Brooksella: what are star cobbles?

Back in the far-off year of 2012, when I was helping to compile instances of paleontological type specimens found in National Park Service units, we had to make decisions about various edge cases. One of these was how to handle names for what later turned out to be pseudofossils. We decided to record the information as historically relevant but did not include the "taxa" in any counts. On this blog we've actually covered a couple of them already, "Lingula calumet" and "Paradoxoides barberi" from within or very near Pipestone National Monument. Another is "Brooksella canyonensis", a putative jellyfish from the Proterozoic Nankoweap Formation of Grand Canyon National Park. It was first reported as such in Van Gundy (1937) and then named, not entirely enthusiastically, in Bassler (1941). "B. canyonensis" has fared poorly as a jellyfish, but has had its supporters as an organic feature (e.g., Glaessner 1969; Kauffman and Steidtmann 1981; Kauffman and Fursich 1983; tentatively Ciampaglio et al. 2006). However, I favor an inorganic interpretation. Admittedly, there are several to choose from: gas-escape structures or compaction (Cloud 1968), "sand-volcano"-type fluid escape (Ford and Breed 1977; Ford 1990), and mud rolls (Fedonkin and Runnegar 1992).

"B. canyonensis" was not the first species in the genus Brooksella, though. Brooksella was named by Charles Walcott for "star cobbles" from the Coosa Valley of Alabama (Walcott 1896), now attributed to the middle Cambrian-age Conasauga Formation (Nolan et al. 2023). In fact, he named three taxa for different forms of cobbles: B. alternata, B. confusa, and Laotira cambria (Walcott 1896). Star cobbles got their name because at their best they look like the stereotypical twinkly pointed things you might doodle. Some of them even have five rays, although six is more typical and they are more lobed than pointed, so it's not a perfect match.

Brooksella (A–D, K) and Laotira (E–H, J) as illustrated by Walcott (1898) and reproduced as Figure 1 in Nolan et al. (2023) (which see for full caption). CC BY 4.0.

Walcott interpreted the objects as representing jellyfish, which are probably not the first thing you think of when fossils come to mind, but jellyfish fossils are in fact known elsewhere. In this case, though, the interpretation hasn't proved tremendously popular over time, and numerous alternatives have been proposed. These alternatives, though, generally involve some kind of organic origin, either as a true body fossils or a trace fossil of some sort. It's not hard to see why: they look like something that *ought* to be organic, even if the identity of that something is unclear. (Anyone who has gone out fossil hunting will probably recognize this feeling. Sometimes you're right, sometimes you're wrong.)

Nolan et al. (2023) have published a detailed reassessment of Alabama Brooksella. As part of it, they prepared a lovely supplemental figure of various hypotheses, with thumbnail evaluations (discussed at greater length in the text). (*Warning*: Hold off on clicking the link if you'd rather not get their solution immediately.) Studies of Brooksella from the past couple of decades have interpreted it as a trace fossil (either a feeding burrow or a coprolite) or a glass sponge (hexactinellid). Nolan et al. subjected star cobbles to about as many tests as can legally be done to rocks in their analysis of the various possibilities, and came to several conclusions, including:

• Brooksella specimens do not have a sponge's anatomy. There aren't spicules, features previously interpreted as ostia (pores) bear a strong resemblance to pitting left behind when lichen are cleaned off, and lobes do not feature opening at their ends for radial canals (which were also not found).
• The orientation of the specimens when found in situ was with the putative central osculum (excurrent vent) down in the sediment, which is an inconvenient place for an osculum. Furthermore, many examples did not even have an "osculum".
  
• The specimens include internal voids and tubes, but these spaces do not correspond to the external form, unlike primary burrows (although this does not preclude the specimens having "captured" parts of burrows that were passing through). Furthermore, the internal features do not include common burrowing structures such as backfill.
  
• The specimens have the same composition as silica concretions from the same rocks, and are very comparable overall, with the same kind of weathering rings, lichen pitting, and random internal voids and tubes.
  

Nolan et al. concluded that Brooksella is no different from the local concretions except for the lobes, and should therefore "be considered a pseudofossil until proven otherwise." A consequence of this conclusion is that Brooksella, not being a glass sponge, would not have been a source of silica for preservation of fossils in the Conasauga. (It's not stated, but it seems that it would have been a sink instead.) It further goes to show that you shouldn't trust strange things in the Cambrian.

Brooksella (A–E) and concretions (F–K) collected from the Conasauga Formation by Nolan et al. (scale bar 1 cm, or 0.4 in); Figure 5. CC BY 4.0.

References

Bassler, R. S. 1941. A supposed jellyfish from the pre-Cambrian of the Grand Canyon. Proceedings of the United States National Museum 89(3104):519–522.

Ciampaglio, C. N., L. E. Babcock, C. L. Wellman, A. R. York, and H. K. Brunswick. 2006. Phylogenetic affinities and taphonomy of Brooksella from the Cambrian of Georgia and Alabama, USA. Palaeoworld 15:256–265.

Cloud, P. E., Jr. 1968. Pre-metazoan evolution and the origins of the Metazoa. Pages 1–72 in E. T. Drake, editor. Evolution and environment. Yale University Press, New Haven, Connecticut.

Fedonkin, M. A., and B. N. Runnegar. 1992. Proterozoic metazoan trace fossils. Pages 389–395 in J. W. Schopf and C. Klein, editors. The Proterozoic biosphere: A multidisciplinary study. Cambridge University Press, Cambridge, United Kingdom.

Ford, T. D. 1990. Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation. Pages 49–70 in S. S. Beus and M. Morales, editors. Grand Canyon geology. Oxford University Press, New York, New York.

Ford, T. D., and W. J. Breed. 1977. Chuaria circularis Walcott and other Precambrian fossils from the Grand Canyon. Journal of the Palaeontological Society of India 20:170–177.

Glaessner, M. F. 1969. Trace fossils from the Precambrian and basal Cambrian. Lethaia 2(4):369–393.

Kauffman, E. G., and F. Fursich. 1983. Brooksella canyonensis: A billion year old complex metazoan trace fossil from the Grand Canyon. Abstracts with Programs - Geological Society of America 15(6):608.

Kauffman, E. G., and J. R. Steidtmann. 1981. Are these the oldest metazoan trace fossils? Journal of Paleontology 55:923–947.

Nolan, M. R., S. E. Walker, T. Selly, and J. Schiffbauer. 2023. Is the middle Cambrian Brooksella a hexactinellid sponge, trace fossil or pseudofossil? PeerJ 11:e14796. doi:https://doi.org/10.7717/peerj.14796.

Van Gundy, C. E. 1937. Jellyfish from Grand Canyon Algonkian. Science 85(2204):314.

Walcott, C. D. 1896. Fossil jelly fishes from the Middle Cambrian Terrane. Proceedings of the United States National Museum 18:611–614.

Walcott, C. D. 1898. Fossil Medusæ. U.S. Geological Survey, Washington, D.C. Monograph 30.

Ordovician updates

A few miscellaneous items to clear out of my Ordovician inbox, involving new members of Club Late Ordovician, conulariids, and pseudofossils:

Camp Nelson National Monument

Since we were introduced to Club Late Ordovician two years ago, two more parks have joined this exclusive organization for National Park Service units that include fossiliferous Upper Ordovician rocks. One is Saint Croix National Scenic Riverway, thanks to some isolated occurrences of the Platteville and Decorah that were protected from erosion by ancient faulting along the St. Croix Valley. These outliers represent the most northerly and landward outcrops of these formations, and are worth a post of their own at some point. The other new member is the newest NPS unit, Camp Nelson National Monument in Kentucky.

Right on cue, it's a map with a giant caption! (It's the map from the 2016 post with the two new points added, #9 and #14.) 1. Death Valley National Park; 2. Great Basin NP; 3. Yellowstone NP; 4. Grand Teton NP; 5. Bighorn Canyon National Recreation Area; 6. Big Bend NP; 7. Chickasaw NRA; 8. Mississippi National River and Recreation Area; 9. Saint Croix National Scenic Riverway; 10. Pictured Rocks National Lakeshore; 11. Effigy Mounds National Monument; 12. Buffalo National River; 13. Hot Springs NP; 14. Camp Nelson NM; 15. Stones River National Battlefield; 16. Natchez Trace Parkway; 17. Katahdin Woods & Waters NM; 18. Saratoga National Historical Park; 19. Delaware Water Gap NRA; 20. Chesapeake and Ohio Canal National Historical Park; 21. Denali NP & Preserve. The grey blob is the known extent of the Deicke K-bentonite.

Camp Nelson is rather more famous for its Civil War history, but the Camp Nelson area is also notable for its geology. In fact, per Andrews (2005), "Because of the quality of exposures, accessibility, and the importance of the site in research and geologic education, the Kentucky Society of Professional Geologists has named the Camp Nelson area as Distinguished Geologic Site 2."

The park overlays four Ordovician formations, in ascending order the Camp Nelson Limestone, Oregon Limestone, Tyrone Limestone, and Lexington Limestone (Wolcott 1969). They are all examples of your classic marine Ordovician limestones; no points for guessing that brachiopods and nautiloids are implicated in Camp Nelson NM joining Club Late Ordovician. (Not surprisingly, these limestone formations are also karst-formers, and caves and sinkholes are common in the area.) The Tyrone Limestone is known to include the Deicke K-bentonite (Kolata et al. 1996), making it partially correlative in time to the Decorah Shale. Redating of the Middle/Late Ordovician boundary may in fact put all four of these limestones into the Late Ordovician.

As the name indicates, the Camp Nelson Limestone was described from the area of Camp Nelson (Miller 1905); the type locality is a little to the south of the monument, around the area of the Kentucky River crossing. You can also find some nice faulting on the south side of the river (Andrews 2005). The Camp Nelson Limestone appears to be the oldest formation exposed in Kentucky. This was a bit surprising to me, and leads to the reflection that, thanks to accidents of geography and erosion, the bedrock at the surface in the entire state of Kentucky is younger than most of the bedrock exposed in Minnesota.

Hiding conulariids

I really hope to have a more complete update later, but suffice it to say that while tracking down references to a conulariid species from the Grand Canyon, I stumbled across George Winston Sinclair's 1948 thesis on conulariids. In this thesis there is a great deal of information about Minnesota's Ordovician conulariids that Sinclair did not publish in his lifetime, including several unpublished species. There is also the implication that at one time the University of Minnesota collections contained two-to-three-dozen conulariid specimens from the Glenwood Formation on up. This came as a surprise, because I did not recall having seen more than a half-dozen or so on my trips. Several possibilities presented themselves:

• The conulariids were being kept separately from the other fossils when I visited, or were just somewhere I hadn't known to look;
• The conulariids have been lost or stolen: if so, not much more can be added;
• The conulariids passed into Sinclair's hands: this seemed worth following, because Sinclair built up a collection of conulariids and I knew that certain elements of the University of Minnesota collections had gone to others. For example, material from Berkey's Taylors Falls work was donated to the Smithsonian in recent years (Yochelson and Webers 2006).

I checked with folks at the University of Minnesota, University of Michigan Museum of Paleontology, and the Canadian Museum of Nature (the latter two institutions associated with Sinclair), but no luck to date. Hopefully this all works out in the wash. By way of apology, here's a photo of the type specimen of Ctenoconularia obex, one of the few Minnesota species Sinclair ended up publishing.

UMPC 6608, holotype of the conulariid Ctenoconularia obex, from the "Sprechs Ferry" (=lower Decorah) of Minneapolis.

One other conulariid note:
It turned out that one of the other conulariids Sinclair named came from the St. Peter Sandstone of Minnesota: Climacoconus humilis, described in Sinclair (1942) and promptly forgotten by virtually the entire universe (it's based on Carnegie Museum 4753 from Faribault, by the way). Maybe it's just my myopia showing, but it doesn't seem right that it's made almost no impression in the literature, even to be refuted. We are, after all, dealing not only with the St. Peter Sandstone, famous for having practically no body fossils, but also an uncommon and distinctive type of fossil!

Dystactophycus revisited

Back in July 2016, I showed a photo of a strange rock with concentric rings, which provided an excuse to riff on Dystactophycus, a possible trace fossil of a swirling crinoid. At the time, I thought it was unlikely that the photographed rock was Dystactophycus, or indeed of organic origin at all. A few months ago, the collector confirmed my general suspicions: he told me that mutual acquaintances have identified it as a modern fracture pattern, consistent with his later observations of features produced by air hammers punching into the Platteville.

References

Andrews, W. M., Jr. 2005. Geology and the Civil War in central Kentucky: Camp Nelson. Field Trip Guidebook, American Institute of Professional Geologists 42nd Annual Meeting.

Kolata, D. R., W. D. Huff, and S. M. Bergström. 1996. Ordovician K-bentonites of eastern North America. Geological Society of America, Boulder, Colorado. Special Paper 313.

Miller, A. M. 1905. The lead and zinc bearing rocks of central Kentucky, with notes on the mineral veins. Kentucky Geological Survey, Lexington, Kentucky. Bulletin 2.

Sinclair, G. W. 1942. The Chazy Conularia and their congeners. Annals of the Carnegie Museum 29(10):219–240.

Sinclair, G. W. 1948. The biology of the Conularida. PhD Thesis. McGill University, Montreal.

Wolcott, D. E. 1969. Geologic map of the Little Hickman Quadrangle, central Kentucky. U.S. Geological Survey, Washington, D.C. Geologic Quadrangle Map 792. Scale 1:24,000.

Yochelson, E. L., and G. F. Webers. 2006. A restudy of the Late Cambrian molluscan fauna of Berkey (1898) from Taylors Falls, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 64.

Follow-up: Pipestone National Monument, Scenella, Cylindrocoelia

Here's a little more information on a few enigmas from previous posts, with some additional photos from the University of Minnesota paleontological collections. First up is Pipestone National Monument's "Lingula calumet", then Scenella, and finally Cylindrocoelia minnesotensis.

Dystactophycus, the crinoid swirl

A few weeks ago, a friend brought in an odd object to the Science Museum, which he had collected from an excavation in the vicinity. The object is a carbonate rock featuring concentric ridges that seem to be radial around a central depression. On the off-chance that it was some kind of exotic trace or trace-like pseudofossil, and that it had been illustrated, I went across the hall, pulled the volume of the Treatise on Invertebrate Paleontology that covers traces, and was pleasantly rewarded in the pseudofossil section with an illustration of something called Dystactophycus. The illustrated specimen was even from the Cincinnatian, which is practically as good as the Platteville/Decorah, being just a couple of million years younger. Of course, when something comes this quickly I'm honor-bound to be suspicious, but in this case the actual existence of something like Dystactophycus is more interesting than the question of whether or not I was right. (I do get back to the question of identity in the last paragraph.)

Dystactophycus, as currently understood, is one of those things that makes perfect sense once you've heard of it, but otherwise would probably never occur to you. It is composed of concentric markings on a conical structure. The initial describers (Miller and Dyer 1878) interpreted it as a seaweed, because the year was 1878 and the campaign to wrap peoples' minds around invertebrate trace fossils was far from being over. In short order, it was dismissed as an impression of a concentrically ringed bryozoan (Monticulipora by way of Lichenalia concentrica, both then considered corals) (James 1885, 1895–1896). More recently, Dystactophycus has been attributed to crinoids (Osgood 1970; Meyer and Davis 2009). As Osgood described it, the earlier researchers had the thing upside down: instead of an upward-pointing cone, the center was a low depression. The stem was partially buried by mud and the rest of it was spun around by swirling currents, causing its arms to sweep out an area that became the depression and quite naturally leave concentric markings. At some point, the crinoid detached, and the depression filled in the absence of the sweeping action. Although kind of like a trace fossil, these markings are not trace fossils because they are not evidence of biological activity. Think of them more like an Ordovician equivalent to marks left by a log on a river bottom (tool marks, in the parlance). (If you'd like to see a true crinoid trace, check this out!)

The specimen in question (I didn't have a scale handy, so my fingertips will have to do). Although similar in some ways to Dystactophycus, I don't think it's an example.

So, do I think that the specimen in hand is an example of Dystactophycus? Although it is superficially similar to the description, there are a couple of characteristics that lead me to say "no". The major issue is that the specimen is not a simple cone shape. Instead, after rising from the center, it slopes again (so kind of a doughnut shape), and the concentric lines are not confined to the cone but continue on the back side. In addition, the markings are not so much grooves and ridges but concentric terraces. My guess instead is some sort of abiotic sedimentary deformation.

References:

James, J. F. 1885. Fucoids of the Cincinnati Group. Journal of the Cincinnati Museum of Natural History 7(4):151–166.

James, J. F. 1895–1896. Manual of the paleontology of the Cincinnati Group, Part VII. Journal of the Cincinnati Museum of Natural History 18(3–4):115–140.

Meyer, D. L., and R. A. Davis. 2009. A Sea Without Fish. Indiana University Press, Bloomington and Indianapolis.

Miller, S. A., and C. B. Dyer. 1878. Contributions to Paleontology 2.

Osgood, R. G., Jr. 1970. Trace fossils of the Cincinnati area. [regrettably, the plates are not included] Palaeontographica Americana 6(41):280–444.

The "fossils" of Pipestone National Monument

This is being posted on Halloween, which is fitting for a topic that lingers like a ghost in the literature. Even today, you can still find stray references to the fossils of Pipestone National Monument. "Occasional small trilobites - Lingula, Paradoxides" sounds promising, doesn't it? If you're reasonably familiar with the fossils of Minnesota, you may be wondering why you never heard of this before. There's a pretty good reason: these forgotten fossils are not trilobites or brachiopods, and odds are they are not fossils at all.