A fizzle or a bang?

What are the origins of animal biodiversity? On one view, complex animal-dominated ecosystems appeared in the blink of a geological eye a the dawn of the Phanerozoic. But detractors argue that this Cambrian ‘explosion’ is an artefact of the fossil record, and the animal phyla diverged closer to a billion years ago.

Read

Our introduction to this topic is a widely cited paper by Cunningham et al. (BioEssays, 2017). This review seeks to explain the discrepancy between the fossil record of early animals and the predictions of molecular clocks.

Discuss

In groups, you'll hold a "journal club" discussion to discuss the findings and implications of the paper, and probe its strengths and weaknesses. Will you and your peers be convinced?

Debate

In the subsequent session, your group will argue either for or against the motion "The Cambrian ‘explosion’ is an artefact of the fossil record".

An integrated view of Cambrian evolution

An introduction to Cunningham et al. (2017)

What the fossil record says

The paper summarises various modes of exceptional fossil preservation to summarise the geological record of the Cambrian explosion. Make sure that at least one member of your group can provide a brief summary of each aspect, with reference to relevant recent papers.

Examples of papers that might provide useful starting points:

  • Recent review of SCF record
  • Controversies regarding biomarkers
  • Comparing Weng'an biota with a Cambrian equivalent

Why clocks tell a different story

The paper proposes some explanations for the mismatch between rocks and clocks. Make sure that one member of your group can summarize each explanation, ideally with reference to current literature.

Papers that might get you started here:

  • Testing the impact of rock volume
  • Thinking about stem vs. crown groups
  • Reliance of the molecular clock on fossil callibrations

Discussing the paper

Your group will pair with one other to discuss each aspect of the paper. What are its strengths and weaknesses? What does it teach us about whether fossils are reliable? How might the paper need updating?

Source: Mogens Engelund / Wikimedia Commons

Debate

In the second of these two sessions, we'll widen our frame of reference to discuss the broader question: What is really represented by the Cambrian explosion?

Preparation

Your group of three or four will be assigned to argue for or against the motion "The Cambrian ‘explosion’ is an artefact of the fossil record". Working in groups allows you to sample a broader selection of the literature than you could alone.

As a team, identify a list of papers that look relevant to the question. Dividing these among yourselves, try to understand and then summarise the key arguments on both sides of the debate. What will your opponents assert, and what are the counter-arguments? What is the strongest evidence in favour of your assigned position? How might your opponents attack these, and how could you respond?

The debate

The debate will follow a structure in which a member of each team in turn puts forwards an uninterrupted five minute statement that helps to build the argument.

Each team should open with an opening statement that puts forwards the outline of their argument in a clear and attention-grabbing way. The subsequent team member(s) will deliver the primary arguments, expanding on the arguments with specific evidence that builds a coherent case. The final speaker on each team will provide a closing statement that rebuts any arguments presented by the opposing team before summarizing the argument for the team's own position.

Only them will you have the opportunity to put questions to members of the opposite team – and your demonstrator may also weigh in with queries to the floor. Be sure to take notes and prepare incisive questions whilst listening to the other team's speeches.

Suggestions for further reading

See the reading list for access to papers and book chapters.

Key Reading

Further literature:

  • Felsenstein, J. (2004). Inferring phylogenies.