Sedimentology and Paleontology Lecture 10: The Cambrian Explosion

Question 1

Describe the Cambrian explosion

Answer 1

The sudden apperance of Large and complex animals with mineralised skeletons

Rapid increase in taxonomic
diversity and abundance

Suggested that marine
ecosystem complexity in
the Cambrian was as
high as the present day

Question 2

When was the Cambrian explosion

Answer 2

Between 540 - 520 million years ago

At the beginning of the Cambrian, in the Terreneuvian series/epoch.

Question 3

How was the Cambrian dated?

Answer 3

Base of the Cambrian dated
by the first appearance of
Treptichnus pedum

• A trace fossil – horizontal
  burrow with horizontal to
  vertical branches
• Type section at Fortune
  Head in Newfoundland,
  Canada

Question 4

What is the Burgess shale
And what is

Answer 4

Sites of exceptional preservation = Konservat Lagerstätten

The Burgess Shale is most famous example of this.

Burgess shale is dated to middle cambrian (508 Ma years)

Discovered in 1909 by Charles Doolittle Walcott

Primarily arthropods and sponges
(Porifera)

Dominated by epibenthic vagile
deposit feeders and sessile
suspension feeders

Question 5

Describe the Doushanto formation

Answer 5

Doushantuo Fm, Guizhou Province, China
Doushanto formation is before the cambrian explosion.

635 - 551 Ma years

Phosphatic microfossils from middle unit (~570 Ma)
* Older than Ediacaran macrofossils

Vernanimalcula (micro-fossil found in the doushanto), Suggested to be the oldest
bilaterian (animals with
bilateral symmetry)

Interpreted to have different
layers, mouth, gut, anus,
and paired sensory pits

Weng’an Biota
(609-570 Ma)
* Algae and acritarchs
* Unlikely that embryolike fossils are sulphur bacteria, animals or
bilaterians

Question 6

Describe the Ediacaran biota

Answer 6

Also before Cambrian explosion
(610-542 Ma)

First complex multicellular
macrofossils
* Worldwide distribution (Canada,
UK, Australia, Namibia, Russia)

Mistaken Point, Canada
* Type section for the PreCambrian-Cambrian
boundary
* Rangeomorphs (fractal
organisms that look like
sea-pens)
* Non-mobile animals that
lived on the seafloor

Similar deposits to Mistaken Point
* Rangeomorphs
* Discovered in 1957 and
identified as similar to
fossils from Australia and
Namibia – older than Cambrian age

Question 7

Describe the Chengjiang biota

Answer 7

After cambrian explosion, before Burgess shale - (~518 Ma, Early Cambrian)

Oldest known assemblage of diverse metazoans. Soft-tissue preservation like Burgess Shale. 100 species in 11 phyla

Primarily arthropods but with
some assemblages with mostly priapulids or brachiopods

• Dominated by epifaunal mobile
  hunters/scavengers, sessile
  suspension feeders, and
  infaunal mobile hunters/
  scavengers

Question 8

Define SCF and SSF

Answer 8

Small shelly fossils (SSFs) - from the latest Ediacaran to
the end of the Early Cambrian

Small carbonaceous fossils (SCFs) – more continuous
record

Question 9

Describe SSF’s

Answer 9

Small shelly fossils (SSFs) – mmsized biomineralized fossils

• Latest Ediacaran to end of the Early Cambrian
• Not a biological clade – whole or
  fragments/distarticulated remains
• Biomineralization due to increase in
  calcium in the oceans or an
  evolutionary arms race

Question 10

Describe SCF’s

Answer 10

Small carbonaceous
fossils (SCFs)

• Fragile sub-mm remains
  of organisms
• Scales, sclerites, and
  feeding appendages
• Cryptic record of
  Cambrian biodiversity

Question 11

Define a trace fossil and a body fossil

Answer 11

Trace fossils are evidence of the activities of organisms

Body fossils are bodily remains of organisms themselves.

Question 12

What is the Agronomic revolution and its significance for the trace fossil record?

Answer 12

Agronomic revolution = change from a PreCambrian matground to Cambrian mixed ground seafloor.

• Evolution of the ability for
  animals to move more
  and burrow vertically
  within the sediment

Therefore allowing significantly more trace fossils to be found in the sediment.

Question 13

Define an ecosystem engineer

Answer 13

Ecosystem engineers =
modifying the environment
in ways that affect other
organisms

Question 14

Describe the entire timing/timeline of the Cambrian explosion and relating biota’s/formations

Answer 14

Doushantuo Fm (635-551 Ma)
Ediacaran Biota (610-542 Ma)
SSFs (~550-520 Ma)
Cambrian explosion (Terreneuvian epoch)
First increase in trace fossils (Fortunian)
Chengjiang Biota (~518 Ma)
Burgess shale (508 Ma years)`

Question 15

What are the supposed causes of the Cambrian explosion?

Answer 15

Unlikely to have been a single cause

Environmental triggers
* Availability of oxygen
* End of ‘Snowball Earth’

Genetic triggers
* Advent of ‘toolkits’ of developmental genes

Ecological triggers
* Co-evolutionary arms races
* Ecosystem engineering