Blastoid Association Plate from Burlington Formation

Name: Echinodermata, Class Blastodea, Orophocrinus stelliformis, Cryptoblastus melo

Age: Lower Mississippian Middle Osagean Stage (345 m.y.a)

Size: mm (25.4mm=1 inch): Orophocrinus: 15 mm by 35 mm, Cryptoblastus: 9 mm by 10 mm on a 65 by 85 mm matrix

Location: Burlington Formation, Pike County, Missouri

Cryptoblastus melo blastoidDescription: Well-preserved example of an unusual spiraculate blastoid known as Orophocrinus stelliformis. It is easy to see where the species derives its name, with its starlike shape. These are hard to find intact, and highly prized by collectors. It is accompanied by an example of the spiraculate blastoids, a close relative of the crinoids, known as Cryptoblastus. The Burlington Formation is as much as 50 m in thickness, and covers a large portion of Missouri as well as Iowa and Illinois along the Mississippi River. It is composed largely of undolomitzied crinoidal limestone. The sediment was an unconsolidated, coarse-grained, and poorly sorted sand to gravel substratum which was episodically mobile. Such a substrate was not suitable for Orophocrinus stelliformis blastoid fossilbrachiopods, bryozoans, or corals, but was apparently ideal for the many types of camerate crinoids that have been found. While the material was episodically disturbed by storms, the rigidly- sutured calyx of camerate crinoids were ideally suited to preservation. Few are found with attached arms or stems, but many parts are also found with the crowns in the “hash” that was preserved. It was also a suitable home for blastoids; indeed, some 20% of the known fossil blastoid taxa are known from the Burlington Formation. Here are 2 of the diverse forms preserved in this wonderful deposit, with the white blastoids a stark contrast to the orange-brown matrix. Notice the many crionoid stem fragments present as well, a reminder of the diverse Echinoderms found.

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