Echinoderms: Cystoids, Blastoids, and Crinoids

Modern Crinoid
Crinoid on the reef of Batu Moncho Island, near Komodo, Indonesia, photographed by Alexander Vasenin

Echinoderms living today include starfish, sea urchins, sand dollars, sea cucumbers, and sea lilies, but there are many other species in the fossil record. Echinoderms have an external calcite skeleton and live on the ocean floor, where they use their tube feet to move and open the shells of their mollusk prey. Starfish and sea urchins are found as early as the Ordovician Period, 490 million years ago. The most prevalent echinoderm fossils in Illinois are cystoids, blastoids, and crinoids (sea lilies).

Fossilized Cystoid
Fossilized cystoid from the Middle Ordovician in northeastern Estonia, photographed by Mark A. Wilson

Cystoids lived from the Ordovician Period, 490 million years ago, to the Devonian Period, 370 million years ago. Their fossils are found in Ordovician and Silurian rocks in Chicago quarries and bluffs along the Mississippi River in northwestern Illinois. Their arms and egg-shaped bodies were irregular in form and they had triangular pore openings. Their bodies were often attached to the substratum by a stalk, with their mouth at the opposite end.

Blastoid from the Lower Carboniferous Period of Illinois
Blastoid from the Lower Carboniferous Period of Illinois, photographed by Mark A. Wilson

Blastoids, which may have evolved from cystoids, date back to the Silurian Period, 425 million years ago. Their mouth was surrounded by small round holes that conducted food in and waste out. Long hair-like brachioles swept food toward the mouth. Blastoids disappeared around 260 million years ago during the Permian extinction. Commonly found in river cliffs and stream banks of western and southwestern Illinois and areas near the Ohio River, blastoid fossils resemble hickory nuts.

Fossilized Crinoid
Fossil crinoid Xenocrinus baeri, an Upper Ordovician crinoid calyx from the Bull Fork Formation, Caesar Creek, Ohio, photographed by Mark A. Wilson

Crinoids are marine animals with a body on the end of a long stem of discs anchored to the ocean floor. Arms sweep food into the mouth at the top of the body, which is made of calcium carbonate plates. Fossil crinoid stem discs are common in Illinois and have been called “Indian beads”. Many limestone beds in Illinois are composed mainly of fossilized crinoids. The earliest crinoid fossils date to the Ordovician Period, 490 million years ago, and they still live deep in the ocean.