The Cambrian Explosion and Beyond

1
The Cambrian Explosion and Beyond
18
2
18.1 The nature of the Fossil Record

• Items we will discuss in this section
• How Organic Remains Fossilize
• Strengths and Weaknesses of the Fossil Record
• Life Through Time An Overview

3
How Organic Remains Fossilize

• Two things to keep in mind.
• Which part of the organism is preserved and
  available for study?

2. What kinds of habitats produce fossils?

• Fossils are very diverse, but there are 5 major
  categories..

4
How Organic Remains Fossilize

• Compression Fossils

• Result when Organic material is buried in water-
  or wind-borne sediment before decomposition
• As a result of the weight of sand, mud, ash etc.
  an imprint is left of the structure.
• This is just like footprints in the mud or leaves
  on wet concrete.
• Two-dimensional fossils.
• Provide information about external surfaces.

5
How Organic Remains Fossilize

• Casts and Molds

• Remains decay after being buried in sediment
• Molds- consist of unfilled spaces
• Casts-form when new material infiltrates a space,
  fills it, and hardens into rock.
• Preserve information about external and internal
  surfaces

6
How Organic Remains Fossilize

• Permineralized Fossils

• Form when porous structures are buried in
  sediments and dissolved minerals precipitate in
  the pores.
• This is just like embedding a tissue in resin
  before sectioning it
• Details of internal structures are preserved
• Examples include fossilized bones and petrified
  wood

7
How Organic Remains Fossilize

• Replacement/ Recrystallization

• Form when entire structures are buried in
  sediments and gradually replaced by other
  minerals.
• No details of internal structures are preserved.
  
• General information about the 3 dimensional
  surface is apparent. Sometimes in detail.
• Examples include many shelled species and
  crinoids.

8
How Organic Remains Fossilize

• Unaltered Remains

• Preserved in environments that discourage loss
  from weathering, consumption by animals, and
  decomposition by bacteria/fungi.
• Examples amber, ice, desiccation.
• 2,000 year old cadavers have been discovered from
  the iron age.
• Woolly mammoths with tissues and fur still
  preserved
• Unaltered remains represent a small fraction of
  the fossil record.

9
How Organic Remains Fossilize

• Key ingredients

• All fossilization processes depend on 3 key
  features of the specimen
• Durability Mostly bones and shells
• Rapid Burial - usually in a water-saturated
  sediment
• Lack of Oxygen- to discourage decomposition by
  aerobic decomposers.
• These factors slow decomposition making the
  specimen more likely to fossilize
• This is the reason why most of the fossil record
  consists of hard structures left in environments
  such as river deltas, beaches, flood plains etc..

10
Strengths and Weaknesses of the Fossil Record

• 3 types of sampling bias

• Geographic
• Produced by the tendency for fossils to come
  from lowland and marine habitats
• Taxonomic
• Marine organisms dominate the fossil record but
  make up only 10 of extant species
• This means that 2/3 of animal phyla living today
  are underrepresented in the fossil record.
• They lack hard parts that are ideal for
  fossilization
• Temporal
• The Earths crust is constantly being recycled
• When mountains erode or plates subduct , their
  fossils go with them
• Older rocks are rare while new rocks are common

11
Strengths and Weaknesses of the Fossil Record

• Studies by Benton and coworkers (2000)
• Suggest that older rocks still contain enough
  fossils to accurately record the order of
  branching events implied by molecular phylogenies
  of living groups.
• This means that the temporal bias does not
  prevent us from understanding lifes diversity
• The fossil record like any source of Data, has
  characteristics that limit the types of
  information that can be retrieved and how broadly
  the data can be interpreted
• Current goals for paleontologists are to
  recognize the constraints and work within them

12
18.2 The Cambrian Explosion
13
Before the Cambrian Explosion

• The Ediacaran Fauna
• Dated 565-544 mya
• None of the fossils found had shells or any other
  type of hard parts
• Present sponges, jellyfish, and comb jellies

14
Bilaterians

• These fossilized embryos support the hypothesis
  that bilaterians evolved prior to the Cambrian
  Explosion

15
During the Cambrian Explosion

• The Burgess Shale Fauna
• Dated 520-515 mya
• Sharply contrast the Precambrian period
• Large, complex, and bilateral symmetric forms
• Present arthropods, mollusks, vertebrates, and
  echinoderms

16
During the Cambrian Explosion

• Chordates (pre-vertebrates)
• Resemble many of the jawless vertebrates today
• Hagfish and lampreys

17
Phylogeny and Morphology

• Diploblasts
• Two embryonic tissue types
• Ectoderm (skin and nervous system)
• Endoderm (gut and associated organs)
• Radial symmetry or asymmetry
• Triploblasts
• Three embryonic tissue types
• Ectoderm
• Endoderm
• Mesoderm (gonads, heart, muscle, connective
  tissue and blood)
• Bilateral symmetry

18
Protostomes and Deuterostomes

• Protostomes
• Gastrulation forms the mouth region first
• Deuterostomes
• Gastrulation forms the anal region first and
  mouth region second
• Both appeared in the Burgess Shale fauna so it
  appears natural selection resulted in the
  different gastrulations

19
Lophotrochozoans Ecdysozoans

• Both are lineages from Protostomes
• Lophotrochozoa
• Contain a feeding apparatus called a lophophore
• Ecdysozoa
• Molting animals

20
What caused the Cambrian Explosion?

• Rising oxygen levels in sea water
• Due to an increase in photosynthetic algae during
  the Proterozoic (Precambrian)
• More oxygen makes higher metabolic rates and
  larger bodies possible
• Larger bodies allow for the evolution of tissues
  and higher metabolic rates are required for
  larger uses of power for increased movement

21
What caused the Cambrian Explosion?

• Rising levels of atmospheric oxygen
• More atmospheric oxygen makes higher metabolic
  rates and larger bodies possible
• Andrew Knoll and Sean Carroll suggest that a mass
  extinction eliminated much of the Ediacaran fauna
• This created an opportunity for the smaller
  organisms to evolve in response to the change is
  conditions
• Both hypothesis (oxygen levels and mass
  extinction) are currently being tested

22
18.3 Macroevolutionary Patterns

• Items we will discuss in this section
• Adaptive Radiations
• Ecological Opportunity as a trigger
• Morphological innovation as a trigger
• Other Examples Adaptive Radiations in Land
  Plants
• Stasis
• Demonstrating Stasis
• Stasis and Speciation in Bryozoans
• What is the Relative Frequency of Stasis and
  Gradualism?
• Why Does Stasis Occur?

23
Adaptive Radiations

• Occurs when a single or small group of ancestral
  species rapidly diversifies into a large number
  of descendants that occupy a wide variety of
  ecological niches
• I.e. The Galapagos finches and Hawaiian
  Drosophila
• Can be seen in a wide array of groups at
  intervals throughout the history of life
• There is a prominent pattern
• It is as if the tree of life suddenly sprouts a
  large number of highly diverse branches
• What factors trigger adaptive radiations?
• Why do only certain lineages diversify broadly
  and rapidly?

24
Adaptive Radiation

• Ecological Opportunity as a Trigger

• Occurs when a small number of species is suddenly
  presented with a wide and abundant array of
  resources, and few competitors
• These conditions favor rapid diversification and
  speciation
• Following extinction events rapid diversification
  occurs
• Extinction of dinosaurs created new
  opportunities for mammals

25
Adaptive Radiation

• Morphological Innovation as a Trigger

• Not associated with ecological changes
• Modifications and elaborations of traits
  increases success
• Occurs when many species occupy the same niche
• Arthropods
• Modification of joint limbs to move more
  efficently and find food

26
Stasis

• Many new species that appear and then persist for
  millions of years without apparent change
• No burst of speciation
• No morphological change
• No gradual change over time in response to
  environmental changes

27
Stasis vs. Darwin

• Gradual nature of evolution by natural
  selection Darwin-

• Stasis created a problem for Darwins theory.
  Why?
• Darwin attributed the sudden appearance of new
  taxa to the incompleteness of the fossil record
• He stated that these gaps would be filled in as
  specimen collections grew showing gradual
  transitions between species
• For a century most paleontologist followed his
  lead.

28
Stasis vs. Darwin

• Niles Eldredge and Stephen Gould

• 1972 broke the Darwin tradition by claiming that
  stasis is a real pattern in the fossil record and
  that most morphological changes occur during
  speciation
• This is called the Theory of Punctuated
  Equilibrium
• This has been hotly debated
• Which is which?

Cambrian Explosion

1. Punctuated equilibrium-all morphological
   variation occurs at the time of speciation
   (branching) event
2. Pyletic gradualism-morphology occurs gradually
   and is unrelated to speciation events. (Darwins
   Theory)

29
Demonstrating Stasis

• Debate spurred paleontologist to ask whether
  stasis is in fact real
• Does the data support the claim that morphology
  occurs at speciation events?
• Is this seen as the predominant feature of
  species histories?
• Rigorous tests for stasis vs. gradualism are
  extremely difficult
• There are certain criteria that must be met for a
  test to be acceptable
• The phylogeny of the clade is known, so
  researchers can identify which species are
  ancestral and which descendant
• Ancestral species survive long enough to co-occur
  with the new species in the fossil record
• Each of these are critical however if the second
  is not fulfilled is impossible to know if
  splitting occurred or it was a rapid evolution in
  the ancestral form without speciation.

30
Relative Frequencey of Stasis and Gradualism

• Doug Erwin and Robert Anstey (1995) wanted to see
  how common stasis was.
• They reviewed a total of 58 studies conducted to
  test the theory of punctuated equilibrium
  spanning a wide variety of taxa and periods
• They concluded that Paleontological evidence
  overwhelmingly supports a view that.
• Speciation is sometimes gradual and sometimes
  punctuated
• No one mode characterizes this very complicated
  process in the history of life
• 1/4th of the studies reported gradualism
  stasis
• Of course this led to more questions and theories
• Is it possible that different types of organisms
  have distinct patterns of change through time?

and
31
Why Does Stasis Occur

• Eldredge and Goulds most prominent claim was
  Stasis is Data
• In other words lack (..) is a pattern that
  needs to be explained
• Studies in some species show that no change
  occurred over millions of years in the fossil
  record.
• Why would morphology remain unchanged for so
  long?
• To approach this focus has been directed to
  living fossils
• Living fossils are species or clades that show
  little or no morphological change over extended
  periods.

• Examples
• Ginko tree leaves
• Current leaves are the same as fossil impressions
  made 40-mya
• Stromatolite-forming bacteria
• Similar to fossils 1,800 mya

32
More Living Fossils

• Horseshoe Crabs- Limulus are identical to fossil
  species 150 mya
• So why have some species remained unchanged while
  the radiation of birds, mammals, and flowering
  plant took place?
• Are they changing or are we only seeing the net
  effect?
• Steve Stanley and Xianging Yang (1987)
• Looked at bivalve species that have shown little
  change over the past 15 million years
• They discovered that the change occurred but
  that there was little net change within species.
• Many had undergone large fluctuations zigzag
  evolution as they called it.
• Changes tended to fluctuate about a mean value so
  stasis was perceived as a result
• Though it might appear static, morphology in a
  lineage may actually fluctuate over time around a
  long-term average.

33
18.4 Mass Extinction

• Represent intervals in which 60 of species that
  were alive went extinct in the span of one
  million years

34
Background Extinction

• While the Big Five are responsible for 4 of
  all extinctions the other 96 are referred to as
  Background Extinctions

35
The K-T Extinction

• What killed the dinosaurs?

36
Believed to be caused by a High-Impact Event

• The best understood of the Big Five extinction
• Evidences include.
• Iridium found in the sediments of the Earths
• Iridium is rare on the earth but highly
  concentrated in meteorites
• Also found shocked quartz either pressurized or
  melted
• Microtektites spherical or teardrop glass
  particles associated with impact sites.

37
Shocked Quartz
Microtektites
38
What was it?

• In 1980 conformation of a 180 km crater from an
  impact on the earth near the Yucatan peninsula of
  Mexico 65 mya
• It was near a town called Chicxulub

Video
39
Killing Mechanisms

• Vaporization of anhydrite and seawater influx
  of enormous amounts of sulfur dioxide and water
  vapor into the atmosphere
• This would form sulfuric acid acid rain
• Sulfur dioxide scatters solar radiation causing
  global cooling
• Cooling also would have occurred from large
  amounts of dust ejected into the atmosphere
  covering the Earth from solar radiation

40
Killing Mechanisms

• Evidence shows the spread of large fires during
  the impact period
• Force of impact may have caused massive
  earthquakes and may have set off volcanoes
• Evidence shows the largest magma deposits date
  back to the extinction during this period
• Impact would have caused an enormous tidal wave
• If asteroid was 10 km wide the wave produced
  would have been 4 km high

41
Impact Effect

• Would have effected many marine and terrestrial
  ecosystems
• Estimates claim 60 to 80 became extinct
• Early hypotheses stated that the target of
  extinction was size selective
• Large-bodied organisms suffered most due to their
  greater nutrition requirements
• Current research shows no correlation between
  extinction and body size
• Research still goes on!!