AQA GCSE Combined Science: Trilogy Biology Paper 2 (Foundation), June 2025: Question 4

✅ Correct Answer (Any 1)

• The skin (or soft tissue) decayed / rotted away.
• Skin was not fossilised / only hard parts (bones) formed fossils.
• There is no fossil evidence / no intact skin found.
• No human has ever seen a living plesiosaur.

💡 Key Knowledge

Fossils commonly form when hard parts (bones, shells, teeth) are replaced by minerals. Soft tissues (skin, muscle, internal organs) decompose rapidly due to decomposers (bacteria and fungi) and oxygen before fossilisation can occur.

🧠 Exam Technique

When asked why scientists "cannot be certain" about external appearances (colour, skin texture, feathers) of extinct animals, always refer to the decay of soft parts or lack of preserved specimens.

❌ Common Errors

Do NOT simply write "the fossils were broken". Be specific that skin decays or soft tissue does not turn to stone.

✅ Mark Scheme Breakdown

• Mark 1 Feature: Limbs / feet / arms are flat(tened), paddle-like, wide, or streamlined (flippers / fins).
• Mark 2 Function: Enables efficient swimming / propulsion / chasing prey / escaping threats.

🧠 Exam Technique: "Explain How"

This is a 2-mark question requiring two linked parts: Structure + Function.

Formula: [Describe the physical feature] so that [state how it helps movement/hunting].

✅ Correct Answer

☑ Behavioural

💡 Types of Adaptations

• Structural: Physical body parts (e.g. sharp claws, paddle-shaped flippers).
• Behavioural: Actions or movements (e.g. moving rapidly upward, migrating, hunting in packs).
• Functional / Chemical: Internal processes (e.g. producing venom, shivering, concentrating urine).

✅ Correct Reading

200 million years ago.

🧠 Graph Reading Tip

On geological timelines, time runs backwards towards the left (older = larger number; present day = 0 on the far right). "First evolved" means the oldest point: the left-hand boundary.

📐 Step-by-Step Calculation

1. Determine scale: The interval from 150 to 200 is 50 million years over 10 small squares. Each small grid square = 5 million years.
2. Find start & end points:
   Start of bar B = 180 million years ago
   End of bar B = 130 million years ago
   1 mark for identifying 180 and 130 (or counting 10 squares × 5).
3. Calculate duration:
   180 - 130 = 50 million years 1 mark .

❌ Common Trap

Misreading the axis intervals. Students often assume each grid square is 1 or 2 units. Always check: 50 million years ÷ 10 squares = 5 million years per square.

✅ Correct Choices

☑ Not all plesiosaur fossils have been discovered.

☑ Some plesiosaur fossils will have been destroyed.

💡 Why the Fossil Record is Incomplete

• Many organisms did not fossilise (wrong conditions).
• Fossils have been destroyed by geological activity (earthquakes, erosion, mountain building, weathering).
• Many fossils remain deeply buried and undiscovered.

❌ Incorrect Distractors

• "All plesiosaurs will have formed fossils" — False, fossilisation is extremely rare.
• "Plesiosaur bones do not decay" — False, bones can decay under standard biological conditions.
• "Plesiosaurs had many predators" — Irrelevant to fossil dating.

✅ Correct Answer

☑ Plesiosaur B existed before plesiosaur C.

🧠 Exam Technique

An ancestor must exist before or overlap with its descendant in the fossil record. Bar B starts at 180 million years ago, whereas bar C starts later at 145 million years ago.

✅ Accepted Marking Points (Any 2)

• Bacteria reproduce very quickly / short generation time (evolution can be observed in days or months).
• Bacteria are still alive / continually evolving.
• Scientists can extract and sequence DNA directly from bacteria (DNA is rarely intact in old fossils).
• Bacteria can be easily cultured in large numbers in a lab (fossil record is scarce and has gaps).

💡 The Power of Bacterial Models

Because bacteria like E. coli can divide every 20 minutes, hundreds of generations can be observed in a few weeks. In contrast, complex vertebrate evolution requires millions of years and relies on incomplete fossil finds.

✅ Correct Reasons (Any 1)

• Infections cannot be treated or cured with regular antibiotics.
• Infections can spread easily and cause severe illness or death.
• Developing new effective antibiotics is very slow and costly.

❌ Common Error: Misunderstanding Immunity

Never write: "The patient becomes immune to the antibiotic" or "Our bodies become resistant."

It is the bacteria that mutate and become resistant to the drug, not human cells or antibodies!