The Cambrian Explosion and Beyond

1
The Cambrian Explosion and Beyond

• Chapter 17

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Fig. 2.18 The geological time scale
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Limitations of the fossil record

• Hard parts shells, bones, teeth most likely
  to be fossilized because they decay slowly and
  are more durable
• In order to be fossilized, a specimen generally
  needs to be covered quickly by water-borne or
  wind-borne sediments (sand, mud, ash)
• Lack of oxygen is favorable for fossilization
• The fossil record consists primarily of hard
  parts left in depositional environments such as
  river deltas, beaches, flood plains, marshes,
  lakeshores, and the sea floor the fossil record
  is biased

4
Early animals Ediacaran fauna(565 544 mya)

• First multicellular animals appear about 565
  million years ago (mya)
• Simple, small, asymmetric or radially symmetric
  sponges, jellyfish
• Few bilaterally symmetric forms, such as
  Kimberella, which has uncertain affinities, but
  appears to be mollusc-like
• Dickinsonia considered by some to be an annelid
  worm, and by others to be cnidarian (jellyfish)
  polyp

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Kimberellahttp//www.ucmp.berkeley.edu/vendian/ki
mberella2.html
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Dickinsoniahttp//www.ucmp.berkeley.edu/vendian/d
ickinsonia.jpg
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The Cambrian explosion

• Cambrian period 543 495 mya
• Explosive appearance of large, complex,
  bilaterally symmetric animals, segmented animals
  with limbs, antennae, shells, external skeletons,
  and notochords including arthropods, molluscs,
  annelids, and chordates within the period 543
  506 mya
• Almost all living animal phyla are present
• Burgess shale fauna (Simon Conway Morris
  Stephen Jay Gould, Wonderful Life, 1989)

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The first animals phylogeny and fossils (Fig.
17.12)
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Macroevolutionary Patterns

• Adaptive radiation
• Punctuated equilibrium versus gradualism
• Extinction
• Taxon survivorship curves
• Mass extinctions
• The Cretaceous Tertiary (K-T) impact extinction
• Anthropogenic extinction

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Adaptive Radiation

• An adaptive radiation occurs when a single or a
  small group of ancestral species rapidly
  diversifies into a large number of descendant
  species that occupy a wide variety of ecological
  niches
• Adaptive radiations can occur when
• A species colonizes a new region where there are
  no or few competitors (i.e., lots of empty
  niches) Galápagos finches, Hawaiian Drosophila
  and silver swords
• A taxon acquires an important adaptation
  evolution of flight in birds
• A taxon is released from competition after
  extinction of a dominant group radiation of
  mammals after extinction of the dinosaurs (?)

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Darwins Finches (Fig. 3.4)
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Adaptive radiation (Fig. 17.13)
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Punctuated Equilibrium

• Niles Eldredge and Stephen Jay Gould, 1972
• The fossil record for some groups reveals that
  morphological evolution consists of long periods
  of stasis with little or no change and very short
  periods during which morphological change occurs
  in association with speciation
• Presented as a challenge to the conventional
  picture of morphological evolution described by
  the modern synthesis

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The controversy

• Phyletic gradualism
• New species arise by transformation of large
  ancestral groups (often without splitting
  anagenesis)
• Transformation occurs over all or a large part of
  the ancestral species geographic range
• Transformation is even and slow
• Evolution occurs more or less at the same rate
  during and between speciation events
• Punctuated equilibrium
• A small subgroup of the ancestral form gives rise
  to a new group by splitting cladogenesis
• New species originates in a small part of the
  ancestral species geographic range peripheral
  isolates model
• New species develop rapidly, then may replace
  ancestral species
• Between speciation events there is stasis

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Patterns of morphological change punctuated
equilibrium and gradualism (Fig. 17.15)
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Explaining the fossil record

• Darwin was aware of this problem and he
  explained the apparent discontinuities and sudden
  transitions in the fossil record as being due to
  the incompleteness of the fossil record
• Eldredge and Gould argued that sudden
  transformations are not artifacts speciation
  occurs rapidly and in small populations and is,
  therefore, unlikely to leave a fossil record

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Testing punctuated equilibrium

• Strong tests of punctuated equilibrium vs.
  phyletic gradualism are difficult
• Need a complete stratigraphic sequence
• Are morphospecies biological species?
• Cryptic species?
• Need multiple specimens and populations of each
  species in order to determine the range of
  variation within species

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Punctuated change in cheilostome Bryozoa (Jackson
and Cheetham 1994) (Fig. 17.16)
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Who wins?

• Erwin and Antsey (1995) review of 58 tests of
  punctuated equilibrium
• Paleontological evidence overwhelmingly supports
  a view that speciation is sometimes gradual and
  sometimes punctuated, and that no one mode
  characterizes this very complicated process in
  the history of life.
• About 1/3 of the studies support a combination of
  gradualism and stasis
• Time-scale effects when the resolution of the
  fossil record is on the order of 10s of
  thousands of years, morphological change may
  appear sudden, but if we had been present
  during a 10,000 year period to witness it, it
  might have seemed gradual

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Genetic and morphological change in two arthropod
clades (Fig. 17.18)a. Horseshoe crabs today are
almost identical to those that lived 150 million
years agob. Hermit crabs and alliesGenetic
distances based on 16s rRNA gene sequences
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Extinction

• Extinction is the ultimate fate of all species
• Mass extinctions
• The big five
• Global in extent
• Involve a broad range of organisms
• Rapid
• Background extinction
• Accounts for the vast majority of extinctions

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Distribution of species extinction
intensities(Raup 1994) (Fig. 17.20)
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Patterns of extinctions of families through time
(Benton 1995) (Fig. 17.21)
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Survivorship curves for taxa

• Leigh Van Valen (1973) showed that the
  probability of extinction of a taxon was
  independent of its age a taxon does not become
  more, or less, likely to go extinct as it gets
  older

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Survivorship curves for genera and families(Van
Valen 1973) (Fig. 17.22)
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How long does a species of marine bivalve exist?
(Jablonski 1986) (Fig. 17.23)Species with
planktonic larvae have longer durations than do
species with direct development
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Geographic range affects the survivorship of
bivalve and gastropod species (Jablonski 1986)
(Fig. 17.24)
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The Cretaceous Tertiary (K-T) extinction

• World-wide iridium anomaly at the K-T boundary
  dated to 65 mya
• Iridium is rare in the Earths crust but more
  abundant in meteorites
• Based on the amount of iridium required to
  produce the deposits seen at the K-T boundary,
  Alvarez et al. (1980) estimated that a 10 km (6.2
  mi) diameter asteroid hit the earth
• The asteroid theory is supported by the presence
  of shocked quartz crystals, glass microtektites,
  and a 180 km (112 mi) diameter crater in the
  ocean near the Yucatán Peninsula

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Iridium anomaly at the K-T boundary (Fig. 17.25 b)
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Location and shape of the Chicxulub crater
(Schultz and DHondt 1996) (Fig. 17.26)
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Effects of a 10 km asteroid impact

• Widespread wild fires
• Acid rain (from release of SO2)
• Darkness and intense cooling from blockage of
  sunlight reduced photosynthesis
• Tsunami (up to 4 km high)
• Severe earthquakes ?
• Increased volcanism ?

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Probability of extinction at the K-T boundary and
geographic range of marine bivalves (Jablonski
and Raup 1995)(Fig. 17.28)
Provinces are graphic regions that share similar
flora and fauna
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Anthropogenic extinctions - 1

• Between 1600 and 1993, humans observed extinction
  of 486 animal species and 600 plant species
  (Smith et al. 1993)
• Most of these extinctions occurred in North
  America, the Caribbean, Australasia, and Pacific
  Islands
• Currently, the taxa with the highest proportion
  of endangered species include
• Palms 925 of 2,820 species (33)
• Gymnosperms 242 of 758 species (32)
• Birds 1,029 of 9,500 species (11)
• Mammals 505 of 4,500 species (11)
• Widespread habitat destruction is probably the
  greatest anthropogenic cause of extinction at
  present

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Anthropogenic extinctions - 2

• Pacific island birds
• Steadman (1995) estimates 2,000 species have
  become extinct over last 2,000 years almost 20
  of all bird species
• 60 endemic Hawaiian species extinct in last 1,500
  yr
• 44 New Zealand species extinct since human
  colonization including 8 species of moas, the
  largest known birds
• On the island of Eua only 6 of 27 land birds
  present before human occupation are still living

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Exinction of forest birds on the island of Eua
(Tonga) (Jablonski and Steadman 1995)(Fig. 17.29)
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Deforestation in the Brazilian Amazon (Skole and
Tucker 1993) (Fig. 17.30)a. 1978b.
1988During this period of time the annual loss
of forested area was about 15,000 km2, an area
approximately equivalent to the state of
Connecticut per year