Decorah craniate brachiopods

One of the more frustrating minor components of the Decorah fauna are the craniate brachiopods. Why are they so frustrating? They're easy to mistake at first glance for all sorts of other fossils (especially when partially buried or partially encrusted), such as:

• Trilobite glabellas
• Echinoderm plates
• Bivalves
• Monoplacophorans
• Small Scenella

One of these...

Craniates, in brief, are brachiopods from the "inarticulate" structural wing but the "calcitic" compositional wing. At the Brickyard section, the two most abundant are Acanthocrania setigera and Petrocrania halli, which can be difficult to distinguish in practice (Rice 1987). The phosphatic inarticulate Schizocrania, which does a lot of the same things, is also present but much rarer. Other inarticulates at the Brickyard, from the phosphatic side, include Craniops minor, Pseudolingula eva, and Trematis sp. (Rice 1987). Other species are cited in museum collections; one which I've seen, "Crania" (now Acanthocrania) granulosa, looks suspiciously like the "raspberry cystoids" in this post.

Several more, in situ. A might be a ringer; I'm not sure. B is an overturned shell (note also the tiny pygidium just above and to the right). C is partially encrusted. D shows what happens when a thin piece of rock and a thin shell meet inexperienced preparation from the underside.

Acanthocrania and Petrocrania have thin domed shells with concentric growth rings. Unlike most of the other Decorah brachs, they do not have strong ridges (and, of course, they don't look much like the other brachiopods in a lot of other ways, too). They seem to have been attractive to encrusters; all of the loose specimens I have are covered with bryozoans, and some of them have either tabulate corals or cornulitids growing on them as well. For their part, craniates are noted encrusters of other brachiopods; presumably the unattached specimens in the photos were also originally attached to other brachiopods, becoming dislodged (probably after the death of the craniate).

What do the five rounded domed blobs of bryozoans have in common? If you turn them over, they all have the heart of a brachiopod.

This strophomenid has a craniate encrusting on the lower right, as well as several cornulitids.

References

Rice, W. F. 1987. The systematics and biostratigraphy of the Brachiopoda of the Decorah Shale at St. Paul, Minnesota. Pages 136–166 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.