Lecture 6 - the Cambrian explosion of animal life

Question 1

What is another word for animals?

Answer 1

metazoans

Question 2

What characteristics do metazoans share?

Answer 2

1. Multicellular body formed from different kinds of cells
2. The ability to manufacture the protein collagen
3. A reproductive cycle with gametes produced by meiosis
4. A nervous system composed of neurones (except in sponges)

Question 3

What have recent advancements allowed in addressing the problem of the origin of metazoans?

Answer 3

1. new fossils
2. phylogenetic analysis of anatomical & molecular data
3. molecular clock studies - assumed molecular evolution is consistent in terms of time (controversial)
4. molecular genetics of animal development

Question 4

How do you classify animals?

Answer 4

• Diploblastic (sponges & archaeocyathids, jellyfish & corals)
• Triploblastic (other animals - bilateria)

Question 5

How many germ layers do Diploblastic animals have?

Answer 5

2 germ layers (ectoderm & endoderm)

Question 6

How many germ layers do Triploblastic animals have?

Answer 6

3 germ layers (layersectoderm, endoderm & mesoderm)

Question 7

`What occurred during the Late Precambrian (Ediacaran) 630-542 Ma?

Answer 7

• the ediacara biota - contain strange markings (fossils found all over the world - e.g. Namibia)
• trace fossils of triplobastic organisms
• egg cases containing embryos
• possible traces of triploblastic animal - they’ve got a gut so triploblastic animal

Question 8

What occurs during the late Precambrian 630Ma?

Answer 8

Large acritarchs (organic-walled egg cases) similar to those in young deposits that contain phosphstized embryo?

Question 9

What occurred during late Precambrian 580Ma?

Answer 9

Phosphatized embryos

Question 10

What occurred during Early Cambrian (Manakayan) 542-530Ma?

Answer 10

Small Shelly fossils

Question 11

What occurred during Middle Cambrian (Tommotian & Atdabanain) 530 - 520 Ma?

Answer 11

The Cambrian explosion (fossilised hard parts appear, including representatives of all modern phyla).

Question 12

What are Small shelly fossils (SSF)?

Answer 12

Small shelly fossils are probably disarticulated elements of a skeletal covering that had yet to evolve into a large discrete ‘shell’ covering the entire organisms.

They are almost certain an adaption against predation

Question 13

What do small shelly fossils (SSFs) represent?

Answer 13

• an extinct phylum close to the mollusca
• some form of annelid

Question 14

What occurred during the Cambrian exceptional preservation of hard & soft parts?

Answer 14

• eyes evolved to evade predation

Question 15

Does the Cambrian Explosion represent?

Answer 15

1. The appearance of fossilisable parts
2. A true evolutionary bust to large size & greatly increased anatomical variety

Question 16

What environmental events at the precambrian-Cambrian transition?

Answer 16

• the long-lived & large supercontinent that had dominated the Pre-Cambrian begins to break apart.
• there are 2 extensive glaciation extending into equatorial latitudes that produce snowball/slushball Earth scenarios.
• there is a mysterious short, isotope excursion (very low C-isotope values) in the latest Precambrian. It coincides with the extinction of skeletal fossils from microbial reefs
• atmospheric O2 levels begin to rise dramatically at the Precambrian/Cambrian boundary - may have triggered the evolution of large animals

Question 17

What could’ve caused the ‘Cambrian Explosion’?

Answer 17

1. Environment
2. Ecology - eyes and/or predation

Question 18

What highly speculative evolutionary progression was seen in the Late Precambrian?

Answer 18

Ediacaran animals inhabit the sea floor. They are unprotected but have no predators. Triploblastic animals are present, but they are very small & their ecology is unknown

Question 19

What highly speculative evolutionary progression was seen in the Early-Middle Cambrian?

Answer 19

Tripoblastic predators with teeth evolve. Most Ediacara organisms become extinct. Other multicellular animals protect themselves by evolving armour (spines, scleritis etc.) or burrowing in the sea floor

Question 20

What highly speculative evolutionary progression was seen in the Late Cambrian?

Answer 20

Predators become more efficient (+ eyes) & multicellular animal evolve better armour (continuous shells & exploit more burrowing niches