Crinoids: Sea lilies
Crinoids are echinoderms, a group that includes the starfish, sea urchins, and sand dollars. Sometimes called sea lilies, crinoids resemble long-stemmed flowers, but they are marine animals. A holdfast at the base of the animal’s stem functions like a root that holds the animal in place. The animal’s cuplike body, or calyx, is composed of a mosaic of geometric plates. The calyx has many arms that open into a fan-like net so the crinoid can feed on microscopic food particles. The flexible stem is composed of a series of button-like discs called columnals. When crinoids die, their stems fall apart, or disarticulate, into columnals. The lumen, a hole in the middle of the stem, contained a tube for carrying nutrients to the stem and holdfast. The mouth of the crinoid is on the top of the calyx.
Crinoids first appeared in the Cambrian and diversified until the Permian extinction, when their numbers were greatly reduced. Complete crinoid fossils may occasionally be found in Wisconsin’s Silurian and Devonian rock, but most crinoid fossils consist of scattered columnals. In the Paleozoic, crinoids lived in colonies in shallow waters, but today they live in deeper regions of the world’s oceans.
Bryozoans: Moss animals
Bryozoans are small, mostly marine animals that form skeletons of a variety of shapes: tiny twigs, nets, domes, and mossy crusts. The zooecium (skeleton) houses highly structured colonies of zooids, or cuplike animals. Bryozoans, like brachiopods, contain a tentacle-bearing lophophore used in feeding and gas exchange. They also contain a U-shaped gut with a distinct mouth and anus.
Bryozoans are inconspicuous fossils, but can be seen most easily on slabs of Ordovician limestone or dolomite with the aid of a magnifying glass or hand lens. Bryozoans are difficult to distinguish from one another. In most cases you may not be able to identify your fossil more specifically than as a bryozoan.
Bryozoans were able to live almost anywhere: on a brachiopod shell, the side of a cephalopod, coating sea plants, or in an individual colony on the sea floor. Many fossils have patches of bryozoans on them. Bryozoans first evolved in the Ordovician and have persisted to the present day; they live in ocean habitats that are similar to those of Paleozoic seas.
Stromatoporoids: Laminated reef builders
Stromatoporoids were colonial marine organisms related to sponges. They formed skeletons composed of thin layers called laminae. Small bumps called mamelons were present on their surface. Stromatoporoids formed reefs in the shallow Silurian seas of Wisconsin. They became extinct 65 million years ago.
Ostracods: Bivalved arthropods
Ostracods are small, two-valved arthropods that resemble small, dark, polished beans. Ostracods first appeared in the Cambrian and still exist today in freshwater, marine, and terrestrial environments. Like other arthropods, ostracods molt their skeletons as they grow. Although they are not very common, ostracods are occasionally found in Wisconsin’s Ordovician and Silurian rock.
Hyolithids: Animals of unknown affinities
Very little is known about these mysterious animals. Hyolithids are an unusual, extinct group of marine animals that had a conical shell with an operculum. They are sometimes found in the digestive tracts of fossil marine worms. Middle Ordovician rock in Wisconsin commonly contains hyolithids.
Receptaculitids: Skeleton-secreting algae
Receptaculitids are another extinct group of marine organisms that scientists have had difficulty classifying. Receptaculitids have been classified as corals and sponges, but they are now thought to be colonial algae. They superficially resemble favositids, the honeycomb corals. Receptaculitid fossils are most common in Ordovician rock.