I was recently out of town for work, and one of the things I saw was Middle Pennsylvanian-age building stone with stem segments from large crinoids:
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Big ol' crinoids |
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It's like bony fingers strewn on the ground |
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You'd think with all this stem, there'd be a calyx somewhere, but no
dice |
At a shade over 1 cm (about 0.4 in) in diameter, the columnals are quite a bit bigger than garden-variety columnals, but still are well shy of world champ columnals, which reportedly exceed 2.5 cm (1 in); certainly much bigger than anything in Minnesota, right?
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Yes! Time for the ironic photo! |
Only yesterday, less than a week after returning from the above trip, I was
visiting a couple of Decorah Shale sites and came across the above specimen. I
happened to be caught short of a traditional scale bar, so you will have to
take my word that the fingernail of the above finger is 1.1 cm (0.43 in)
across at its widest point. Therefore, that columnal is 1.5 cm (0.59 in)
across, which is pretty darn big for anything in the Decorah except for certain trilobites. In fact, it made
me wonder if the stone might be a ringer transported from another formation, by
glacier, river, or what-have-you. (Not impossible at all; here's a
neat report
on all kinds of exotic rocks and fossils found in Mississippi gravel,
including Lake Superior agates and Sioux Quartzite; closer to home, a piece of
an Upper Cretaceous ammonite was once found at the Brickyard, as related in
Cobban and Merewether 1983:19.) However, the chunk shows no evidence of
transport, and lithologically it looks the same as any piece of thin limestone
eroded out of the Decorah. Were it not for the great honking columnal, I
wouldn't have thought twice about its legitimacy. (I wouldn't even have thought once!) My guess is that this particular specimen originated from higher in the formation than the stuff I usually see, or that great honking crinoids were a very minor part of the Decorah fauna and this just happens to be my first encounter.
References
Cobban, W. A., and E. A. Merewether. 1983. Stratigraphy and paleontology of mid-Cretaceous rocks in Minnesota and contiguous areas. U.S. Geological Survey, Washington, D.C. Professional Paper 1253.