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 shaleEdiacaran

Fauna and FloraCambrian Explosion

Charles WalcotStephen Jay GouldAdaptive 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 non-burrowing 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 byCharles WalcottIn 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 Date % families % speciesExtinction (MYA) lost lost end-Ordovician 439 26 85 late-Devonian 367 22 83 end-Permian 250 52 96 end-Triassic 215 22 80 Cretaceous- 65 16 76Tertiary (K-T)

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 HypothesisOcean 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.