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Name:
Platycrinites hemisphericus
Age: Mississippian
Formation: Edwardsville Formation
Location: Crawfordsville, Indiana
Size: Crinoid is 4.3 inches long
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This is a special crinoid fossil with a story to tell. It is a large Platycrinites hemisphericus from the Mississippian of Indiana. This type of camerate crinoid has a cup composed of large, ornamented plates and multi-branching, biserial arms - this 3-dimensional specimen exhibits all of these features in fine fashion. The plate structure of the crinoid is clearly defined, and the long, curving arms are beautifully displayed. Another striking feature of this crinoid is the unique, twisted, ''corkscrew'' stem (indicative of the genus) still tightly attached to the base of the cup. But that's not all - there is a large gastropod, Platyceras, attached to the cup. This gastropod fed off the excrement from the crinoid, and in doing so, kept the environment clean so that the crinoid could keep living. This is a rare example of a true symbiotic relationship in the fossil record. The crinoid is well showcased on the large, sculpted plate of limestone. This is a extremely uncommon example of an ancient animal relying on another animal for its sustenance.
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