Historical Geology - Paliozoic (Cambrian Explosion)

Question 1

What do we mean when we refer to the “Cambrian Explosion”?

Answer 1

The appearance in the fossil record of most major animal body plans about 543 million years ago. The new fossils appear in an interval of 20 million years or less.

Question 2

When did the first hard parts on organisms start to develop?

Answer 2

530 Ma Found in the Tommotian Fauna (small shelly fauna)

Question 3

Why were the development of hard parts on organisms important?

Answer 3

• Response to predators - Hard parts provide protection from predators - Jaws and claws had not evolved yet!
• Increase in size - Hard part support squishy parts
• Locomotion - Can attach muscles to hard parts to move them around

Question 4

what did the Cambrian Explosion result in?

Answer 4

Increase in the diversity and complexity of trace fossils in the Cambrian.

Question 5

What was the preferred environment of the Ediacaran or early Cambrian faunas?

Answer 5

Ample shallow shelf in Cambrian

Question 6

Name two Cambrian localities that provide the largest windows into the past.

Answer 6

Burgess Shale, Canada (~505 Ma)

Chengjiang Fauna, China (~522 Ma)

Question 7

Who discovered and excavated the Burgess Shale?

Answer 7

Charles Doolittle Walcott

- primary fossil site named Walcott Quarry

Question 8

How was the Burgess Shale discovered and what is it?

Answer 8

• The railway arrives in Field, BC in mid 1880’s
• Canadian Pacific hotel builders reported “stone bugs” (Trilobites)
• Fossils were traced up the mountain
• marine shale with an abundance of well preserved fossils middle Cambrian in age (~505 Ma)

Question 9

What was the depositional setting of the Burgess shale?

Answer 9

• fine-grained calcareous mudstone

Question 10

What is Lagerstätten?

Answer 10

fossil sites with exceptional preservation

Question 11

What is required for the high level of preservation in Fossils?

Answer 11

• Stagnation or incomplete recycling
• Rapid burial
• Rapid diagenesis (chemical alteration/conversion)

Question 12

Do scientists know why the Burgess shale fossils were preserved so well?

Answer 12

• Chemistry and environmental conditions that allow
  for exceptional preservation are still a matter of
  debate and under study by numerous researchers
  so……
• Scientist speculate that the organisms were rapidly covered by layers of mud with little or no oxygen and then the mud was covered over with a layer of calcium carbonate cement which prevented microbs from degrading the soft tissue completely.
• preservation was greatly aided by enhanced calcium carbonate concentrations in the Cambrian oceans and by depletion of oxygen and sulfate.
• Studies have shown that the Burgess Shale sediments were not anoxic (without oxygen)
  confusing??

Question 13

Name the types of Burgess Shale Flora.

Answer 13

• Algae
• worms
• trilobites
• sponges
• onychophora
• arthropods
• chordata

Question 14

What is the significance of the Burgess Shale?

Answer 14

• One of the best early windows into newly developed diverse Cambrian ecosystems.
• Demonstrates major body plans, life habits, and phyla were all accounted for by the mid-Cambrian
• One of the few sites worldwide to preserve soft tissues

Question 15

What is the geologic setting of the Chengjiang Fauna?

Answer 15

• mudstones of the Qiongzhusi Formation

- though to represent a shallow sea with a muddy bottom

Question 16

What is the deopsitional environment of the Chengjiang Fauna?

Answer 16

• Shallow water
• Close to terrestrial sediment input
  -Affected by tidal influences and episodic terrigenous
  sediment input
• Probably a partially brackish environment (delta?)

Question 17

Describe the preservation of the Chengjiang Fauna.

Answer 17

• Lagerstratten - exceptional preservation
• three dimensional (unlike Burgess Shale)
• soft tissue is preserved

Question 18

What is the faunal composition of Chengjiang?

Answer 18

• similar to Burgess Shale (sponges, arthropods,etc)

Question 19

What are the implications of Chengjiang Fauna?

Answer 19

• Chordates and vertebrates had evolved, and were diversifying by 522 Ma
• Early divergence consistent with molecular evolutionary models

Question 20

What are some of the biases we need to take
into account when discussing a rapid increase in
diversity (explosion)?

Answer 20

• Diverse early faunas, combined with divergence dates from molecular phylogenies, suggest the ‘explosion’ was a more gradual process
• Appearance of rapid diversification during early Cambrian may be a sampling bias in the fossil record

Question 21

What are the implications of the Cambrian Lagerstätte?

Answer 21

• very diverse faunas even during early Cambrian
  
  • Pelagic - Nektonic and planktonic
  • Benthic - Epifaunal and infaunal
• Primary feeding habits were deposit feeders
• Modern body plans (bilateral symmetry, encephalization, organ systems) had already evolved
• Early Cambrian ‘Small Shelly fauna’ suggested many animals had some sort of skeletal component (However,