RocksdayBrachiopod vs Trilobite
How to tell them apart
Fossil shells of marine animals that look like clams but are only distantly related.
Photo: James St. John, CC BY 2.0
Fossil sea arthropods with three-lobed bodies that lived for nearly 300 million years.
Photo: James St. John, CC BY 2.0
Quick tests
- Acid (vinegar or dilute HCl): Brachiopod fizzes; Trilobite doesn't.
Key differences
- Trilobites are segmented, not two shells.
- Brachiopods are two-part shells with no segments.
Side by side
| Property | Brachiopod | Trilobite |
|---|---|---|
| Type | Fossil | Fossil |
| Mohs hardness | 3 | 3–4 |
| Luster | Dull | Dull to vitreous |
| Colors | Gray, Brown, White | Black, Gray, Brown |
| Crystal system / structure | Two-part calcite shell | Calcite exoskeleton |
| Specific gravity | 2.6–2.8 | 2.6–2.8 |
| Found in | Cincinnati area, Ohio; Kentucky | Wheeler Shale, Utah; Anti-Atlas, Morocco |
Properties that are the same for both are grayed out.
How to identify brachiopod
- Two shells of different sizes.
- Each shell is symmetrical down its middle.
- Ribs or growth lines fanning out.
Structure: Two-part calcite shell
Full Brachiopod guide →How to identify trilobite
- Oval body split lengthwise into three lobes.
- Distinct head, segmented middle and tail shield.
- Often curled up or found as molted pieces.
Structure: Calcite exoskeleton
Full Trilobite guide →Could you tell them apart from a blurry close-up?
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