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Fossils in thin section

In limestones, most grains are pieces of organisms. Shape, size, wall structure and how the grain goes extinct under crossed polars tell you which group it came from, and that tells you what kind of rock and environment you're looking at.

Quick tips

  • A grain that goes extinct all at once as a single crystal, often with a dusty mesh texture: echinoderm (crinoid) plate.
  • A thin, curved shell ending in a hook ("shepherd's crook") with sweeping extinction: trilobite.
  • A spar-filled mold with only the outline left: originally aragonite, usually a mollusc.
  • Multiple small chambers in a row or coil, under about 1 mm: foraminifera.
  • Tiny two-valved shells with overlapping edges: ostracods.

Groups

ForaminiferaBrachiopodsMolluscs: bivalvesMolluscs: gastropodsMolluscs: cephalopodsEchinoderms (crinoids, echinoids)BryozoansCoralsTrilobitesOstracodsSponges and spiculesCalcareous algaeStromatolites and stromatoporoidsno open-licensed photoRadiolariansno open-licensed photoCalcareous nannoplanktonBarnacles

Overview plates

Shelled fossils: brachiopods, bivalves, barnacles, trilobites
Shelled fossils: brachiopods, bivalves, barnacles, trilobites. Page Quinton via Wikimedia Commons, CC BY-SA 4.0. Source
Chambered fossils: cephalopods, sponges, gastropods, foraminifera
Chambered fossils: cephalopods, sponges, gastropods, foraminifera. Page Quinton via Wikimedia Commons, CC BY-SA 4.0. Source
Colonial fossils: tabulate and rugose corals, bryozoans, sponges
Colonial fossils: tabulate and rugose corals, bryozoans, sponges. Page Quinton via Wikimedia Commons, CC BY-SA 4.0. Source
Crinoid plates and columnals, brachiopod spines, calcispheres, spicules, conodonts
Crinoid plates and columnals, brachiopod spines, calcispheres, spicules, conodonts. Page Quinton via Wikimedia Commons, CC BY-SA 4.0. Source

Photos are hosted by their original sites and shown under their open licenses (CC BY, CC BY-SA, public domain or Open Government Licence). Click a photo for its source page.

Sources

Rygel & Quinton, Sedimentary Geology 9.7: Fossils in Thin Section · SEPM Strata: Skeletal Grains · Rygel & Quinton, Sedimentary Geology 9.5: Microfossils