Friday, April 15, 2011

Stephanocrinus - Case of the Triangular Stem

Over the last year or so while collecting fossils in the Silurian period Waldron Shale of Indiana, I have encountered pea size fossils.  These little flower bud looking fossils have one unique characteristic that makes them stand out.  The base of the fossil has an almost equilateral triangle shape. How often do you see this type of shape in organic creatures?

Early paleonotology studies done by George Greene (1894-1906) of New Albany, Indiana and James Hall (1878-1891) of New York noted this triangular shape.  No explanation given as to why the creature evolved this way.  It was thought to be crinoid but has characteristics of a blastoid.

In 1962, Robert Fay of the Oklahoma Geological Survey published a paper Ventral Structures of Stephanocrinus angulatus showing the crinoid belonged in the order of Coronata.  Fossils in this order had be identified as crinoids, blastoids and even cystoids.  He notes the characteristics of the order are: "All have three basal plates, with the azygous one in the right interior interradial position, five radials and five interradials that extend into coronal processes, and an anal opening on the adoral side of the coronal process at the junction of two adjacent radial limbs with the anal interradial. High ridges, in the form of pore-rhombs, extend at right angles to the sutures on the sides of the calyx, giving the appearance of a pore-rhomb cystoid. These ridges are superficial and are not extended in depth."

Dr. Carlton Brett of the University of Cincinnati did research of Rochester Shale (similar to Waldron Shale) fossils in which he comments on the Stephanocrinus in the 1981 book Colossal Cataract" The Geologic History of Niagara Falls edited by Irving Tesmer.   "The elongate, pyramidal thecae of this genus are among the most abundant echinoderm remains in the bryozoan-rich layers of the lower Rochester shale. Stephanocrinus possesses prong-like structures at the top of the theca and rather tiny coiled armlets. The stem, which is rarely preserved, was relatively short and attached by pad-like structure at its lower end."  He goes to say that maybe this an early version of a blastoid found later in the period.

In 2002, it was classified as a

See Stephanocrinus fossils else where on the Internet:

Stephanocrinus angulatus of the Rochester Shale

The New York Geological Survey web site showing it as a blastoid.  That is a nice image of a specimen!

Stephanocrinus documented in Naubug Beds of Kashmir