Sunday, December 10, 2017

Platteville moss

Sometimes it's hard to remember, but the early Paleozoic wasn't a complete water world. We've seen a bit of that up in Taylors Falls, where the sea surrounded basaltic islands during the late Cambrian. A less obvious example is at the contact of the Shakopee Formation and St. Peter Sandstone, which represents a few million years of exposure and erosion between marine cycles. The unusual thickness of the Decorah Shale in the Twin Cities has been interpreted as a result of nearby landmasses supplying sediment (the Transcontinental Arch, running northeast–southwest through the state). Land in the early Paleozoic gets a rap as an uninhabited wasteland (in fact, absence of land plants is sometimes put forward as one of the conditions necessary for the great sand sheets of the Cambrian into the Ordovician), and it's not like we have a lot of big showy fossils to dispute this perception. There are some hints, though.

Our oldest records of actual multicellular land plants are dominated by spores, which is not too surprising given that spores stand a much better chance of preservation than your typical terrestrial non-vascular plant (mosses, liverworts, etc.). These show that plants were colonizing the land by at least ~470 Ma, about the time of the Shakopee–St. Peter hiatus, and that vascular plants were around by the end of the Ordovician. It's been thought that these first land plants were not major influences on their environment, lacking roots and such, but modern mosses are no slouches at chemical weathering, and weathering from early plants may be implicated in Ordovician glacial cycles (quite a bit of debate there; see for example Lenton et al. 2012, Quirk et al. 2015, Porada et al. 2016). Early plants also appear to have given atmospheric oxygen the last kick toward the modern level, from the Ordovician into the Early Devonian, by causing more carbon burial (Lenton et al. 2016).

If plants were abundant enough to do these things, we ought to be able to find some body fossils to go with their spores. Where to look? In a recent study, Cardona-Correa et al. (2016) went to our old friend the Platteville, focusing on an outcrop in Dane County, southern Wisconsin. The formation and location were chosen for specific reasons. First, the Platteville was chosen because of the presence of fungal microfossils in the slightly younger Guttenburg Formation (roughly equivalent to the lower Decorah of the Twin Cities) (Redecker et al. 2000). These fungal microfossils appear to represent glomalean fungi, which are often symbiotic with plants today. The location is about 50 km (30 mi) from the ancient Baraboo Range, which was probably somewhat more imposing 455 million years ago when it was one of the few terrestrial areas in the region. That "terrestrial" bit is the important part, because the closer to shore, the more likely you could get transported plant remains.

Cardona-Correa et al. dissolved about 13 lb (6 kg) of rock in hydrochloric acid to retrieve organic microfossils. Among the foraminifera (basically "amoebas with shells") and acritarchs (a true "wastebasket" of organic microfossils that defy further classification) were a handful, fewer than 20, of multicellular fragments. Most of these fragments were polygonal arrays of cells in sheets, with a few flattened cylinders of cells. Cardona-Correa et al. compared these to the leaves and stems of peat moss, respectively (recognizing that moss leaves and stems aren't quite the same as the leaves and stems of vascular plants). If correct, not only do these fossils help to put a "face" to the early spore-producers, as well as confirm the general timing of plant groups diverging from each other as estimated through molecular clocks, but they are also evidence for very early peatlands.

These aren't the kind of fossils you're liable to stumble across, unless you too have access to facilities for treating samples as well as a good microscope and a lot of patience (13 pounds of rock for <20 fragments on the order of a few hundred microns across), but if they can be found once, it should only be a matter of time and effort to find more.

References

Cardona-Correa, C., M. J. Piotrowski, J. J. Knack, R. E. Kodner, D. H. Geary, and L. E. Graham. 2016. Peat moss–like vegetative remains from Ordovician carbonates. International Journal of Plant Sciences 177(6):523–538.

Lenton, T. M., M. Crouch, M. Johnson, N. Pires, and L. Dolan. 2012. First plants cooled the Ordovician. Nature Geoscience 5:86–89.

Lenton, T. M., T. W.Dahl, S. J. Daines, B. J. W. Mills, K. Ozaki, M. R. Saltzman, and P. Porada. 2016. Earliest land plants created modern levels of atmospheric oxygen. Proceedings of the National Academy of Sciences of the United States of America 113(35):9704–9709.

Porada, P., T. M. Lenton, A. Pohl, B. Weber, L. Mander, Y. Donnadieu, C. Beer, U. Pöschl, and A. Kleidon. 2016. High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician. Nature Communications 7, article 12113. doi:10.1038/ncomms12113.

Quirk, J. J. R. Leake, D. A. Johnson, L. L. Taylor, L. Saccone, and D. J. Beerling. 2015. Constraining the role of early land plants in Palaeozoic weathering and global cooling. Proceedings of the Royal Society B 282(1813):20151115. doi

Redecker, D., R. Kodner, and L. E. Graham. 2000. Glomalean fungi from the Ordovician. Science 289(5486):1920–1921.

Sunday, December 3, 2017

The News in Hadrosaur Dietary Paleobiology

I don't follow dinosaurs quite as avidly as I once did, but one topic I've kept an eye on is the paleobiology of hadrosaurs. If you're looking to work on paleobiology in dinosaurs, hadrosaurs are pretty much ideal. Many species are known from good remains of numerous individuals of various ages, giving about the best sample sizes you can get for nonavian dinosaurs. In addition, with all the bells and whistles like crests, soft tissue impressions, and the unique feeding adaptations, there's plenty of room for arguments. I've picked a few recent papers, focusing on feeding: what they ate and how they ate it.

Edmontosaurus at the Denver Museum of Nature and Science wonders how you get by with just two sets of teeth.

Sunday, November 19, 2017

Stegomosuchus longipes, the terrestrial croc relative of Massachusetts

If you're the kind of person who reads this blog regularly, you're probably also the kind of person who's got at least one rock laying around. Maybe you've got dozens. Maybe you've got too many. Who am I to judge? The point is you've got rocks. Odds are, though, there isn't a potential type specimen in your yard.

Sunday, November 12, 2017

The first fossils described from Dinosaur National Monument

The first fossils described from Dinosaur National Monument (to the best of my knowledge) were not dinosaurian. They weren't vertebrate. They weren't from the Morrison Formation. They weren't from the Jurassic, or even the Mesozoic. You may not realize it, but the monument has a geologic record extending from the Neoproterozoic to the Quaternary (see for example Untermann and Untermann 1954, Gregson et al. 2010, or Santuci and Kirkland 2010). For this bit of history, we're also going back in geologic time.

Echo Park, at the confluence of the Green and Yampa rivers (NPS/Jacob W. Frank). Why this landmark? Read on!

Sunday, November 5, 2017

Rhabdodontidae

French geologist Philippe Matheron named Rhabdodon priscus in 1869, making it among the oldest dinosaur names still in use. In terms of public interest, Rhabdodon and its close relatives have definitely been late bloomers. From 1869 through the 1990s, we've had a few papers, but the only person who seems to have put much energy into rhabdodontids during that time frame was the inimitable Baron Franz Nopcsa. The fortunes of the group have picked up in the past few decades; several new species have been described since 2000, old species have been reevaluated, and there was even an appearance in a segment of a TV documentary special. (Of course, there were the usual drama-related inaccuracies, and the rhabdodonts had to go in disguise as "dwarf Iguanodon", but at least they were there.) The latest news is a reevaluation of rhabdodontid paleobiology that makes them into something more than stock small ornithopods.

Sunday, October 29, 2017

Halloween: Platteville in disguise

Let's start with a pair of photos, one of typical Platteville Formation fossils and the same for the Decorah Shale:

Welcome to the Ordovician! Hope you like brachiopods!

...or bryozoans and crinoids

No points for guessing which is which. The first photo shows natural molds and casts in the Platteville, while the second shows the well-shredded fossils of the thin limestone beds of the Decorah. Now try this one:

No peeking!

Despite the preservation, this piece is from the Platteville. Specifically, it comes from a bed about halfway up the Mifflin Member. Somehow this bed managed to escape the worst of the heartbreak of dolomitization, at least in a small area. Little glimpses like this show a richer picture of the Platteville fauna than we get from the natural molds and casts. (The tiny crinoid columnals are an interesting touch, as is the relative scarcity of bryozoans.)

They're in there. It's a proper hash.

The strophomenids might be a little smaller, too.

There are also snails, ostracodes, and some elongate triangular things. Some of these are probably small nautiloids, others might be hyoliths.

The example near the center bottom of the photo is one I suspect is a hyolith.

There are a lot of subtleties to the rocks and faunal assemblages once you start looking!

(Also, having found a decent place to see the Glenwood up close, I've added a couple of photos to the old post.)

Sunday, October 22, 2017

Sunday morning Decorah fossils

I've recently made a small collection of Decorah Shale pieces from spoil piles at a construction site. Most of them will go to other people and groups for use in education, but while I've got them I'm certainly going to take the opportunity to photograph them. Incidentally, construction can be a good source of fossils in the Twin Cities, if you don't mind disruption of the original stratigraphic context (which tends to happen anyway with the Decorah around here). Of course, as always, you'll want to ask for permission, and it's advisable to make collections when someone is working there, so you aren't mistaken for a trespasser or other nefarious sort.