Lecture Twenty Five - Life in the Cambrian

Question 1

What was significant in the organisms in the Cambrian period?

Answer 1

They were the first to have hard shells (actually a few in pre-cambirian, but much more in the cambrian).
All phyla with hard parts (except Bryozoa) began the Cambrian, plus many more without hard parts).
By 540 may, shell bearing organisms abundant on every continent.
Most types of invertebrates were represented in the Cambrian.

Question 2

Why was there a sudden increase in life forms in the Cambrian period?

Answer 2

Rising sea-levels increased shallow seas.
Exoskeleton provided leverage and protection.
General waring trend through Cambrian.
Rising atmospheric oxygen level 3-10% of PAL (present atmospheric levels).
Movement of continents - organic carbon ratios fluctuate - change of ocean circulation.
Empty niches.

Question 3

Why was there an explosion of life in the Cambrian period?

Answer 3

Environmental changes.
Simple genes - easier for evolution to have wild experiments.
First predators - leads to evolutionary arms race.
Development of in fauna (burrowing organisms), and teeth.

Just before and during the Cambrian the precursors to nearly all the modern metazoan (multi-celled animals) phyla evolved.

Question 4

What is Burgess shale?

Answer 4

About 530 myo.
Base of a large ancient reef.
Organisms were buried quickly and preserved - small clay sized grains, low energy = less destruction of organisms, even soft ones.

Terbidites and shales are deposited in a deep water basin next to an enormous algal reef with a vertical drop off of several hundred meters - ideal for fossilisation.

These fossils can be and have been found at the Walcott Quarry in the Canadian rocky mountains.

Question 5

What happened for the rest of the Palaeozoic Era?

Answer 5

Almost exponential growth in animals diversity through the early Palaeozoic, stabilising in the Ordovician period.

Ordovician/Silurian: 
High diversity of fauna. 
Trilobites begin to reduce. 
Archeaocyathids now extinct. 
Dominated by filter feeders. 
Neutiloids main predators. 
Expansion of vertebrates. 

Devonian:
Rise and expansion of fish - evolution of jaws.

Late Devonian:
Land plants evolve and take over the land in the carboniferous and permian.

Late Silurian to early Devonian:
Vertebrates and invertebrates colonise the land as well.

Permian:
Evolution fo reptiles and mammal like reptiles.
Precursors to dinosaurs and mammals.
The end of the Palaeozoic is defined by the Permo-Triassic mass extinction event:
- Formation of Pangaea triggers glaciation during Carboniferous-Permian (larger deserts means more heat lost to atmosphere, therefore rapid Earth cooling).
- Volcanic eruptions intensify at the end of the Permian.
- Stratified, anoxic oceans during glaciation.
- Sudden increase in CO2 levels in shallow waters at the end.
- Asteroid/comet impact.

Killed 90-95% of all known groups of organisms.
More damaging to marine groups.