4 - The Cambrian Explosion

What is the Burgess shale

A Canadian fossil deposit from the mid Cambrian which preserved almost an entire ecosystem including soft tissues

List 8 organisms found in the Burgess Shale

Trilobites

Brachiopods

Annelidia → segmented worm

Priapulida → penis worm

Ottoia → worm containing a through gut

Hallucigenia → lobe limbed spiky worm

Metaspriggina → early vertebrate

Anomalocaris → arthropod

What is the defining fossil of the Cambrian

Treptichnus → worm burrow trace

Why is Metaspriggina considered a vertebrate

Has gill arches, paired eyes, muscles and a notochord

What was Gould’s interpretation of the Cambrian fauna

Classified each new weird and wonderful organism as its own new phyla → now outdates/dismissed approach which was understandable at the time

Current interpretation of the fauna

Most organisms are stem group members of modern phyla

How many modern phyla are there

30

What is a phyla

A group of organisms sharing a unique body plan

Modern interpretation of the Cambrian fauna in the context of evolution

Still a rapid ‘explosion‘, not to the same degree as Gould imagined (still getting progressively more varied and complex over time as Darwin imagined) just still more sudden and rapid tan he originally hypothesised

What was the rate of animal diversification in the Cambrian compared to normal

5x faster

List the main causes of the Cambrian explosion

Break up of Rodinia

Red queen hypothesis

Hox gene mutations

limited early competition so lots of available niches waiting to be filled

Explain how Rodinia breaking up contributed to the Cambrian explosion

Sea levels rising → shallow flooded continental shelves make great habitats

Regolith erosion → increased carbonate input into the ocean increasing alkalinity → more habitable and the origin of biomineralisation

Explain how the red queen hypothesis contributed to the Cambrian explosion

The predator prey arms race meant that they each had to evolve quickly to evade/catch the other respectively → spurs on the diversification of the other group increasing the complexity of the food web

Explain how hox gene mutations contributed to the Cambrian explosion

Hox genes are responsible for dictating what genes are expressed so how the body plan develops → relatively new so still ‘finding their feet‘ and ‘experimenting‘ → lots of new body plans in the early days of diversification → over time become more common and less deadly

Features of Anomalocaris

Large arthropod

shrimp like shape

Raptorial appendages → put food in mouth

Swimming appendages

Weaker than thought → could only eat soft prey

Define crown group

All living members of a clade, its common ancestor and all its descendants

Define Stem group

Extinct organisms outside the crown group with the shared most recent common ancestor

Example of a stem group organism

Metaspriggina is an extinct vertebrate on the stem linneage