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 I used Fenton and Fenton (1924) quite a bit, recognizing that the taxonomy is outdated (brachiopod taxonomy does not sit still).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.

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.

Rockford, Part 1: The Site

A few weeks I was able to join a Geological Society of Minnesota field trip to the Fossil & Prairie Park Preserve of Floyd County, Iowa, which to some of you may be more familiar as the Rockford site. We ended up having a practically ideal day: pleasant temperatures, clear skies, dry conditions, and a light breeze. If you have an itch to collect fossils (an itch that's becoming difficult to legally scratch in Minnesota these days), I highly recommend a visit.

A view into the old quarry directly south of the parking area.

On the eating of one's words

Back in the day, when I was an undergrad at the University of St. Thomas, I was still very much a vert paleo chauvinist, just getting my toes wet in the Decorah. I'd been going through some of the old journals and textbooks, and was dismayed by the lack of coverage of vertebrates (well, dinosaurs). Everything seemed to be about invertebrates, particularly those with some kind of useful economic function (biostratigraphy) or with extensive fossil records permitting the testing of pet evolutionary hypotheses. While discussing this with my professors, I said something to the effect of "A brachiopod can't bring you love. A trilobite, maybe, but not a brachiopod."

Two decades later, I am the proud namesake of a brachiopod, specifically (in both senses) Ivdelinia (Ivdelinia) tweeti Blodgett et al. 2022: "The species name is in honor of Justin S. Tweet, paleontologist dedicated to the documentation, preservation, and study of National Park Service fossils." Thank you, Robert, Valeryi, and Vince!

A handsome fellow, isn't it? (Scale bar is 1 cm; Figure 6 in Blodgett et al. 2022).

I. tweeti comes from the Emsian-age (late Early Devonian) rocks of the Shellabarger Limestone in Denali National Park. The formation itself is also newly minted in Blodgett et al. (2022), and is part of the Mystic sequence of the Farewell Terrane. If you're not familiar with the geology of Alaska, it's almost entirely made up of bits and pieces of crust that collided with each other during the Phanerozoic. Characteristics such as biogeography have been used to reconstruct where the crustal fragments came from and the timing of their journeys. In this case, the Shellabarger Limestone brachiopods and other invertebrates show more of an affiliation to northeast Russia than to North America, indicating the fragment rifted from Siberia before arriving at what became Alaska (Blodgett et al. 2022).

References

Blodgett, R. B., V. V. Baranov, and V. L. Santucci. 2022. Two new late Emsian (latest Early Devonian) pentameridine brachiopods from the Shellabarger Limestone (New Formation), Shellabarger Pass, Denali National Park and Preserve, south-central Alaska. New Mexico Museum of Natural History and Science Bulletin 90:73–83.

More fun with ammonoids

A few years ago I posted on the joyful eccentricities of the heteromorph ammonites, who refused to be bound by conventional notions of what ammonites were supposed to look like. Ammonoids also experimented with some interesting morphological choices in their early days. Here are a couple of examples of inspired ammonoids you might have come across scuba diving in the Late Devonian: 

Below is Parawocklumeria, looking like a Paleozoic premonition of the globigerine foraminifera (although somewhat larger).

Parawocklumeria paradoxa, Plate 19 from Wedekind (1918) (described as Wocklumeria paradoxa)

Despite the inflated appearance, Parawocklumeria is still doing the typical coiling. Another ammonoid, Solicylmenia, seems to have decided that coiling is all well and good, but it's even better when done triangularly. What exactly this taxon got out of this, I'm not sure; it seems to have worked well enough for Soliclymenia but not well enough for anyone else.

Solicylmenia paradoxa, Plate 16 from Münster (1839) (described as Cylmenia paradoxa). I realize the quality isn't great, but on the other hand it's fun to track down the original illustrations. (And yes, both chosen species here happen to be paradoxa; I wasn't being lazy with cut and paste.)

References

Münster, G. Graf zu. 1839. Nachtrag zu den Clymenien des Fichtelgebirges. Beiträge zur Petrefactenkunde 1:35-43.

Wedekind, R. 1918. Die Genera der Palaeoammonoidea (Goniatiten). Palaeontographica 62:85–184, pl. 14–22.

The Matagamon Sandstone

I recently had occasion to go to north-central Maine for work. If you've never been there, the geology is the tectonic equivalent of taking a bunch of little leftover bits of colorful modeling clay and smooshing them together: the area was on the margin of the North American craton during the Paleozoic and thus was the recipient of a conveyor belt of crustal fragments. Of course, when this happens, you get all kinds of interesting structural features and metamorphism, which does unfortunately tend to obscure the original geology. Tack on the Pleistocene glaciations and subsequent dumping of drift, followed by the growth of forests, and you can see how things can get complicated and confusing to follow.

One of the geologic units I observed in this region is the Lower Devonian Matagamon Sandstone. The Matagamon has been interpreted as part of a deltaic system that advanced to the northwest during the Acadian Orogeny (Hall et al. 1976; Pollock et al. 1988). We've got a pretty good idea of when its deposition ended because it transitions upward into the Traveler Rhyolite (Rankin 1965), the explosive component of a supervolcano that erupted approximately 407 million years ago (Seaman et al. 2019). Curious about the guts of that volcano? Look no further than Katahdin.

Maine's tallest mountain? The crystallized heart of one of North America's largest volcanoes? Why not both?
(Incidentally, they told us this was the best time of year to do geologic work in north-central Maine. They were right!)

Anyway, the Matagamon is a fossiliferous unit, with an assemblage dominated by brachiopods. Clarke (1909) described a few assemblages from this formation, which was then identified as the Moose River Sandstone (it did not receive its present name until Rankin 1965). The fauna includes plant fragments, corals, brachiopods, monoplacophorans, bivalves, nautiloids, gastropods, tentaculitids, trilobites, crinoids, and invertebrate trace fossils. Brachiopods certainly seemed to be the most abundant fossils in the outcrops I saw.

A fairly large brachiopod with Leptaena-type ridges.

A bulbous shell on the left and a cylindrical object of unknown origin on the right.

A shell bed.

Fossils tended to be abundant in localized areas, mostly preserved as molds, external casts, and steinkerns, with occasional shell material in the brachiopods. (Overall, the rocks, the fossils, and their preservation rather reminded me of the somewhat younger Mahantango Formation from the Delaware River valley.) In some cases, the fossils had been stained bright orange, very appropriate for autumn and Halloween.

Slightly orange small flat ribbed brachiopods, resembling potato chips.

Two strongly orange brachiopods: a small shell with few but heavy ribs on the left, and a much larger brach with many fine ribs in the center.

Some calcitic material remains with these shells.

References

Hall, B. A., S. G. Pollock, and K. M. Dolan. 1976. Lower Devonian Seboomook Formation and Matagamon Sandstone, northern Maine: a flysch basin-margin delta complex. Pages 57–63 in L. R. Page, editor. Contributions to the stratigraphy of New England. Geological Society of America, Boulder, Colorado. Memoir 148.

Pollock, S. G., A. J. Boucot, and B. A. Hall. 1988. Lower Devonian deltaic sedimentary environments and ecology: examples from the Matagamon Sandstone, northern Maine. Pages 81–99 in R. D. Tucker and R. G. Marvinney, editors. Structure and stratigraphy. Maine Geological Survey, Augusta, Maine. Studies in Maine geology: papers to commemorate the 150th anniversary of C. T. Jackson’s reports on the geology of Maine. Volume 1.

Rankin, D. W. 1965. The Matagamon Sandstone–a new Devonian formation in north-central Maine. U.S. Geological Survey, Washington, D.C. Bulletin 1194-F.

Seaman, S. J., R. Hon, M. Whitman, R. A. Wobus, J. P. Hogan, M. Chapman, G. C. Koteas, D. Rankin, A. Piñán-Llamas, and J. C. Hepburn. 2019. Late Paleozoic supervolcano-scale eruptions in Maine, USA. GSA Bulletin 131(11–12):1995–2010.

Ammonicrinus

Nothing too heavy today; I just wanted to draw your attention to a very unusual crinoid, Ammonicrinus from the Early and Middle Devonian of Europe and north Africa. While we're used to the idea that crinoids are either stalked things with spindly arms, or free-floating things with spindly arms, not all of them stuck to this body plan. Some of them evolved an enrolled body plan, such as Myelodactylus, which as a fossil looks a bit like a curled-up millipede. Some of them went even farther, like the subject of today's post (and see Bohatý 2011 for much more information).

Yes, this is a crinoid, just... different. Figure 1 in Bohatý (2011). CC-BY-4.0.

Ammonicrinus came in three great flavors: stalked and with a shielded but exposed crown ("exposed roller-type", seen only in the early history of the genus); stalked and with an entirely enrolled crown, most of the animal laying on the seafloor ("encased roller-type"); and barely stalked with an entirely enrolled crown, perched on a brachiopod shell ("settler type"). For the remarkable "encased roller-type", the base was a holdfast attached to something, which was followed by several large, bead-like columnals. Then the columnals began to widen and flatten into broad concave-convex structures, shaped something like brackets in cross-section. These bracket-shaped columnals then turned into a much narrower section which connected to a stocky crown. The crown and the thinner columnals were wrapped up within the broader bracket-shaped segments, with just enough space on either side to pass water through. For good measure, the segments were also decorated with long, articulated, echinoid-like spines (Bohatý 2011).

An early Ammonicrinus, of the "exposed roller-type". Figure 6 in Bohatý (2011). CC-BY-4.0.

What could have possessed the ammonicrinids to go in this direction? One possibility is that everything is tied to making an end-run around other crinoids. Typical crinoids would filter higher in the water column; Ammonicrinus could have the lower levels all to itself. The drawbacks are that the crinoid would be more exposed to predation from benthos (it's hard to believe that anything would willingly eat a crinoid, but there's no accounting for taste), and would be more vulnerable to fouling from the muddy bottom. Ammonicrinus addressed the issue of predation by protecting its crown via enrolling; the spines would have also offered protection. The spines would have also helped to brace the animal against the unstable bottom environment. To keep water flowing and to clear itself of sediment, it could rock the enrolled part of the skeleton, which would force water through the crown (Bohatý 2011).

An Ammonicrinus doing its thing, rocking to promote a current. Figure 14C in Bohatý (2011). CC-BY-4.0.

References

Bohatý, J. 2011. Revision of the flexible crinoid genus Ammonicrinus and a new hypothesis on its life mode. Acta Palaeontologica Polonica 56(3):615–639.

A Devonian reef

When I visited Delaware Water Gap National Recreation Area and the surrounding area back in 2017, one of the places I stopped was at a Lower Devonian sponge-coral bioherm. A bioherm is a geological term for a mound-like feature made up of fossils. The useful thing about "bioherm" is that it just refers to the shape, rather than making any conclusions about how that structure came about. (You see, there are reefs, and then there are things that are like reefs but aren't reefs, and there are also just plain old accumulations of skeletal fragments, and there's actually a lot of nuance involved that can tell you things about depositional environment and so forth.)

Looks like circular stromatoporoid colonies with tabulates between them—or are there also some corallites in the circles? Things get complicated in a reef, especially 400 million years and some weathering later.

Anyway, this particular bioherm is one of a group identified as "patch reefs", which is more or less what it sounds like: a relatively small discrete reef. There's a handful of these scattered throughout the area, found in the Shawnee Island Member of the Coeymans Formation. They are described as including a central core up to 160 x 70 m (525 x 230 ft) in area and 15 m (15 ft) thick, surrounded by flank beds of skeletal debris. They are primarily formed of stromatoporoid sponges and favositid tabulate corals, and grew on a marine carbonate shelf (Monteverde 2001; Precht 1988). The rugged modern topography was still in the future; the Acadian Orogeny hadn't even kicked in yet and the previous topography generated by the Taconic Orogeny had been well eroded by the early Devonian.

This one's a bit easier to tell as a Favosites chunk (even if the photo isn't that sharp).

There hasn't been much occasion to talk about stromatoporoids previously. The first thing to know about stromatoporoids is that despite the confusingly similar name, they have nothing to do with stromatolites. Stromatolites, like Cryptozoon rosemontensis from back in 2014, are layered sedimentary structures left by microbial colonies. Stromatoporoids also show a layered appearance in cross-section, but that's because of layered body tissues: they were sponges (even if this is not immediately apparent), and produced body fossils, not trace fossils. The body, mineralized as calcium carbonate, consists of horizontal laminae supported by vertical pillars. A basic stromatoporoid plan has been adopted several times, including by some modern sponges, but classic Stromatoporoidea had its heyday in the middle Paleozoic as a reef-forming group. I don't think anyone has attempted to identify the Coeymans Formation stromatoporoids to genus or species since White (1882) invoked Stromatopora, but I might have missed that reference.

Not sure what's going on here; maybe a branching-type stromatoporoid (e.g., Amphipora) has gotten into the act as well.

On the other hand, the common Coeymans patch reef tabulate has been identified to the genus level as Favosites (Weller 1903; Willard et al. 1939; Epstein et al. 1967). Swartz and Swartz (1941) put it in F. helderbergiae, which is appropriate for the time and place, but you wouldn't be able to tell just walking by (unless you carry your own thin-sectioning equipment and microscope with you when you're out for a walk). Favosites, the "honeycomb coral", is rather more impressive than most of the tabulates in the Ordovician of Minnesota, rare Foerstephyllum colonies excepted. The reason for "honeycomb coral" isn't obvious from these photos, but if you did have a loose colony sitting in front of you, you'd be able to see the logic: it's a coral colony that resembles a chunk of honeycomb. Each "cell" of the "honeycomb" is a corallite that held a polyp, much smaller than the polyps of horn corals and other rugose corals but still much larger than the tiny animals of a bryozoan colony.

Not dinosaur skin, just weathering across a favositid that's been more effective on the corallite walls than the fill, producing a pebbly-looking "negative".

This photo shows a clear demarcation between favositid on the left and not-favositid on the right.

References

Epstein, A. G., J. B. Epstein, W. J. Spink, and D. S. Jennings. 1967. Upper Silurian and Lower Devonian stratigraphy of northeastern Pennsylvania, New Jersey, and southeasternmost New York. U.S. Geological Survey, Washington, D.C. Bulletin 1243.

Monteverde, D. H., leader. 2001. Road log and stop descriptions; Day 1, Stop 5: Montague mini-mall fossil site; flank of a coralline bioherm in the Coeymans Formation. Pages 191–198 in J. D. Inners and G. M. Fleeger, editors. 2001: a Delaware River odyssey. Field Conference of Pennsylvania Geologists, Harrisburg, Pennsylvania. Guidebook for the Annual Field Conference of Pennsylvania Geologists 66.

Precht, W. F. 1988. Lower Devonian reefs of the Coeymans Formation in the northern Appalachian Basin. Pages 514–519 in H. H. J. Geldsetzer, N. P. James, and G. E. Tebbutt, editors. Reefs; Canada and adjacent areas. Canadian Society of Petroleum Geologists, Calgary, Alberta. Memoir 13.

Swartz, C. K., and F. M. Swartz. 1941. Early Devonian and Late Silurian formations of southeastern Pennsylvania. Bulletin of the Geological Society of America 52:1129–1192.

Weller, S. 1903. The Paleozoic faunas. New Jersey Geological Survey, Trenton, New Jersey. Report on Paleontology 3.

White, I. C. 1882. The geology of Pike and Monroe Counties. Geological Survey of Pennsylvania, Harrisburg, Pennsylvania. Report of Progress 9(6).

Willard, B., F. M. Swartz, and A. B. Cleaves. 1939. The Devonian of Pennsylvania. Pennsylvania Geological Survey, Harrisburg, Pennsylvania, 4th series. General Geology Report 19.

Rectogloma problematica

What sounds like a kidney disorder, looks kind of like a flattened straight nautiloid possibly with a little curl (or maybe a Devonian Hot Pocket), and remains enigmatic more than a century after its description? It's Rectogloma problematica, which illustrates that even though we're past the Cambrian, the fossil record can still throw some oddballs.

From Van Tuyl and Berckhemer (1914). It's difficult to maintain "natural size" a few generations removed from original illustrations; this is one reason why you should use scale bars instead!

The only reported specimens of Rectogloma problematica were collected by Columbia University students on a field trip to the Delaware Water Gap area. They were found in red shale in the Upper Devonian Catskill Formation in a cut along the Delaware, Lackawanna & Western Railroad near the Henryville station, per Van Tuyl and Berckhemer (1914) ["Henrysville" of their spelling]. The fossils were discovered by Van Tuyl and another student, C. W. Honess, which just goes to show that you should always pay attention on field trips. A couple of the better specimens were collected, and have today made their way into the collections of the American Museum of Natural History (AMNH FI 22377A and B).

The fossils, as illustrated and described by Van Tuyl and Berckhemer (1914) and Conway Morris and Grazhdankin (2005), are a few cm long, roughly rectangular but broader at one end than the other, and have an oval cross-section. They were found oriented vertically, with the wide end pointed up, which as noted by Conway Morris and Grazhdankin (2005) indicates that they are not flattened by compression, but really did have an oval cross-section in life. The broader "sides" are marked with numerous parallel lines 1 to 3 mm apart, each gently arched. Van Tuyl and Berckhemer (1914) described them as "sutures", but they don't go all the way around the fossil. Only one of the two presumed "ends" can be seen in the fossils, so it's not known how the wide end, er, ended. The other end that *can* be seen is also controversial. Van Tuyl and Berckhemer (1914) illustrated a sort of "spit curl" or coil coming off an otherwise rounded termination, but Conway Morris and Grazhdankin (2005) could not confirm this.

Figuring out what Rectogloma problematica is has proven difficult. Van Tuyl and Berckhemer (1914) thought it kind of resembled an abnormal cephalopod, but noted that the incomplete "sutures", lack of a siphuncle, and undivided coil argue against this. (Incidentally, a putative second species, "Rectogloma" zaplensis Turic et al. 1982, did prove to be a nautiloid [Cichowolski 2008].) Knight (1941) noted that it only superficially resembled a gastropod, which is true, but at the same time it's unclear who thought it was a gastropod to start with. Knight instead thought it was more likely a coprolite, which he repeated in 1960 (Knight et al. 1960). While the specimens do indeed appear to be reasonably poop-shaped lumps in basic form, it is rather difficult to produce droppings with smooth sides marked by closely spaced parallel thin features that do not continue all the way around, and a coprolitic identity has been rejected (Häntzschel 1975; Conway Morris and Grazhdankin 2005). Conway Morris and Grazhdankin (2005) did not make any firm classifications; about all they were willing to say was that it may have been "some sort of tube, with a relatively thick wall" of unknown composition. The apparent rarity of Rectogloma fossils and absence of obvious relatives suggest to me that Rectogloma lacked mineralized structures and just lucked out this one time in terms of fossilization. The "sutures" seem more like slits or structural features involved in flexure. Maybe it was some kind of tubular or goblet-like soft-bodied creature, stuck to the sea floor? Or perhaps some kind of fish egg case?

References

Cichowolski, M. 2008. The orthocerid Dawsonoceras? (Nautiloidea) from the Lipeón Formation (Silurian), northwestern Argentina. Ameghiniana 45(4):791–793.

Conway Morris, S., and D. Grazhdankin. 2005. Enigmatic worm-like organisms from the Upper Devonian of New York: an apparent example of Ediacaran-like preservation. Palaeontology 48(2):395–410.

Häntzschel, W. 1975. Treatise on invertebrate paleontology. Part W. Miscellanea. Supplement I. Trace fossils and problematica. Geological Society of America, Boulder, Colorado, and University of Kansas Press, Lawrence, Kansas.

Knight, J. B. 1941. Paleozoic gastropod genotypes. Geological Society of America Special Paper 32.

Knight, J. B., R. L. Batten, E. L. Yochelson, and L. R. Cox. 1960. Supplement. Paleozoic and some Mesozoic caenogastropoda and Opisthobranchia. Pages I310–I331 in R. C. Moore, editor. Treatise on invertebrate paleontology. Part I. Mollusca 1. Geological Society of America, Boulder, Colorado, and University of Kansas Press, Lawrence, Kansas.

Turic, M. A., V. A. Ramos, and J. Oliver Gascón. 1982. "Rectogloma" zaplensis (problemática) de la Formación Lipeón, Provincia de Jujuy, Argentina. Revista del Instituto de Ciencias Geológicas 5.9–14.

Van Tuyl, F. M., and F. Berckhemer. 1914. A problematic fossil from the Catskill Formation. American Journal of Science, 4th series, 38:275–276.