Evidence of Biodiversity Change: Mass Extinctions and Diversification

The fossil record reveals that biodiversity has not increased smoothly over time — it has been marked by dramatic crashes (mass extinctions) and rapid rebounds (adaptive radiation). Understanding this pattern requires interpreting physical evidence preserved in rock layers.

Reading the Fossil Record

Paleontologists reconstruct past biodiversity using:
- Species richness in rock strata of known ages
- First and last appearances of taxa in stratigraphic sequences
- Diversity curves — graphs plotting number of taxa over geological time

Limitations of the record:
- Preservation bias: marine invertebrates with shells are over-represented; soft-bodied organisms are rarely fossilised
- Sampling bias: some geological periods are better studied than others
- Taxonomic uncertainty: species concepts are difficult to apply to fossil material

Despite these limitations, patterns in the fossil record are robust enough to identify major events.

Evidence for Mass Extinctions

Stratigraphic Boundaries

Mass extinctions are identified by abrupt disappearances of many taxa across multiple phyla at the same stratigraphic level. The clearest example is the Cretaceous-Paleogene (K-Pg) boundary:
- A global layer of iridium (rare on Earth’s surface but common in asteroids) marks the boundary
- Below the boundary: non-avian dinosaurs, ammonites, many marine taxa are abundant
- Above the boundary: they disappear entirely
- Supporting evidence: shocked quartz and glass spherules from an impact event; Chicxulub crater in Mexico

The ‘Big Five’ in the Record

Each mass extinction is characterised in the fossil record by:
- Simultaneous disappearance of many lineages
- Disproportionate loss from specific environments (e.g. marine tropics in the Permian)
- Evidence of a triggering mechanism (volcanism, impact, glaciation)

Evidence for Periods of Rapid Diversification

The Cambrian Explosion (~540 MYA)

• Within ~20 million years, representatives of most modern animal phyla appear in the fossil record
• Previously, only simple multicellular organisms existed
• Evidence: the Burgess Shale (British Columbia) and Chengjiang (China) Lagerstätten preserve extraordinary soft-body detail
• Explanations: rising atmospheric oxygen, ecological opportunity, evolutionary innovations (hard parts, eyes, active predation)

Post-Extinction Radiations

• Following each mass extinction, surviving lineages diversify rapidly into vacated niches
• After the K-Pg extinction, mammals radiated explosively — from small, nocturnal insectivores to whales, bats, primates and elephants within ~10 million years
• This is recorded as a sharp increase in mammalian diversity in early Paleogene strata

Interpreting Diversity Curves

Plotting the number of known families or genera over geological time produces a characteristic pattern:
- Gradual increase through most of history
- Sharp drops at the five mass extinctions
- Rebounds that eventually exceed pre-extinction diversity levels
- Steep increase in the Cenozoic, particularly for marine animals and flowering plants

Key skill: VCAA may provide fossil diversity data and ask you to interpret trends, identify extinction events, or describe the rate of change.

Rates of Change

• Background extinction rate: ~1–5 species per million species per year
• Mass extinction rate: potentially thousands of times higher
• Recovery from mass extinctions: typically 5–10 million years to return to pre-extinction diversity levels

EXAM TIP: When asked to use the fossil record as evidence, be specific — name the extinction event, describe the pattern in the record (e.g. ‘sudden disappearance of taxa across a stratigraphic boundary’) and link to a plausible mechanism. Avoid vague statements like ‘the fossil record shows species went extinct’.