4th Grade Science worksheets - Edform - Free Printable

Let’s go through each fossil one by one and match it to the correct type: mold, cast, trace, or true form.

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1. Fish imprint in mud → hardened into stone
The fish body decomposed, but left an *imprint* (a hollow shape) that filled with minerals. That’s a mold — it’s like a negative space where the organism used to be.

→ This is a mold fossil.

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2. Dinosaur footprint stuck in mud
It’s not the dinosaur itself — it’s evidence of its activity (walking). Footprints are classic examples of trace fossils.

→ This is a trace fossil.

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3. Frog trapped in sticky sap → hardened into amber
The whole frog was preserved inside amber — no decomposition, just encased. That’s a true form fossil, because the actual organism (or most of it) is still there.

→ This is a true form fossil.

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4. Fern leaf fell in puddle → dried and decayed → left imprint
Again, the plant didn’t stay — only the impression remained. That’s another mold fossil.

→ This is a mold fossil.

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5. Shell left imprint in sand → sand hardened → shell washed away
Same idea — the shell made a hole/imprint, then disappeared. The rock now has the shape of the shell. That’s a mold.

Wait — actually, let’s double-check: if the shell left an imprint and then was gone, and the rock kept that shape… yes, that’s still a mold. But sometimes people get confused — if something fills that mold later, it becomes a cast. Here, nothing filled it — so it’s still a mold.

→ This is a mold fossil.

BUT — wait! Let me re-read: “This shell left an imprint in the sand. After many years, the sand hardened to form rock. The shell was long gone, but its imprint was left in the rock.”

Yes — that’s definitely a mold.

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6. Snake slithered through clay → track preserved
Another example of movement — not the snake itself, but its trail. That’s a trace fossil.

→ This is a trace fossil.

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7. Animal died → body decomposed → imprint left behind → rocks kept same shape as bones
So the bones rotted away, but their shape was preserved in the rock. That’s again a mold — the space where the bones were.

But hold on — if the rocks remain the same shape as the bones, does that mean the bones turned to stone? No — it says “its body decomposed” and “the rocks that remain are the same shape”. So it’s the impression — still a mold.

Actually — this might be tricky. If the original material is gone and only the shape remains in the surrounding rock, it’s a mold. If mineral replacement happened, it could be petrified (true form), but here it says “decomposed”, so likely mold.

→ This is a mold fossil.

Wait — let’s compare to #1: same thing — body decomposed, imprint left, filled with minerals? In #1 it said “filled with minerals which hardened into stone” — that would make it a cast? Oh no — I think I made a mistake earlier!

Let’s clarify definitions:

- Mold: impression/cavity left after organism decays.
- Cast: when that mold gets filled with sediment/minerals, creating a 3D copy.
- Trace: evidence of behavior (footprint, burrow, etc.)
- True form: actual remains preserved (amber, ice, tar, etc.)

Re-evaluating #1:
“When this fish died, its body left an imprint in the mud. The body decomposed but the mold was filled with minerals which hardened into stone.”
→ So first, mold formed. Then it got filled → that makes it a cast!

Oh no — I misread that initially. Important detail: “the mold was filled with minerals” → so it’s no longer just a mold — it’s a cast.

Similarly, #5: “shell left imprint... shell was long gone, but its imprint was left” → no mention of filling → so that’s still a mold.

#7: “body decomposed and the imprint it left behind was filled with minerals. The rocks that remain are the same shape and size as the bones were.” → Again, imprint + filled with minerals = cast

Okay — let’s redo all with careful reading:

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1. Fish: Imprint → mold → then filled with minerals → hardens → that’s a cast

→ cast

2. Dinosaur footprint: Just the print — no filling mentioned → trace

→ trace

3. Frog in amber: Whole organism preserved → true form

→ true form

4. Fern leaf: Left imprint, dried, decayed → only imprint remains → mold

→ mold

5. Shell in sand: Left imprint, sand hardened, shell gone → only imprint → mold

→ mold

6. Snake track: Movement痕迹 → trace

→ trace

7. Animal bones: Decomposed → imprint → filled with minerals → rocks keep bone shape → cast

→ cast

8. Mammoth frozen in ice: Body completely preserved → true form

→ true form

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Final check:

1. Cast
2. Trace
3. True form
4. Mold
5. Mold
6. Trace
7. Cast
8. True form

That matches standard definitions.

Final Answer:
1. cast
2. trace
3. true form
4. mold
5. mold
6. trace
7. cast
8. true form