Unknown from the Minturn Formation
Identified: crinoid anal sac plates

This fossil was found near McCoy, Colorado in the Minturn Formation, which is known
for its Pennsylvanian invertebrates. The photos show both sides of the fossil, which is
1.5 x 2.0 cm. So far, I have not been able to identify it and would appreciate help
with identification.

Click on pictures to Magnify




Dennis Gertenbach

Viewer Comments

1.  In looking at your fossil I see multiple spheres connected together. A few of
them appear to have either an opening or attachment point. From that info I
would say that what I am looking at is a group of sponges. Girtyocoelia is a
possibility and I have found them in the Pennsylvanian of Kansas and Texas.

2.  I bet Dennis a dollar it was crinoid. Never even considered it might be sponge.

3  My dollar bet also goes to Crinoid . It looks to have single crystal calcite plates
and possible five fold symmetry. Both characteristics of crinoids along with the
abundance of crinoids in the Minturn.

4  I'll take a piece of that bet too. Some of the plates have ridged faces, which
could be articular facets, suggestive of a crinoid. I was going to suggest a holdfast
rather than a calyx, however I believe the plate sutures of a holdfast are generally
inflexible, so they wouldn't have ridged articular facets.

Not sure about Girtyocoelia & related sponges, but I always thought the spheres
were generally in a single line like a string of beads, not in a bunch like Dennis's

5   I would suggest that the Minturn Mystery Fossil is several plates from an
anal sac from some crinoid such as Triceracrinus or Utharocrinus. I have
several crowns of these with anal sacs intact, and these are made up of
bulbous or nodose plates.

6  I don't know much about anal sacs. Do the plates sometimes (or always?)
have ridged articular facets? I thought that was pretty much limited to the column
and arms.

7  The plates on the sac have articular facets such that the sac could remain
flexible and could be tipped outside the food gathering areas when the crinoid
had to go potty.

Mystery solved - crinoid anal sac plates

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