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Ordovician · 485.4 – 443.8 Ma

Ordovician fossil sites

The Ordovician was a period of warm seas and explosive marine diversification. Trilobite morphology became extravagant. The first true reef-building corals appeared. Nautiloid cephalopods grew to over five meters. The period ended with one of the largest mass extinctions in Earth's history.

A limestone bedding surface crowded edge to edge with brachiopod shells
Photo: James St. John, CC BY 2.0, via Wikimedia Commons

Ordovician seas were warm, shallow, and crowded. Reefs built by bryozoans and corals spread across tropical shelves. Brachiopods carpeted the sea floor in numbers that modern shellfish beds rarely match. The period ended in a severe glaciation and one of the largest extinctions in the whole fossil record.

The rocks reflect that abundance. Ordovician limestone often breaks into slabs packed edge to edge with shells. Brachiopods outnumber everything else. Bryozoan colonies appear as lacy sheets or twiggy stems. Trilobites are less common but far more sought after, and Isotelus grew larger here than in almost any other period.

Coverage here runs to twenty-one Ordovician sites. The Cincinnati region of Ohio supplies five of them, and its roadcuts and lake spillways rank among the most productive fossil grounds in North America. Morocco supplies four more, worked commercially for trilobites. Eight of the twenty-one allow free collecting and four require a permit, so read the access note before you travel.

21 fossil sites

Updated 1 August 2026

Life of the period

Three to know from the Ordovician

You will not find all of these yourself. They are the animals and plants that make the period worth reading about.

  • A section of fossil nautiloid shell laid on a plain surface, with pale bands across it marking the walls between chambers.
    Straight-shelled nautiloidCameroceras

    A cephalopod in a straight cone rather than a coil. Length estimates have fallen over the years. It remains among the largest animals of its time, and it hunted in shallow tropical seas.

    James St. John, CC BY 2.0

  • A diorama of an Ordovician sea floor with a large trilobite beside orange coral.
    Giant trilobiteIsotelus

    A specimen found in Manitoba in 1998 measures over 70 centimetres and is the largest complete trilobite ever recorded. Most trilobites you find fit on a thumbnail.

    James St. John, CC BY 2.0

  • A small grey fossil trilobite photographed against a white background beside a centimetre scale.
    Enrolled trilobiteFlexicalymene

    Rolls into a tight ball for defence, exactly as a woodlouse does. Enrolled specimens are the ones collectors want, because the animal died in the act and preserved in three dimensions.

    Smithsonian National Museum of Natural History, CC0

All Ordovician sites

Frequently asked questions

Where is the best Ordovician collecting?
The Cincinnati Arch, which covers southwest Ohio, southeast Indiana, and northern Kentucky. Roadcuts and lake spillways expose shell-rich limestone and shale. Weathered slopes at the base of an exposure usually produce more than the fresh rock face above.
What is the most common Ordovician fossil?
Brachiopods, by a wide margin. Look for two shells of unequal size that are symmetrical about a line down the middle. That symmetry separates them from clams, which are instead symmetrical between the two shells.
Are Ordovician trilobites easy to find?
Whole ones are not. Fragments are common in the right beds. Flexicalymene and Isotelus are the two genera most collectors hope for around Cincinnati, and enrolled Flexicalymene specimens are the prize.