Biology • Year 11 • Module 3 • Lesson 10
Fossil Evidence
Apply stratigraphic reasoning, interpret a rock column, evaluate transitional fossil evidence, and assess the strengths and limitations of the fossil record.
1. Interpret a stratigraphic column
The table below represents a vertical section through undisturbed sedimentary rock layers. Each layer contains fossils as labelled. Use the data to answer the questions. 8 marks
| Layer (from surface downward) | Fossils present | Relative position |
|---|---|---|
| Layer 1 (surface) | Mammal bones | Shallowest / youngest |
| Layer 2 | Reptile-like fossils | |
| Layer 3 | Amphibian-like fossils | |
| Layer 4 | Fish fossils | |
| Layer 5 (base) | Marine invertebrates | Deepest / oldest |
1.1 Describe the principle of superposition and explain how it applies to this rock column. 2 marks
1.2 Explain what the sequence of fossils from Layer 5 to Layer 1 suggests about biological change over time. Refer to at least two layers in your answer. 3 marks
1.3 Why does this stratigraphic pattern constitute evidence for evolution rather than simply evidence that organisms died in the past? 3 marks
2. Cause-and-effect chain — why gaps exist and what they mean
Complete the cause-and-effect chain below. Each cause is given; fill in the effect. 5 marks
Cause 1: Most organisms die in environments where rapid burial does not occur, and soft tissues decay quickly.
Effect 1 / Cause 2:
Cause 3: Erosion, metamorphism and tectonic activity destroy or distort rock strata.
Effect 3 / Cause 4:
Cause 5: The preserved fossils that do exist show a time-ordered pattern consistent with evolutionary theory.
Effect 5 (Overall conclusion):
3. Case study — Tiktaalik and the fish-to-tetrapod transition
Read the scenario, then answer the questions. 6 marks
Scenario. Tiktaalik roseae was discovered in 2004 in late Devonian sedimentary rocks approximately 375 million years old. It has scales and fins like a fish, but also possesses a neck, robust ribs, and limb-like fin bones (including a wrist-like joint) capable of supporting weight in shallow water. It was found in rock layers consistent with a shallow-water, wetland environment, filling the predicted time gap between fully aquatic fish and early tetrapods (four-limbed land vertebrates).
3.1 Explain why Tiktaalik is classified as a transitional fossil. Include at least two specific features in your answer. 2 marks
3.2 Explain how the geological context (rock type, approximate age, environment) supports the claim that Tiktaalik represents the fish-to-tetrapod transition. 2 marks
3.3 A student says “Tiktaalik must be the direct ancestor of all tetrapods.” Evaluate this claim using the lesson definition of a transitional fossil. 2 marks
Q1.1 — Superposition and the rock column (2 marks)
The principle of superposition states that in an undisturbed sedimentary sequence, deeper layers were deposited before the layers above them and are therefore older [1]. In this column, Layer 5 (marine invertebrates) is the oldest and Layer 1 (mammal bones) is the youngest because Layer 5 is deepest and Layer 1 is at the surface [1].
Q1.2 — What the fossil sequence suggests (3 marks)
The sequence shows biological change over time consistent with descent with modification [1]. The oldest layer (Layer 5) contains marine invertebrates, while the more recent layers successively contain fish, amphibian-like organisms, reptile-like organisms and ultimately mammals [1]. This progression from aquatic invertebrates to vertebrates to air-breathing tetrapods matches the predicted order of evolutionary branching rather than being a random assortment of organisms [1].
Q1.3 — Why this is evolution evidence, not just death record (3 marks)
If organisms had simply been dying and leaving remains, we would expect to see a random assortment of species across all layers, not a consistent time-ordered progression [1]. The fact that more complex and derived forms appear consistently in younger layers, while simpler or earlier forms dominate older layers, shows that the pattern reflects change over time [1]. This directional, time-ordered pattern is consistent with the predictions of evolutionary theory — it is evidence of descent with modification [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1 / Cause 2: The fossil record is biased toward organisms with hard parts such as shells and bones. Soft-bodied organisms rarely fossilise, so their remains are underrepresented or absent in the record.
Effect 3 / Cause 4: Parts of the rock record are lost or altered, creating additional gaps. Some lineages that did fossilise may have their evidence destroyed before it can be found.
Effect 5 (Overall conclusion): The fossil record is incomplete, but its incompleteness does not make it unreliable. The preserved pieces show a consistent time-ordered pattern that aligns with evolutionary predictions. An incomplete record that still produces a coherent pattern is a strong form of evidence.
Q3.1 — Why Tiktaalik is a transitional fossil (2 marks)
Tiktaalik is a transitional fossil because it shows a combination of features expected near the fish-to-tetrapod transition [1]. It has fish features (scales, fins) alongside tetrapod-like features (a neck, robust ribs, and limb-like fin bones with a wrist-like joint capable of supporting weight) — this mix is exactly what would be predicted at the boundary between fully aquatic fish and early land-capable tetrapods [1].
Q3.2 — Geological context (2 marks)
The fossil was found in late Devonian sedimentary rock approximately 375 million years old — a time period that falls between the last known fish and the first known tetrapods, consistent with a transitional position [1]. It was found in rocks from a shallow-water wetland environment, which is precisely the type of habitat where the predicted transition between aquatic and early semi-terrestrial life would have been occurring [1].
Q3.3 — Is Tiktaalik the direct ancestor? (2 marks)
The claim overstates the case. A transitional fossil only needs to show the predicted mix of traits expected near a branching transition — it does not need to be a direct ancestor [1]. Tiktaalik could be a close relative of the fish-to-tetrapod transition line rather than the exact ancestral lineage of all tetrapods. The significance of Tiktaalik is the combination of features it shows, not whether it is the direct ancestor [1].