The Cambrian Explosion

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The Cambrian Explosion

• Rare Earth, Chapter 7
• (Ward and Brownlee)

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Introduction

• for first 4 By only primitive life had developed
  on Earth
• at 545 My plant and animal species suddenly
  increased dramatically, fossils from 500 My are
  widespread and abundant
• rate of evolution and diversity has never been
  equaled since
• what was special about this event?

Many figures from Bentons book
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Reminder - Precambrian
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Reminder - Fossils
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Fossils
Fossils can form on land, especially in soft
muddy conditions such as the shores of lakes and
river bottoms.
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Fossils

• Land organisms may become embedded in soft
  sedimentary deposits that later become buried,
  compressed, and possibly folded, heated, or
  transported great distances
• Marine organisms naturally fall to the ocean
  bottom at the end of their lives, or can be
  buried by cataclysmic events (earthquakes,
  turbidity currents, etc.)
• As long as the rock is not melted, it can
  reappear millions (or billions) of years later
  bearing the imprints of the organisms (fossils)

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Reefs
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Reefs
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Reminder - Biological Classification
species
family
phyla
kingdom
domain
Eukaryota!
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When did the CE Take Place?
Ediacaran (late Precambrian EON, or late
Proterozoic) ERA 565-543 My Cambrian (PERIOD) or
Paleozoic (ERA) 543-490 My
Age estimates of the Ediacaran, Cambrian, and
Early Ordovician (International Subcommision on
Cambrian Stratigraphy)
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What was the CE?

• CE marked by numerous large fossils
• in many locations, large trilobites appear
  suddenly in the stratigraphic sequence, with no
  small or intermediate sized precursors
• challenge to Darwins theory of continuous
  evolution
• fossils became the age markers for geology,
  worldwide
• geologists encountered sequences with no fossils,
  interbedded with sequences of abundant fossils

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what?

• base of Cambrian used to be marked as first
  trilobites, not the first trace of animal
  behavior
• Darwin claimed there must be pre-cursor fossils,
  but never found them
• now we know that precursors were abundant, but
  not well enough developed to produce fossils
• rocks now dated more precisely by radioactive
  means, so geological strata can be given absolute
  ages
• Ward and Brownlee consider the CE as a milestone
  marker for complex life in the galaxy

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What Animals were Involved in the Cambrian
Explosion?

• CE took place in the shallow oceans,
  stromatolites had been consumed by higher
  eukaryota

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543 My
490 My
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Four Stages of DiversityI - Ediacarans

• these were soft bodies jellyfish, worms, sea
  anemones and soft corals
• primitive in the sense of few body parts
• discovered in 1940 (Ediacaran Hills of
  Australia), dated at
• some very large (up to 40), varied in shape
  (Figure 7.1)
• thought by many to be fore-runners of modern
  animals (Simon Conway Morris), but others think
  they are related to plants, fungi, or lichens
• later found to be world-wide distribution
• possibly an extinct side branch of evolution
• no evidence they were the source of food for
  other Eukaryota enigmatic relationship with
  other Cambrian animals

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Ediacarans
What was life like 560 million years ago? The
Vendian marks the first appearance of a group of
large fossils collectively known as the "Vendian
biota" or "Ediacara fauna."
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Ediacarans

• Fossils from the Ediacara Hills
• On the left is Arkarua, a small disc-shaped
  animal that may be the oldest echinoderm.
• At center is an unusual disc-shaped form with
  three-part (triradial) symmetry. Named
  Tribrachidium heraldicum, its affinities are
  still mysterious, although distant relationships
  have been proposed with either the Cnidaria or
  the Echinodermata.
• On the right is the fossil Spriggina, a possible
  relative of the arthropods.

Photos of Arkarua and the Ediacara site by
Lisa-ann Gershwin
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Ediacaran Fauna
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II - Trace Fossils and Shelly Fossils

• these are not true fossils, only impressions of
  their movements
• demonstrated locomotion, possibly wormlike
• many details lacking, they seem to be an
  intermediate phase in the CE
• Also many small creatures up to ½ inch long, with
  mineralized skeletons (true fossils), Shelly
  Fossils

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Small Shelly Fauna

• Cambrian fossiliferous strata in Siberia, China,
  Europe, and Australia have yielded an unexpected
  range of well-preserved phosphatic microfossils.
• These microfossils have become familiar as the
  "small shelly fossils."

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Continents during Cambrian

• After the breakup of Rodinia (1400 800 My) the
  continental fragments moved apart
• Two new supercontinents were in the making
  Gondwana and Laurentia
• The process of plate tectonics, responsible for
  this motion, will be discussed in Chapter 9

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III - Trilobite Faunas

• this group is the best known of the Cambrian
  fossils trilobites, brachiopods, mollusks and
  echinoderms
• ages approx 530 My 500 My
• trilobites were spiny, hard shelled animals with
  large eyes that crawled on the sea bed
• others were immobile coral-like animals
  (archeocyathids) that formed reefs, as others do
  today
• seemed to have become extinct at about 500 My
• Burgess Shale is the most famous location of
  Cambrian fossils, where a whole cliff of sediment
  collapsed onto the sea floor and buried the
  creatures (even the softer parts)
• The remarkable fossils of Chenjiang, of latest
  Early Cambrian age, are just now being described
  in detail. 

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Burgess Shale, British Columbia Canada
NB This is not the Burgess Shale!
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Burgess Shale, British Columbia Canada
Mount Wapta is the peak visible in the distance.
The escarpment in the Cathedral Formation is
visible just a few metres beyond the edge of the
quarry on the left. It occurs at the contact
between the white dolomite and the grey
slate/shale of the Stephen Formation. If you have
access to older photographs of the Walcott
quarry, it is worth comparing them to this photo
-- the quarry has doubled in size as a result of
continued work in the last few years, mainly by
Des Collins at the Royal Ontario Museum.
http//www.geo.ucalgary.ca/macrae/current_project
s.html
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• This photo shows details of the bedding within
  the Burgess Shale.
• The bedding occurs a the scale of a few
  centimetres, and consists mostly of graded beds.
• The yellowish bands are slightly calcareous,
  while the darker grey bands are more pelitic
  (clayey).
• Both lithologies are very fine grained (silt size
  grains or smaller).
• These lithological differences are obvious only
  where the weathering of the face of the quarry
  has altered the carbonate-rich beds.
• The graded beds represent individual
  sediment-input events, possibly representing
  storms that disturbed muddy sediment high on the
  escarpment, and caused it to rain down into the
  deeper water at this location, burying and
  smothering organisms carried along for the ride.
  The scale bar is 30 centimetres.

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• This is the largest of several species of
  trilobites found in the Burgess Shale, some of
  which have been preserved with soft appendages.
• This specimen is missing its left free cheek (on
  the head), but its skeleton is otherwise
  complete, and I think one of the posterior
  antennae is present, but it is not easily visible
  in the photograph.
• The orange colour is produced by iron oxides.
• This specimen was wetted for the photograph.

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Chenjiang

• The remarkable fossils of Chenjiang, of latest
  Early Cambrian age, are just now being described
  in detail. 
• The fossils occur in rocks of the Qiongzhusi
  Formation, cropping out at Maotianshan near
  Chengjiang, Yunnan Province, South China (fig. 1)
  where they were accidentally discovered in 1984.
  

Fig. 2 Eoredlichia intermedia with preserved
antennae and mid-gut diverticula (dark blobs in
the anterior part of the rhachis). Qiongzhusi
Formation, Chengjiang. Image courtesy of Dr.
Gerd Geyer,  Institut für Paläontologie,
Bayerische Julius-Maximilians-Universität,
Würzburg, Germany.
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Chengjiang Microdictyon

• It is not only the diversity and early appearance
  in the fossil record which makes the Chengjiang
  assemblage fabulous, but also the fine
  preservation which offers the opportunity to
  learn more about the morphology of these early
  creatures.
• One of the most outstanding examples might be
  Microdictyon, the isolated sclerites of which
  were known from numerous localities on various
  continents, but none of the specialists had any
  idea how this creature could have been organized.
• The discovery of these net-like scales of
  Microdictyon on a worm-like animal resolved the
  question.

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Was the Cambrian Explosion Inevitable?

• The Darwinian theory of evolution rests on two
  assumption (1) that all life came from a common
  ancestor, and (b) that various species descended
  from this ancestor by a process of modification
• evolution does not necessarily imply that life
  evolves continuously to more and more complex
  organisms
• the Archaea and Bacteria have not evolved to
  greater complexity
• the Eukarya have evolved extensively, on this
  planet, to form the basis of all plant and animal
  life (including ourselves)
• it appears from Earths record, that forming
  animal life is the hardest part of evolution

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Inevitable?

• evolution appears to have proceeded by long
  periods of stability and then sudden dramatic
  changes
• in 40 My all the elements of complex animal life
  seemed to have formed (Figure 7.2)
• evolution produced only a relatively few body
  plans PHYLA (about 30 for all the tens of
  millions of animal species on Earth today)
• is this special for Earth, or are other
  possibilities founds on other planets?
• no new phyla (body plans) have appeared since the
  end of the Cambrian, in fact there were more
  phyla then ( 100) than now

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What if Anything Triggered the Cambrian
Explosion?

• Scientists seek to understand whether genetic
  development dominates evolution, or whether
  environment plays the crucial role (sounds
  familiar?)

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Environmental Causes

• Oxygen reached a critical value
• after the global glaciati0on, the level of O2
  passed a threshold that enabled animals to
  develop hard skeletons
• collagen (fingernail material) might have formed
  earlier, but calcium and silicon based shells and
  skeletons required an oxygen rich atmosphere

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Environmental Causes

• Abundant Nutrients became available
• particularly important nutrients are phosphorous,
  nitrates and iron
• previously these had been buried in the bottom
  sediments
• perhaps plate tectonics (the break up of Rodinia)
  allowed these minerals to be re-cycled into the
  mid-ocean ridges

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Environmental Causes

• - Temperatures moderated after the last deep
  freeze
• the second cycle of temperatures accompanied the
  deep freeze of 600 My
• temperatures became warm enough to trigger the
  diversity
• homo sapiens also appeared after a glaciation -
  the last Pleistocene that ended 20 000 years ago
• sea level changes probably significantly affected
  the CE

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Environmental Causes

• Inertial Interchange Event
• some scientists (including R. Ripperdan) believe
  that a profound change accompanied by a 90 degree
  change in continental positions relative to the
  spin axis
• so the continents at the equator and the poles
  were exchanged
• evidence comes from paleomagnetism- the study of
  the position of the Earths magnetic field with
  time
• the motion of the continents during such an event
  could have been rapid (15 My) and this would have
  changed the climatic conditions dramatically
• theory is consistent with distribution of Carbon
  isotopes (different forms of elemental carbon)
  associated with release of organic carbon in
  sediments

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Environmental Causes

• All of them?
• Authors contend that if all environmental
  conditions are required to be present, then the
  CE might be a very rare planetary event!

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Biological Causes

• Hard skeletal systems are essential
• development of a skeleton is difficult and
  requires change in respiration mechanism
• higher oxygen levels required for skeletons
• is this not the same as environmental factor?
• the second cycle of temperatures accompanied the
  deep freeze of 600 My

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Biological Causes

• - Conditions favorable for larger sizes
• larger sizes, enabled by the environmental
  factors, were crucial to biological development

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Consumers - size matters!
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Biological Causes

• The predation hypothesis
• evolutionary development was accelerated by the
  appearance of predators that forced greater
  diversity (skeletons, mobility, etc.)

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Is the Cambrian Explosion Simply an Artifact of
the Fossil Record?

• do we see the CE only because of the appearance
  of skeletal forms in many phyla that had already
  been established?
• this does not avoid the fact that there was an
  abrupt development of hard parts that later
  became fossils
• the many phyla that use shells should have
  evolved only after the development of shells.

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Cambrian Explosion, Cambrian Cessation

• why were there no new body plans developed after
  the CE?
• were all ecological niches occupied by the
  Cambrian trilobite fauna, leaving no further
  development possible?
• but, even after later extinctions, when large
  numbers of species disappeared, no new phyla
  developed
• what made the CE so special?
• perhaps after later extinctions, there were
  enough representative of the various phyla to
  re-populate the planet
• or perhaps there is some critical complexity in
  DNA that prevents further evolution a sobering
  thought!

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Diversity and Disparity

• diversity refers to the number of species
  present, and their relative abundance
• disparity is the number of distinct body plans,
  types, or forms (phyla)
• in the CE there were lots of phyla, but few
  species (variations of each)
• Stephen Jay Gould remarks about the significance
  of this fact
• is this the way evolution would work elsewhere in
  the galaxy?
• Gould believes disparity was at its maximum in
  the CE. Conway Morris believes that it has been
  increasing intermittently (Figure 7.3)

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Disparity
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After the Cambrian Explosion the Evolution of
Diversity

• ecosystems changed along with the progression
  from prokaryota to eukaryota
• two other periods of diversification have been
  identified the Ordovician (following the
  Cambrian) and the Tertiary (after the 65 My
  extinction)
• are such episodes the normal pattern of galactic
  evolution?

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Relevance to the Frequency of Life on Other
Planets

• if Earth is typical, then the CE required an
  oxygenating atmosphere and then a large number of
  phylogenetic (biological) changes
• these require time (how much?) and a hospitable
  planetary surface without major catastrophes
  (mass extinctions)