More from Graf

I talked about the Ordovician rocks exposed at Graf, Iowa last week, including the spectacularly rich deposits of the cephalopod Isorthoceras sociale. But that was just the tip of the iceberg; there are lots of other unusual goodies to be found at Graf.

On several trips to Graf I found unusual features that I couldn’t quite interpret in the field, such as the two examples shown above. These are elongate, almost paper-thin textured patches. They are roughly the size and shape of a flattened specimen of Isorthoceras, but these patches lack the segmentation associated with nautiloid cephalopod shells. Even so, they occasionally seem to be associated with the nautiloid shells in some way.

A closeup reveals some details of the surface texture:

Here’s a patch under a microscope:

This looks more familiar. In fact, I’ve seen similar patterns all over other Ordovician rocks, and even on a whale bone from the Pliocene. This appears to be an encrusting bryozoan colony.

Sure enough, a literature search turned up a description of Spatiopora iowensis by Ulrich, 1893*, which reads, in part:

Spatiopora iowensis, n. sp.

Zooarium spread as an exceedingly thin crust over the cones of Orthoceras sociale Hall….

In the dark shales at Graf, Iowa, this bryozoan is preserved as a thin gladiolus leaf-like film, the Orthoceras grown upon being compressed to such a degree that its original presence may not be suspected.”

Ulrich didn’t include a figure, but his description is quite clear and matches these specimen perfectly. His type specimens were even collected at Graf! So there’s no great mystery here; these patches were identified and published over 100 years ago, although it seems they haven’t been studied much since then; so far I’ve not been able to turn up a single online image of this species or even any technical references except ones that were simply citing Ulrich.

Reference:

Note that this reference is generally cited as 1893, but the scanned copy linked below lists 1895 as the publication date. I’m not sure of the reason for the discrepancy.

Ulrich, E. O., 1893. On Lower Silurian Bryozoa of Minnesota. In Geology of Minnesota Final Report, V.3, Part 1., pp. 96-332 (Spatiopora iowensis description on p. 321).
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