Crinoids (cry'-noids) are called "sea lilies," but they are animals rather than plants. They look like plants, however, because the body skeleton or calyx generally is on the end of a stem made of button-like discs and held on the sea floor by either a stony anchor or root-like arms. The mouth, on top of the body, is surrounded by arms that sweep food into it. The body is made of calcareous plates that fit together like irregular bricks.
When the animal dies, the plates and discs tend to fall apart and sink to the sea floor. Crinoid stems or stem discs are common throughout the Midwest, and their popular names include "Indian beads" or "fish bones." Many limestone beds are composed mostly of crinoid plates and discs. The complete calyx is a highly prized fossil. Well-preserved specimens are found in the limestone cliffs along the Mississippi River between Burlington and Alton.
The oldest crinoids come from Ordovician rocks. Some crinoids live today, mainly in deep parts of the ocean, but they are not nearly as common as in the past.
Burlington Limestone
The Burlington Formation is a marine limestone, found in rock layers laid down during the Mississippian Period (325-360 million years ago). The Mississippian Period was relatively short-lasting only 35 million years-and its fossil record is almost entirely marine. This is due to the flooding of North America called the Kaskaskia transgression, which began during the Devonian Period (380-450 million years ago). The Mississippi Valley was covered by this clear, shallow, warm inland sea that advanced from the south. During this period, limestone was deposited to a total thickness of more than 2,000 feet in some areas.
The Burlington Limestone is widespread throughout the mid-continent region. It is known from Iowa to northwestern Arkansas and from western Illinois to western Kansas. As observed in most exposures, the Burlington Limestone is unusually course-grained, crystalline, crinoidal limestone. Its texture is sufficiently distinctive and persistent to permit recognition of the formation commonly on this basis alone. The Burlington Limestone is made of almost entirely on the remains of various fossils, by far the most important of which are crinoids.
A large proportion of the crinoid species are restricted to the Burlington Formation. The Burlington Formation contains a record of the continuation and acceleration of crinoidal development; the clear, shallow waters of the Mississippi Valley being apparently a region of maximum differentiation and dispersal of crinoids. Crinoids flourished because they were filter feeders, and most of the particles in the clear oceans would have been bits of food. They also needed warm water to produce their elaborate skeletons, since warm water can hold more dissolved calcium carbonate than cold, making it easier to precipitate.
One can imagine the crinoids growing in extensive marine meadows, rippling in the waves on their long, thin, graceful columns, like garden flowers in the wind. On rare occasions, they were preserved whole, flattened to the bottom by a storm and quickly covered with lime mud. Unfortunately, the organic matter connection the plates and columnals of the stem nearly always rotted in the water and plates were scattered and sorted by the waves to form crinoidal limestones
Logging this cache
At the listed coordinates, you will find some horizontal pieces of Burlington Limestone with crinoid fossils visible on the upper surface.
Please send us a message stating whether these fossils are more cylindrical in shape or more flowery in shape.
Further interest
The gray retaining wall a few feet north of the listed coordinates is also constructed of Burlington Limestone. Close inspection will reveal numerous crinoid fossils throughout.
You are currently very, very close to the Des Moines County Heritage Center which offers a crinoid exhibit with additional information.
Crinoids are so abundant and important that Burlington was named “The Crinoid Capital of the World.”
For more information about crinoids: https://en.wikipedia.org/wiki/Crinoid
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