Rockford, Part 3: Brachiopods

In the previous two posts on the paleontology of Rockford, Iowa, we looked at the site itself and the majority of the fossils, leaving the brachiopods for now. I ended up identifying 13 brachiopod taxa to genus or species. They're written up in alphabetical order below, with a brief description of what I found to be the most important distinguishing characteristics for those of us who know a thing or two about brachiopods but are not conversant with fine anatomical details. I have to admit that the photos are a bit disappointing, which is in part because we're dealing with strongly three-dimensional objects. Pictures are great, but there are things that just don't quite make sense unless you're holding a fossil in your hand. I used Fenton and Fenton (1924) quite a bit, recognizing that the taxonomy is outdated (brachiopod taxonomy does not sit still). I also consulted online galleries of Rockford fossils, e.g., this, this, and this, and had recourse to Ma and Day (2000) for the spiriferids. All in all, I'm happy with the identifications in a broad sense, but inevitably there are a few individual specimens that I'm not sure about.

Conispirifer cyrtiniformis

Triangular with numerous fine ribs (plications), a well-defined sulcus and fold (many brachiopods have this, a valley [sulcus] down the midline of one valve matched with a ridge [fold] on the other), and a flat posterior surface with a steep triangular gap (delthyrium). Overall this is similar to Cyrtina iowensis but much larger and with finer ribbing (plications).

Anterior view, showing pronounced sulcus and fold (gives the appearance of a lip, or tongue sticking out, or an exaggerated philtrum). Ribs are difficult to see on the upper valve here, but are visible on the lower.

The prominent triangular delthyrium. At the tip the beak (the "baby shell", basically) is visible by a change in texture.

Cyrtina iowensis

Very tiny triangles, much coarser and fewer ribs than Conispirifer cyrtiniformis but broadly similar.

These three specimens show the tiny size and rough similarity to Conispirifer cyrtiniformis. The left shell is in anterior view (rotated 90 degrees), the center shell shows the posterior (and triangular delthyrium), and the right shell is viewed from the triangle point down, showing the thick ribs.

Cyrtospirifer whitneyi

Plump, triangular to D-shaped, moderately coarse ribs, well-defined sulcus and fold like Conispirifer cyrtiniformis, overhanging hooked beak region.

The left and lower shells show the overhanging hooked beak, and the right shell shows the sulcus.

Same shells, reoriented.

Devonoproductus walcotti

Inflated shape with fine ribs and growth lines, finely pustulate (the little bumps would have supported spines. These have usually broken off long before collection).

Several valves here. If you're familiar at all with productids, you'll probably have a mental image of a brach with a big rounded valve and a much flatter valve, plus fine ribs and often a finely "warty" surface, and that's what you get here.

Nervostrophia

Several species of small, strongly inflated D-shaped shells with fine ribs and sometimes points on the corners have been reported from Rockford, and honestly I can't tell them apart from the descriptions. They are often described as Nervostrophia (or Sulcatostrophia) camerata, although what I have seems to be just as easily described as Nervostrophia canace. For all I know, both (or additional similar species) are present. Two of mine have extended points or the bases of extended points at the corners of the hinge line.

Little D-shaped finely-ribbed shells were fairly common, despite their fragility. Note the "cat ears" on the corners of the hinge lines of some specimens.

The left valve has the best hinge point of any I collected. The valve on the right has an obvious extension, but most of the point has been lost (which happens to points in general around here, so it's in good company!).

Platyrachella macbridei

Similar to Cyrtospirifer whitneyi but not as inflated, ribs not as prominent, beak not as overhanging, sulcus not as strong, and hinge-line corners sometimes extended to points.

The upper left valve is probably the best example, but the shell on the right has both valves and shows the overhang.

The upper left and lower valves are reoriented. This species has a prominent delthyrium.

Pseudoatrypa devoniana

One of the more abundant species, P. devoniana can be recognized by moderately fine ribs with strong concentric growth lines (but not as frilly as Spinatrypa rockfordensis), inflated and broad to elongate D-shape, and flattened margins. Some have a slight sulcus. It's a variable species, and seems to be prone to deformation.

These are typical, undeformed examples.

Schizophoria iowensis

These shells are relatively large and inflated, with fine ribs (although my most complete specimen is quite flattened). The most notable feature is the somewhat heart-shaped profile.

Some are more heart-shaped than others.

Schuchertella parva

D-shaped with ribbing, nearly flat hinge line except for an overhanging triangle, and stepped profile (I don't know if they count as ruga or not).

The ribbing of this species gives a corduroy appearance, which with the concentric ridging puts me in mind of a folded knit cap.

Spinatrypa rockfordensis

This, like Pseudoatrypa devoniana, is another abundant species that tends to have been vulnerable to deformation (particularly flattening). It can be recognized by coarse ribs, well-developed concentric growth lines, "frilly" appearance of growth lines where they cross ribs, and nodes (sometimes with small spines, difficult to prepare). Most that I found were flattened but they can be somewhat inflated, and some have a slight sulcus.

These ornamented shells are easy to spot in the field. The specimen in the center left still has bits of spines, which is why there is still a lot of matrix on it.

Strophonelloides reversa

This one resembles the strophs of the Twin Cities. Examples are D-shaped with a prominent fluted ridge around the rim, fine ribs, and generally flat.

The one on the left is my best example of Strophonelloides, with the characteristic fluted marginal ridge.

Tenticospirifer shellrockensis

This is another of the small triangular species, in this case featuring numerous fine ribs, sometimes a modestly defined sulcus, and a posterior surface that is slightly curved with an overhang. This species has finer ribs and larger size than Cyrtina iowensis; a different delthyrium, curved posterior surface, and less steeply triangular profile compared to similarly sized Conispirifer cyrtiniformis; and a poorly developed sulcus and narrower shape compared to Cyrtospirifer whitneyi or Platyrachella macbridei.

These were more challenging to photograph because they will only sit in a couple of ways, and the angles provided aren't the most informative.

Theodossia hungerfordi

Inflated, roughly D-shaped, medium ribs, prominent hooked beak; not the most exciting, kind of standard-issue brachiopod.

They're brachiopods, all right.

References

Fenton, C. L., and M. Fenton. 1924. The stratigraphy and fauna of the Hackberry Stage of the Upper Devonian. Contributions from the Museum of Geology, University of Michigan 1.

Ma, X., and J. Day. 2000. Revision of Tenticospirifer Tien, 1938, and similar spiriferid brachiopod genera from the Late Devonian (Frasnian) of Eurasia, North America, and Australia. Journal of Paleontology 74(3): 444–463.