The small, disc-like object with a central hole you found is likely a piece of a crinoid stem, one of the world’s most common fossils. These are the mineralized remains of an ancient marine animal’s stalk, which was a long, flexible column. The abundance of these fossils in certain rock layers indicates that vast underwater forests of these creatures once populated the seafloor.
The Crinoid Animal
Crinoids are often called “sea lilies” due to their plant-like appearance, but they are animals related to modern starfish and sea urchins. They belong to the phylum Echinodermata, and their bodies are organized in patterns of five, a characteristic known as pentameral symmetry. The skeleton is composed of calcite plates, or ossicles, embedded within its skin.
An ancient crinoid’s body had three main parts. A root-like holdfast anchored the animal to the sea bottom. From this anchor rose the stem, or column, which elevated the main body into water currents. At the top of the stem was the crown, which included a cup-shaped calyx and feathery, branching arms used for filter-feeding on plankton.
While most crinoid fossils belong to extinct, stemmed species, crinoids still exist in today’s oceans. Many modern forms are free-swimming species called “feather stars,” which lack the long stalk of their ancient relatives. The shift away from stemmed forms in shallow water may have been influenced by increased predation pressure during the Mesozoic Era.
Anatomy of the Stem
A crinoid’s fossilized stem is not a solid piece but a column of stacked, disc-like plates called columnals. When the crinoid was alive, ligaments held these ossicles together, and the disc surfaces often had radiating ridges to help them interlock. Individual columnals are found frequently because the ligaments decayed after death, causing the stem to disintegrate.
Columnals are most often circular but can also be pentagonal, square, or oval. Each disc has a central hole, called the lumen or axial canal, which once contained the soft tissues and nerves running the length of the stem. The shape of this lumen is useful for identification and can be round, star-shaped, or divided into multiple canals.
Finding Crinoid Fossils
Crinoid fossils are common because their calcite skeletons were durable and fossilized easily. They are abundant in rocks from the Paleozoic Era, especially the Silurian and Carboniferous periods, when they thrived in shallow marine environments. The remains were sometimes so plentiful they became the primary component of the rock, forming crinoidal limestone.
These fossils are found in sedimentary rocks that formed in ancient oceans, such as limestone and shale. Fossil hunters can find them where these rock layers are exposed, including creek beds, quarries, and roadside cuts. In some areas, they can be found by looking through shingle and gravel.
Certain crinoid species are useful to geologists as “index fossils” to help determine the age of rock layers. This is because they were prevalent only within specific time intervals. For instance, some species from the late Cretaceous period are so abundant in narrow bands that they define distinct geological time markers called biozones.