Download What caused the Cambrian explosion?

Objectives:
1. Understand the physical characteristics and
fossil evidence of the Burgess shale.
2. Analyze the major causes of the Cambrian
explosion.
3. Apply and interpret Macro evolutionary
patters such as Adaptive radiation and
punctuated equilibrium.
Burgess shale
Ediacaran
Fauna and Flora
Cambrian Explosion
Charles Walcot
Stephen Jay Gould
Adaptive radiation
Punctuated Equilibrium
Cambrian Period
• 542 – 488 mya (million years ago)
• “the age of trilobites”
• The Cambrian Period was the time period
when most of the major groups of animals
first appear in the fossil record.
Ediacaran Fauna
• Flora: plant live
Fauna: Animal life
• dates to about 560 MYA.
• are exclusively soft-bodied (sponges,
jellyfish, comb jellies, etc) and nonburrowing organims.
Vernanimalcula, found in China in 2004
Dates to 40 – 55 million years before Cambrian
Elrathia kingi, found in the Wheeler shale, in the
town of Delta Utah
Burgess Shale Fauna
Discovered by
Charles Walcott
In 1909
Burgess Shale Fauna
• found near Field, B.C., dates to 520 MYA
• all but one of the 35 existing phyla
dramatically “appear” – this is the
Cambrian explosion.
• entirely new modes of locomotion evolve
(i.e., swimming, burrowing, climbing).
Anomalocaris
Wiwaxia
Opabinina
Hallucigenia
Pikaia
Stephen Jay Gould (1941-2002)
what caused the Cambrian explosion?
1. Increase in the oxygen content of seawater
• allowed organisms to achieve increased sizes and
metabolic rates.
• large size is clearly a prerequisite for the evolution of
predators.
What caused the Cambrian explosion?
2. Origin of hard parts (shells and mineralized
exoskeletons).
• some of the earliest shells have holes bored through
them by predators!
• strong selection pressures by presence of predators
would have favored mineralized shells.
What caused the Cambrian explosion?
3. The evolution of eyes
• proposed by Andrew Parker in his 2003 book, “In the
blink of an eye”.
• eyes first appear in trilobites about 540 MYA.
• large predators with eyes make for better predators!
What caused the Cambrian explosion?
4. Genetic changes
• did the diversification of homeotic genes drive the
Cambrian explosion?
• homeotic genes encode for transcription factors.
• they activate suites of genes that control body plans
during early development.
Macroevolutionary patterns
1. Adaptive Radiation
Definition (Mayr 1963): adaptive radiation rapid
diversity and speciation caused by a new
adaptation.
- this process results in an array of species
exhibiting different morphological and
physiological traits with which they can exploit a
range of divergent environments
Some generalizations about adaptive radiation
2. Facilitated by the absence of competitors
and predators
• island archipelagoes, such as the Galapagos. are
prime areas for radiations.
Some generalizations about adaptive radiation
3. May involve “general adaptations”
• general adaptations enable exploitation of new
adaptive zones.
Example: evolution of the jaw
2. Punctuated equilibrium (PE)
• first proposed by Stephen Jay Gould and Niles
Eldredge in 1972 to account for “gaps” in the fossil
record.
Punctuated Equilibrium
Phyletic Gradualism
2. Punctuated equilibrium (PE)
• first proposed by Stephen Jay Gould and Niles
Eldredge in 1972 to account for “gaps” in the fossil
record.
Two characteristics:
1. Periods of rapid morphological change co-occur
with periods of rapid speciation.
2. After species are formed they exhibit “stasis”
period of relatively little change.
3. Mass extinctions
• identified when extinction rates raise well above
normal “background extinction”.
The “Big Five”
Mass
Extinction
Date
% families
(MYA)
end-Ordovician
439
26
85
late-Devonian
367
22
83
lost
% species
lost
end-Permian
250
52
96
end-Triassic
215
22
80
CretaceousTertiary (K-T)
65
16
76
What caused the end-Permian mass
extinction?
1. Glacial Cooling Hypothesis
Such a decline in temperature would destroy
Cambrian fauna which are intolerant of cooler
conditions.
significant continental glaciation would bring
large amounts of ocean water onto the land
resulting in the decrease of sea-level and the
withdrawal of shallow seas.
What caused the end-Permian mass
extinction?
2. Oxygen Depletion Hypothesis
Ocean cooling would also result in
stratification of the water column and a
change in ocean currents resulting in a
depletion of dissolved oxygen and other
nutrients.
Who survives mass extinctions?
1. generalist species outsurvive specialized
species.
2. Temperate marine species outsurvive tropical
species.
•
sometimes called the “nowhere-else-to-go”
hypothesis.
3. Small-bodied species outsurvive large-bodied
species.