Rockford, Part 2: Fossils (exclusive of brachiopods)

Before we get into the festivities, I've recently written an article for the online magazine Agate, about identifying common Paleozoic fossils of Minnesota. It's a compact summary that covers the most abundant groups, so if you're looking for something like that, go have a look!

In our previous post we had a look at the geology of the Fossil & Prairie Park Preserve of Floyd County, Iowa, also known as the Rockford site. For this post I'm going to briefly detail the fossils I collected, with the exception of the brachiopods, which will get a post of their own. For most of the non-brachiopods, I didn't get too far into the weeds on taxonomy, because many of the groups don't lend themselves to simple eye-checks for genera and species. Horn corals and bryozoans, for example, usually require thin sections, and crinoid columnals are generally only diagnostic of the presence of crinoids. I did, though, have recourse to Fenton and Fenton (1924) and other peoples' identifications to get some ideas.

Sponges

Stromatoporoid sponges were an important part of the fauna here, but their structure was, well, spongy, so their fossils aren't necessarily very obvious. Look for the vaguely briquette-like objects with dark, rough surfaces.

Like this one.

Corals

The Rockford site has horn corals, colonial rugose corals (particularly the singular-looking Pachyphyllum, which resembles a cratered moonscape with radiating furrows around each crater), and tabulates, especially tiny encrusting Aulopora-type tubes. Horn corals were fairly common, and the fossils included both conical forms and more tubular forms. Sizes of both general forms varied.

A smattering of typical horn corals.

Given the long history of the site, I don't expect to have come up with anything new to science (I'm nowhere near pure of heart enough to have that kind of luck). I did, though, come up with this knot of horn corals, which is pretty weird if nothing else. I picked it up with only a couple of the individual corallite apertures showing, thinking it might be some kind of Pachyphyllum. Preliminary preparation showed it definitely wasn't bog-standard Pachyphyllum (although it's not *too* far off from some specimens identified by Fenton and Fenton as P. crassicostatum). Fenton and Fenton (1924) included a compound horn coral, Diphyphyllum tubiforme, which also isn't a great fit but shows that beasts of a similar ilk were out there.

The horn coral bouquet.

Part of a typical Pachyphyllum (lower right part of rock); not quite the same thing.

Bryozoans

Bryozoans are essentially an accessory component. I found long thin pieces mostly on larger rocks, loose bits of twiggy bryozoans, and fragments of net-like bryozoans on other things.

An assortment of bryozoan fragments.

 

I found a small number of these flat rectangular rocks with scattered twig-like fragments of bryozoans, all in the same area. They don't seem to fit together, but who knows? Maybe I missed the connecting pieces. Anyway, there may also be some sponge action going on here.

Brachiopods

The brachiopods were where the action was. From comparisons with Fenton and Fenton (1924) and other peoples' identifications, I can pick out about a dozen genera and species in my collection. Given that, I've chosen to give them their own post later.

For the heck of it, though, here's a small hash with some shell fragments.

Bivalves

Rockford bivalves mostly pertain to the variable genus Paracyclas, and that was my experience.

Paracyclas, probably mostly if not all P. sabini.

I've got a small number that are bigger and more irregular, but are still identifiable as bivalves, and it would not surprise me if they are also Paracyclas.

Bigger bivalves.

Snails

The Cerro Gordo is noted for palm-sized snails, often flattened, pertaining to the genus Floyda (also known as Floydia, including in Fenton and Fenton 1924). Confusingly, there is a similar but distinct genus, Westerna (also known as Westernia, including in Fenton and Fenton 1924). I came back with one of this sort plus much smaller snails of a couple of different forms, including a bellerophont, a bulbous coiled form that might just be young Floyda, and something with a loose-looking slender whorl (perhaps a result of excess matrix hiding the rest of the whorl). The snails seemed to be steinkerns or external casts.

Big snail roadkill. Floyda? Westerna?

The flip side of a flat snail.

Baby giant snails?

Or are these the young? (Or a mix of small adult snails and young giant snails?)

Diminutive bellerophonts, of the classic "brass instrument swallowing itself" shape.

These have interesting slender whorls, and remind me a bit of microconchids.

Crinoids

Crinoid columnals were small and pretty plain, generally simple cylinders, some with occasional more prominent rings.

Pretty typical stuff.

Trace Fossils

I also found a few burrow pieces, which are reminiscent of Rauffella filosa without the "filosa" part. Fenton and Fenton (1924) included "fucoids" under the "algal" genus Gracilerectus, and these look the same, so I'm happy calling them that (with the understanding they're actually invertebrate burrows and not seaweed casts).

Well, maybe not the slender smooth one in the lower right; that one's kind of weird and might be something else.

Other

A coupe of pieces had tiny tube networks. My first instinct was the tabulate coral Aulopora, which is certainly known from Rockford, but the site also has similar fossils of bryozoans (Hernodia, Vinella), and Hederella, which is an extinct colonial animal that appear to have been related to the bryozoans and brachiopods. However, the Rockford taxa of these more obscure forms all appear to have had smaller tubes than my fossils, so I'm going to go with Aulopora for now. The "sessile colonial animal encrusting other animals in networks of small tubes" niche seems to have been popular at Rockford.

A horn coral (left) with a tube network belt.

The left side of this rock is littered with tiny tubes. If you embiggen, you'll also notice a ribbed cylinder left of center near the bottom margin; it makes me think of an itty-bitty tentaculitid (or maybe a little strand from a crinoid).

Finally, cephalopods and echinoids are also known from the site, but I didn't find examples of either (as far as I know).

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.