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To solve this worksheet, we need to identify which images represent fossil casts.

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🔍 What is a fossil cast?

A fossil cast forms when:
1. An organism (like a shell, bone, or plant) is buried in sediment.
2. The original material decays or dissolves away, leaving a cavity or mold in the rock.
3. Minerals carried by groundwater fill that cavity and harden, forming a 3D replica of the original organism — this is the cast.

> ✔ A cast is a solid, positive impression — it looks like the original object, but is made of mineral deposits, not the original material.

Compare this to:
- Fossil mold: Just the hollow impression left behind (negative space).
- Original preservation: The actual shell or bone is still there (e.g., permineralized or unaltered).
- Trace fossil: Footprint, burrow, etc. — not the body itself.

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## 🧐 Let’s analyze each image:

We’ll go row by row, left to right.

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Row 1

1. Top-left (fish fossil):
→ Shows fish skeleton preserved in rock. This looks like original bones or permineralization, not a cast.
✘ Not a cast.

2. Top-middle (ammonites):
→ These are impressions in rock — you can see they’re sunken into the surface. This is a mold, not a cast.
✘ Not a cast.

3. Top-right (ichthyosaur skeleton):
→ Again, this shows actual bones preserved in rock — likely permineralized remains.
✘ Not a cast.

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Row 2

4. Middle-left (fern-like plant):
→ This is a carbon film or compression fossil — organic material flattened and preserved as carbon.
✘ Not a cast.

5. Middle-middle (dragonfly):
→ Very thin, flat impression — this is a mold or compression fossil.
✘ Not a cast.

6. Middle-right (ammonite close-up):
→ This is raised, solid, and detailed — looks like it was filled in after the shell dissolved. This is a cast.
✔ CAST

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Row 3

7. Bottom-left (trilobite):
→ Solid, 3D, raised trilobite — very likely a cast formed in the mold left by the original.
✔ CAST

8. Bottom-middle (many ammonites):
→ These look like molds — impressions pressed into the rock. Some might be casts, but most appear sunken.
✘ Not clearly casts.

9. Bottom-right (spine/vertebrae):
→ Looks like actual bones preserved — possibly permineralized.
✘ Not a cast.

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Row 4

10. Bottom-left (large ammonite on rock):
→ Raised, solid, detailed — classic example of a cast.
✔ CAST

11. Bottom-middle (boxed ammonites):
→ These are real fossils — many are original shells or molds. Since they’re collected specimens, some may be casts, but visually, they look more like original or molds.
✘ Not clearly casts.

12. Bottom-right (ammonite close-up):
→ Clearly raised and solid — this is a cast.
✔ CAST

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## ✔ Final Answer: Circle these 4 images

You should circle:

- Image 6 (middle-right ammonite)
- Image 7 (trilobite)
- Image 10 (large ammonite on rock)
- Image 12 (close-up ammonite)

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## 📝 Summary Explanation for Student:

> A fossil cast is a 3D copy of an organism that formed when minerals filled in the space left after the original body decayed. Look for fossils that are raised up from the rock, look solid, and have detailed, positive relief — like a sculpture of the original. Avoid flat impressions (molds) or actual bones/shells (original preservation).

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✔ CIRCLE IMAGES: 6, 7, 10, 12

*(Note: Image numbers refer to position in grid: Row 1 = images 1–3, Row 2 = 4–6, Row 3 = 7–9, Row 4 = 10–12)*

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