who was out there, how far away, how fast and in which direction they were moving

• How did the eye evolve?

  • Common ancestor with photoreceptors

    • Eyespots – protists, flatworms

  • Eyes then developed independently in the different groups – mollusks, arthropods, chordates

  • Increased in complexity as part of evolutionary arms race – few million years

• Fast forward to the Burgess Shale and all the ferocious arthropod predators have well developed eyes

• Trilobite eyes have lenses of calcite, transparent mineral crystal

  • Preserves extremely well

Evolution of Predators (theory of Cambrian Explosion)

• Both predators and their prey also had abundant spines. Suggests that even predators themselves needed protection from larger or fiercer predators

• predation could have been a big step that led to Cambrian Explosion

  • Predation created new, intense selective pressures, created new niches

  • Favored organisms that could make shells, burrow deeply, quickly, swim away rapidly

  • Evidence of predation is common in Cambrian fossils, including bite marks etc.

  • Sudden presence of vertical burrows suggests a need to hide from predators

  • Sudden, widespread evolution of shells also circumstantial evidence of increased predation

chordate (cambrian explosion)

possesses a notochord, the precursor to a backbone

Pikaia

• World’s first chordate

• found in the Burgess Shale

• is a close relative of vertebrate ancestors, but is not an ancestor.

Haikouella

• one of the earliest members of the vertebrate family tree

• one of the earliest chordate ancestors

• found in the Maotianshan shale

• seems to be the same species as Pikaia (has teeth but no jaw or bones)

Haikouichthys

• the first fish

• Very primitive, but had a notochord and cartilaginous vertebrae –Maybe first vertebrate!

• Cartilage evolved to allow it to swim more efficiently

• Did very well in Ord. – precursor to other fish and vertebrates (maybe first vertebrate)

Maotianshan Shale

• 10 million years older than the Burgess Shales

  • chinese version of the british shales; has everything just way older

• the world's most important for understanding the evolution of early multi-cellular life, particularly phylum Chordata, which includes all vertebrates.

How did the Asteroid that killed the Dinosaurs affect the earth?

• The impact and explosion would dwarf the energy of the worlds’ entire nuclear arsenal

• Through the winds of the upper atmosphere the dust shroud spreads

• Dust quickly envelops the Northern Hemisphere, obscuring the sun

• Many animals are killed outright, pulverized by the explosion or swept away by a monstrous tidal wave over 100 feet tall

• Tsunami deposits have been found all over the Caribbean, including evidence in Cuba of a 100 foot tidal wave

• Darkness settles over the Earth at high noon

• Photosynthesis stops, and plankton and terrestrial plants die of starvation

• Foraging in the dark for ever fewer scraps of living vegetation, the herbivores gradually succumb.

• Herbivores are followed shortly by the carnivores that preyed on them

• For a brief period, detritivores and scavengers rule the world

• In the sea, a major long-term ecological disruption is underway

  • The increasingly acid ocean waters kill the calcareous plankton, and dissolves calcareous sediments on the ocean floor

    • Calcareous – composed of calcium carbonate

• This releases immense amounts of carbon dioxide into the atmosphere, adding to the outgassing of numerous volcanoes and combustion gases from the raging forest fires

• Adding to the carnage and chaos, the Deccan Traps pour a river of lava over much of what is now India

• Additional dust, carbon dioxide, add another knockout punch

• A feedback loop between ocean and atmosphere creates a greenhouse effect, further stressing the organisms that have managed to survive the initial blow

• Massive pulse of organic carbon from dead terrestrial organisms floods into the sea, causing anoxia, stagnation

• The Cretaceous skies were full of diverse species of winged pterosaurs

• The oceans teaming with plesiosaurs, ichthyosaurs, elasmosaurs…

• Along with them went 50% of the large invertebrates, most of the biomass of land plants, and most of the marine plankton

• The dinosaurs have vanished, and the wealth of ecological niches they formerly occupied are opened up for the rapidly radiating mammals

• Finally, when the dust settles, the planet is transformed • Tropical conditions have given way to a temperate climate

• For a while, ferns dominate the land, “fern spike” at KT boundary reminiscent of spike seen after Mt. St. Helen’s eruption

• Ferns slowly give way to pines and other gymnosperms and finally to the recovering flowering plants.

• The rules, however, have changed forever…

extinction

the death of species

• a normal and ongoing process, an inevitable consequence of natural selection and environmental change

Why do species go extinct?

• there’s an upper limit to adaptation

  • If new species are adaptively superior, then new species should outlast ancestral species, but new species have the same extinction rate as ancestral species

• environments constantly change, organisms must constantly adapt to these changes

• But if the environment changes too rapidly species can go extinct

  • Sooner or later every species fails to keep pace with local or global changes

• Environment never stops changing, species never stop evolving. Eventually even the most varied gene pool will be exhausted, species goes extinct

• Only a handful of species and genera show long life in the fossil record

  • Most disappear in a relatively short period of time

  • Reflects the basic pattern of biodiversity

  • Most genera have relatively few species

  • Most species have relatively few populations

  • Most species live in narrow geographic areas

  • Makes for a “low stake”, a perilous existence for most species…

Modes of Extinction (list the 4 ways in which smaller populations are vulnerable to extinction

• Demographic randomness

• Genetic deterioration

• Social dysfunction

• External limiting factors

• May be a minimum viable population, below which the population cannot sustain itself

  • Concept comes from conservation biology

  • Much of the research on why modern species go extinct applies to past species

  • Important principle of conservation biology is the relationship between biodiversity and geographic area

• Mass extinction events reduce the amount of habitable areas

  • Greatly reduces the overall number of species

  • Easy to see the effects of loss of habitat on modern animals – main cause of ongoing extinctions

• Species tend to become very specialized

  • When things change, specialization can become deadly

  • If the diet or habitat you rely upon is gone, so are you (ex. plant / pollinator)

  • When everything changes at once, specialized species die in large numbers

Demographic randomness (mode of extinction)

random accidents can have a much greater effect on small populations.

Genetic deterioration (mode of extinction)

less variability, highly inbred, can’t always adjust to rapid environmental change

social dysfunction (mode of extinction)

• Harder for males to find females in small populations

• Social facilitation in colonial breeders

External limiting factors (mode of extinction)

• fires, floods, etc. are density-independent

  • Whether they occur is not affected by population size

  • But have greater effect on small populations

What makes some organisms more vulnerable than others? (modes of extinction)

• small population

• restricted range, habitat

• specialized niche

Divergent evolution

divergence of new species from a common ancestor

background extinction

The ongoing normal rate of natural extinction

• example: 1 species of an animal would be expected to go extinct every 400 years; long-term, gradual disappearance of a species due to natural processes like habitat change, competition, or climate shifts, as opposed to a sudden, cataclysmic event.

• opposite of mass extinction

mass extinction

a rapid turnover of fossil organisms, entire communities, abrupt changes in the nature of the very rocks themselves. everything changing suddenly is indicative of this event

• specialists are even more vulnerable than usual

• Adaptation has very little to do with survival during mass extinctions

  • Chance plays an even greater role in survival than usual

• Burrowing animals, deep sea fish would survive, but only because they were hidden

• Species whose habitat was at or near an impact event would be wiped out regardless of their adaptive superiority

• Natural selection is not a factor

• Precambrian extinction may have wiped out the Ediacaran fauna, paved the way for the Cambrian explosion

  • At least one or two minor extinctions in the Cambrian

  • Second Cambrian extinction took out early trilobites (Agnostus), first reef formers

• Widespread environmental stress might not be sufficient to cause mass extinction

  • Most organisms could escape to refugia, migrate to a new habitat

  • Most can migrate faster than climate can change, or shorelines rise or fall

list the 2 major causes of mass extinction

cosmic causes & geologically normal causes

cosmic causes of mass extinction

• Cosmic collisions – asteroids, comets, meteors

• Cosmic radiation – nearby supernovae

geologically normal causes of mass extinction

• Climate change

• Sea level change (regression)

• Extensive volcanism

• predation, diseases, parasites

• Competition

Kill Curve (mass extinctions)

• plots the probability of percentage of species killed in a mass extinction event over time

• We can use it to predict:

  • Likelihood of major extinction events occurring

  • Average waiting time between mass extinctions

common features of a mass extinction event

• Many species go extinct in a short period of time (30% or more)

• Vanished species span all habitat types, sizes - marine and terrestrial, large and microscopic

• fossil records

Evidence of Mass extinctions: what is the evidence for large impacts?

• Big craters (duh)

• Anomalous levels of iridium and other elements

• Shocked quartz or other minerals

• Tektites in sediment

• Soot particle

Anomalous levels of iridium and other elements (evidence for large impacts)

• Some elements, like iridium, are rare on Earth’s surface, but common in outer space

• Walter Alvarez and his team measured the iridium in exposed rocks spanning the boundary between the Cretaceous and the Paleogene

  • discovered a significant spike in iridium levels right at the KT boundary

• a massive increase in cosmic iridium in an area comes from an asteroid impact

• KT, Permian and late Devonian extinctions (3/5) clearly show an iridium spike

Shocked quartz or other minerals (evidence for large impacts)

Atoms in crystals are arranged in neat, ordered rows. In the stress of collision, these rows of atoms can become dislocated. Shocked minerals show characteristic pattern of displaced atoms. Multiple intersecting shock planes are characteristic of meteoroid impacts

• shocked minerals have been discovered at the KT boundary layer

tektites (evidence for large impacts)

• Small glassy spheres of rock, liquefied by the heat and pressure

• Thrown off as liquid splashes, harden into tektites

soot particles (evidence of large impacts)

thin layer of soot at the KT boundary suggests global wildfires started by the impact

big 5

• Ordovician

  • death rate: 85%

  • time: 445 mya

  • likely causes: rapid global cooling and falling sea levels

  • results: costal areas destroyed, chemical reactions affected by cold

• Devonian

  • death rate: 70%

  • time: 340mya

  • likely cause: asteroid impact(s), rapid global cooling

  • results: local destruction from debris, ocean life affected by temperature

• Permian

  • death rate: 95%

  • time: 250mya

  • likely cause: volcanic activity, increase in methane and CO2, rapid global warming

  • results: oxygen removed from oceans, desertification of land

• Triassic

  • death rate: 76%

  • time: 200 mya

  • likely cause: increase in methane and CO2, rapid global warming

  • results: desertification of land, frequent heat waves

• Cretaceous (KT):

  • death rate: 80%

  • time: 65 mya

  • likely cause: asteroid impact, volcanic activity, falling sea levels

  • results: widespread fires, plants disrupted by global ash cloud (can’t photosynthesize), “nuclear winter”

name the 6 mass extinctions in order (on the test, will need to put them in order and probably name the causes and big evolutionary changes that happen in each)

1. The Odovician/Silurian

2. Late Devonian

3. Permian

4. Triassic/Jurassic

5. Cretaceous/Tertiary (KT)

6. current extinction rates for humans, mammals, amphibians, birds, and reptiles

The Ordovician/Silurian Extinction (name causes and big evolutionary changes)

• caused by changes in ocean chemistry - heavy metals

  • rapid global cooling and falling sea levels

The Devonian Extinction (name causes and big evolutionary changes)

Caused by asteroid impact (maybe multiple), with climatic changes and major changes in sea level and ocean chemistry — Woodleigh Crater - Australia

***Permian Extinction (name causes and big evolutionary changes)

• the biggest mass extinction (96%)

• much ocean life lost, including trilobites

• evidence of shocked quarts and several possible impact sites (all large)

• this extinction was caused by asteroid impact

  • Plume Tectonics – a giant pulse of heat between Earth’s mantle and core rises toward the surface as a plume

    • Generates volcanic eruptions that generate hundreds of thousands of Kg of Basalt – Flood Basalt

Triassic/Jurassic Extinction (name causes and big evolutionary changes)

• Most archosaurs and therapsids and all large amphibians went extinct, allowing dinosaurs to take over

• cause is unclear — impact event and volcanic activity?

Cretaceous Tertiary (K-T) (name causes and big evolutionary changes)

• caused by impact event (Chicxulub) and volcanoes

• Ammonites, plesiosaurs, mosasaurs, dinosaurs, and pterosaurs went extinct.

• Allowed mammals and birds to diversify

characteristics of survival of a mass extinction

• Being very widespread - greater chance that someone will be spared

• High population densities - more numerous you are, higher chance some will make it through

• Being lucky…blind chance

The 6th Mass Extinction

• humans are the single greatest threat to global diversity in the history of life

• modern rates of extinction are extremely high

  • Climate change, habitat loss, pollution, introduction of invasive species, hunting, disease

• since 1600, about a thousand organisms have gone extinct due to human action and the rate of extinction is increasing

• Current extinction rates for mammals, amphibians, birds, and reptiles are faster than or as fast as all rates that would have produced the Big Five extinctions over hundreds of thousands or millions of years

How do we know we’re in a mass extinction? (Sonn said could be asked on exam)

We are well above the mass extinction rate.

fossil

any evidence of life that is at least 10,000 years old

how do fossils form?

• Death – the organism dies.

• Decay – soft tissues rot, leaving hard parts.

• Burial – rapid burial in sediment protects remains from scavengers and oxygen.

• Diagenesis – pressure and chemical changes turn sediments to rock and remains to fossils (via mineralization, replacement, or molds/casts).

• Preservation – sometimes whole organisms are preserved in amber, ice, or tar.

what factors work against the preservation of fossils?

• Behavioral – scavengers, detritivores

• Physical – transport, burial of remains

• Chemical – geochemistry of environment (aerobic ex.)

scavengers

animals that eat dead animals

• break bones and other hard parts into smaller pieces, and scatter the remains – disarticulation

bioerosion

Wearing away of hard parts by other organisms burrowing or boring through them – barnacles, worms, mollusks

• Scavengers break bones and other hard parts into smaller pieces, and scatter the remains – disarticulation

detritivores

animals that eat particulate organic matter and help break down decomposing organisms

factors that affect rate of decay

• Available oxygen: – bacteria require oxygen for respiration, breaking down organic molecules for “fuel”

• Temperature: – higher the temperature, the higher the rate of decay

• Acidity: – the higher the acidity, the slower the rate of decay

• Type of organic carbon: Types of organic compounds in the tissues affect decay. Some organic molecules break down very quickly, others are more stable

physical factors that work against preservation

once scavengers move on, the remains begin to decay; exposed remains now subject to physical processes — wind, water, sun, etc

Disarticulation (Scavengers break bones and other hard parts into smaller pieces, and scatter the remains)

Transport (remains become scattered by wind, water, animals)

Fragmentation (remains can become fragmented, broken into smaller pieces)

Abrasion (friction from contact with the sediment or other fragments of remains, grinds away outer edges, removes surface, find details)

chemical factors working against becoming a fossil

• corrosion ex: dissolves unstable minerals

• water s often slightly acidic

  • water absorbs carbon dioxide, makes carbon acid (weak acid)

How fossils form

• once scavengers move on, the remains begin to decay

• the remains begin to decay; exposed remains now subject to physical processes — wind, water, sun, etc

• any surviving remains are now buried, either rapidly or slowly

  • burial itself further alters the remains

diagenesis

sum total of physical and chemical changes undergone by fossils and the rocks that contain them

carbonization

compression between fine layers of sediment

(soft parts can be preserved under ideal condition — volatile chemicals disappear, leaving a thin film of carbon(ex is species in Burgess Shales))

replacement (definitely on the exam)

gradual replacement of original tissues (shells etc.) by minerals during fossilization (= mineralization - petrified wood ex.)

recrystallization

Some fossils, especially shelled organisms, are preserved by _______, when the original material in the shell is unstable and recrystallizes into a more stable atomic structure. Process doesn’t usually alter the surface of the shell and sometimes even preserves some internal features.

mold (definitely on the exam)

• The entire organism may decay after burial, leaving a detailed impression of itself in the surrounding sediment

• can be internal or external (made by the inside of the shell or by the outside – ex. a clam filled with mud)

cast

molds that are filled with minerals (space between the internal and external mold becomes filled with minerals)