RocksdayAmmonite vs Orthoceras
How to tell them apart
Coiled shells of extinct squid-like sea creatures that vanished with the dinosaurs.
Photo: Llez (H. Zell)., CC BY-SA 3.0
Straight, cone-shaped shells of ancient nautiloids, often polished in black limestone plates.
Photo: Parent Géry, CC BY-SA 3.0
Key differences
- Orthoceras shells are straight, not coiled.
- Ammonites are coiled.
Side by side
| Property | Ammonite | Orthoceras |
|---|---|---|
| Type | Fossil | Fossil |
| Mohs hardness | 3–4 | 3–4 |
| Luster | Pearly to dull | Dull (polished in shops) |
| Colors | Brown, Gray, Gold, White | Black, Gray, Red |
| Crystal system / structure | Coiled shell (aragonite, calcite or pyrite) | Straight chambered shell (calcite) |
| Specific gravity | 2.6–2.9 | 2.6–2.8 |
| Found in | Jurassic Coast, England; Mahajanga, Madagascar | Erfoud, Morocco; Öland, Sweden |
Properties that are the same for both are grayed out.
How to identify ammonite
- Flat spiral shell with ribs.
- Wavy, frilly suture lines where internal walls met the shell.
- Often split and polished to show chambers.
Structure: Coiled shell (aragonite, calcite or pyrite)
Full Ammonite guide →How to identify orthoceras
- Long straight cones with chamber walls across them.
- Often in polished black or red limestone.
- Fizzes in acid.
Structure: Straight chambered shell (calcite)
Full Orthoceras guide →Could you tell them apart from a blurry close-up?
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