How to Identify Common Fossils: A Visual Guide
How to Identify Common Fossils: A Visual Guide
Found a fossil but not sure what it is? This guide covers the most commonly encountered fossil types, with key identification features to help you narrow down what you've discovered.
Step 1: Determine the Fossil Type
Before identifying the organism, determine how it was preserved:
- Three-dimensional body fossil: Actual preserved remains (shells, bones, teeth)
- Impression/compression: Flat imprint on rock, possibly with a dark carbon film
- Mold: Hollow space where the organism dissolved away
- Cast: Mineral-filled replica of the original organism
- Trace fossil: Evidence of behavior (footprints, burrows), not body parts
The preservation type helps narrow identification because it tells you something about the original organism and its environment.
Step 2: Marine or Terrestrial?
Check the rock type:
- Limestone, chalk, marine shale → Marine organism (most common)
- Sandstone, terrestrial shale, coal measures → Could be terrestrial (plants, insects, vertebrates)
- Freshwater limestone or mudstone → Freshwater organism (fish, plants, snails)
Marine invertebrate fossils are by far the most commonly found.
Common Marine Fossils
Brachiopods
What they look like: Two shells (valves) joined by a hinge. Superficially resemble clams but differ in key ways: brachiopod shells have a line of symmetry running from top to bottom (each valve is different), while clam shells have symmetry running along the hinge (each valve is a mirror of the other).
Where found: Marine limestone, especially Paleozoic. Extremely common.
Key features: Often have radial ribs or growth lines. Some have a hole (pedicle foramen) at the beak for attachment.
Crinoid Stems
What they look like: Circular, star-shaped, or pentagonal discs, often with a hole in the center. Individual segments stack together like a roll of coins.
Where found: Marine limestone, especially Carboniferous. Very abundant — some limestone beds are essentially made of crinoid fragments.
Key features: Regular geometric shape (circular or pentagonal). Central hole. May show a star pattern on the surface.
Trilobites
What they look like: Three-lobed body divided into head (cephalon), body (thorax), and tail (pygidium). Compound eyes often visible.
Where found: Marine shale and limestone. Paleozoic only (they went extinct 252 Mya).
Key features: Three longitudinal lobes (hence the name). Segmented body. Often found enrolled (curled up defensively).
Ammonites
What they look like: Coiled spiral shells, often with prominent ribs or knobs. Cross-sections show chambers divided by intricate suture lines.
Where found: Marine shale and limestone. Mesozoic (Jurassic and Cretaceous are best).
Key features: Spiral coiling (usually in a single plane). Ribbed surface. Complex suture patterns visible on worn specimens.
Echinoids (Sea Urchins)
What they look like: Round or heart-shaped tests (shells) covered with small bumps where spines attached. Five-fold symmetry pattern visible.
Where found: Chalk and limestone. Mesozoic and Cenozoic.
Key features: Five-rayed symmetry. Tubercles (bumps) for spine attachment. Mouth on the underside.
Gastropods (Snails)
What they look like: Spiral shells, usually coiling in three dimensions (unlike the flat spiral of ammonites).
Where found: Limestone, marine and freshwater deposits. All periods.
Key features: Asymmetric spiral coiling. Aperture (opening) on one side.
Corals
What they look like: Varied — honeycomb patterns (tabulate corals), horn shapes (rugose corals), or branching forms.
Where found: Marine limestone. All periods, but different types dominate different eras.
Key features: Regular repeating pattern of cells or chambers. Radial symmetry within individual corallites.
Common Terrestrial Fossils
Plant Fossils (Leaves, Ferns)
What they look like: Leaf shapes with visible vein patterns, often as dark impressions on shale or mudstone.
Where found: Shale, mudstone, coal measures, sandstone.
Key features: Recognizable leaf or frond shapes. Vein patterns. Dark carbonaceous film.
Shark Teeth
What they look like: Triangular, pointed teeth with a root at the base. Hard enamel surface, often glossy.
Where found: Beach deposits, marine sediments, phosphate beds.
Key features: Triangular shape. Serrated edges (in many species). Hard enamel distinct from surrounding rock.
Petrified Wood
What they look like: Wood that has turned to stone but retains visible growth rings, bark texture, and cellular structure.
Where found: Volcanic sediments, arid regions.
Key features: Wood grain and growth rings visible. Heavy (much denser than normal wood). Often colorful due to different mineral deposits.
When to Seek Expert Help
Contact a local museum, university geology department, or fossil club if you find:
- Vertebrate fossils (bones, teeth of fish, reptiles, or mammals) — these may be scientifically significant
- Unusually well-preserved specimens with soft-tissue detail
- Fossils you cannot identify after using guides and online resources
- Large or complex specimens that might require professional excavation
Use our interactive Fossil Identification Guide for a step-by-step identification process, or browse the Eon Codex collection to compare your find with cataloged specimens.