QUESTION 154 (7 marks)
The two graphs summarise a synthetic fossil dataset. A mass-extinction episode is identified here when the extinction rate exceeds the shown background rate. Each separated above-background peak is one episode. Ages are measured in millions of years before the present.
a) Identify the number of mass-extinction episodes and estimate their ages. [2 marks]
b) Describe the change in family diversity from 250 to 200 million years ago, using values. [1 mark]
c) Explain how an extinction episode can create conditions for later evolutionary radiation. [3 marks]
d) State one limitation of inferring all historical diversity from preserved fossil counts. [1 mark]
Practice marking scheme
Answer
Three episodes occur, near 450, 250 and 65 million years ago. After the 250-million-year episode, family diversity increases from about 40 to 250 by 200 million years ago. Loss of competitors can open niches and allow surviving lineages to diversify.
Working
The above-background extinction peaks are separated by intervals below the threshold. The later rise in family count is consistent with diversification after a major loss, rather than resurrection of extinct families. Reduced occupation of ecological niches can allow surviving lineages to expand and diverge under different selection pressures. Fossil preservation/sampling can also affect observed counts.
Marking criteria
- Identifies three separated above-background episodes. [1 mark]
- Gives ages near 450, 250 and 65 million years ago. [1 mark]
- Describes increase from about 40 to 250 families. [1 mark]
- Identifies loss of occupying species/competitors. [1 mark]
- Explains availability of ecological niches/resources. [1 mark]
- Links occupation of different niches and divergence to diversification of surviving lineages. [1 mark]
- States incomplete/unequal fossil preservation or sampling, or another relevant fossil-record limitation. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.