Placoblastus obovatus
Class Blastoidea, Order Spiraculata, Family Nucleocrinidae
Cyclocystoides sp
Class Cyclocystoidea, Order Cyclocystoididae
Geological Time: Middle Devonian
Size (25.4 mm = 1 inch): Blastoid: 33 mm by 20 mm Cyclocystoid: 4 mm across on a 45 mm by 65 mm matrix
Fossil Site: Thunder Bay Limestone, Alpena County, Michigan
Code: AW72
Price: Sold
 Description:
Here is an association piece of two classes of Echinoderms with
wildly disparate morphology. Blastoids are extinct stemmed echinoderms
whose morphology bears a resemblance to a hazel nut that arose
during the Ordovician. They reached their pinnacle of diversity
during the Mississippian, only go extinct by the time of the Great
Dying at the end of the Permian. This one is a member of the genus
Placoblastus. Note the ambulacral groves that housed the brachioles
with which the blastoid filtered particulates from the water column.
Interestingly enough, the second class represented is not commonly
seen, an example of an enigmatic family of Echinoderms known as
the cyclocistoids. The group is little known and scantily understood.
The digestive system is incompletely known. They consist of a completely
plated disc encircled by a pleated peripheral skirt. It is thought
that the mouth was oriented on the ventral surface to allow for
collection of the microscopic food that they presumably fed upon.
They are found from the Middle Ordovician of North America to the
Middle Devonian of Europe. The only other example I have had came
from the Ordovician of New York. |
