One small mystery, and another

Item One: Always pay close attention to your hash slabs

I've recently done some educational events, and I usually bring a few hash slabs. When I'm done, I usually give them a once-over to check for damage incurred from transportation, handling, and so forth (there's a bryozoan that has been breaking on one of the slabs, for one thing). I noticed something unusual on one of the slabs: a small triangular object that on close inspection proved to have interesting surficial markings of fine, gently curved lines running between the long axes of the object. The object is approximately 6.5 mm long by 3 mm wide. What there is of it appears to be either triangular in cross section, or two flaps of material (kind of like a lopsided tent) draped on the matrix. I say this because the tip overlaps a small circular fragment, and because there does not appear to be anything continuing down into the matrix from the two sides that are visible. The lining of the surface does not appear to be segmenting, but looks almost like it defines slightly imbricated sections of the structure. The wide end appears to be open, with the two visible edges slightly overhanging infilling matrix. The edges do not define a flat plane, but curve in slightly where they meet. The sides appear to be flat, making allowance for a slight overall curve and skew to the object. My interpretation is that the structure is a hollow or partially object of triangular cross-section, composed of material that grew in layers from the tip, with the wide end open. The host slab is one of those I collected last spring from the construction pile. It is certainly from the Decorah Shale, and probably came from the lower third or so of the formation, based on geography. Behold (scale bar in mm):

Earthquakes in Minnesota

I was co-leading a field trip for a Minnesota Master Naturalist group over the weekend, and at one point we strayed onto a tangent involving faulting and earthquakes in Minnesota. Given we've already covered one of the rarest of the geological rare birds of the state with "Minnesota's dinosaurs", let's add another notch to the rock hammer with the subject of earthquakes in the Land of 10,000 Lakes.

Gonioceras: when a nautiloid is also a shovel-flounder

In a previous post, we were briefly introduced to the local nautiloids, which for the most part inhabited straight, gently tapered shells. Plectoceras coils, but otherwise the local squidlings are reasonably predictable. There is one other notable exception, though: Gonioceras is one of the oddest-looking nautiloids you can run across. Picture the blade of a shovel: it's pointed, it's flat, it's got a ridge at the broad end that turns into a socket for the handle. Now, let's modify it a bit. First, trim the blade into a triangle. Now, chop off the fitting for the handle where it goes beyond the blade. Next, take the rest of the fitting section and extend it down to the tip, and make it hollow, so you've got a tube running the length of the blade. Taper it, and keep the underside of the blade flat. Finally, put an appropriately sized nautilus body in the tube. This is essentially what a Gonioceras looked like. One other thing: the septa (the divisions between chambers) are arranged in sine-wave-like curves, concave behind the aperture and convex on the flanges. The illustration below includes the central portion and one of the flanges of a shell, with the aperture toward the top, and should make things more clear:

Gonioceras occidentale from Illinois, plate LVII of Clarke (1897). This specimen shows part of the wedge-like flaring of the shell and the curved septa.

Of course, in the field, you're liable to just find a chunk with those distinctive wavy septa:

Gonioceras in the wild, skooshed flat and otherwise worse for wear, from the lower to middle Mifflin Member of the Platteville Formation.

Although Gonioceras certainly cannot be accused of simply doing what the other nautiloids were doing, its innovative structure does not appear to have caught on. In Minnesota, Gonioceras is primarily a Platteville Formation concern (Stauffer and Thiel 1941; Catalini 1987). It is generally thought of as a bottom-feeder, sometimes interpreted as a crawler but perhaps analogous to a flounder (Vickers Rich et al. 1996). Like other nautiloids, its propulsion would have moved it primarily backward, so the pointed end was the leading end of the animal when it had to get going.

References:

Catalani, J. A. 1987. Biostratigraphy of the Middle and Late Ordovician cephalopods of the Upper Mississippi Valley area. Pages 187–189 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Clarke, J. M. 1897. The Lower Silurian Cephalopoda of Minnesota. Pages 760–812 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Vickers Rich, P., T. H. Rich, M. A. Fenton, and C. L. Fenton. 1996. The fossil book: a record of prehistoric life (corrected republication of 2nd edition). Dover Publications, Inc., Mineola, New York.

Requiem for ACM 41109, the first holotype of Anchisaurus

ACM 41109, also known as AM 41/109, was until recently the holotype or defining specimen of Megadactylus polyzelus, better known as Anchisaurus. ACM 41109 includes eleven back and tail vertebrae, a partial shoulder blade, most of the right hand, part of the right forearm, much of the left leg, and parts of both ischia. These are all that remain of a basal sauropodomorph that lived about 200 million years ago. Unlike the overwhelming majority of its brethren, the mortal remains of ACM 41109 were not only buried and fossilized, but through sheer inconceivable luck were buried in a place where, 200 million years later, they would be uncovered by earth-moving operations carried out by a species that was not even a hint of a glimmer of a twinkle in the eye of one of the therapsids that lived at the same time as the dinosaur. Its rebirth in 1855 was difficult: it suffered blasting and its bones were then dispersed by workmen who were preparing improvements to the great Springfield Armory, a timely effort given the Civil War to come a few years in the future. As much of the skeleton as could be salvaged was sent to pioneering paleontologist Edward Hitchcock of Amherst College, which is how the specimen came to be labeled "ACM"; the college's museum is today the Beneski Museum of Natural History. Although the first reports came out in the 1850s, a formal name would have to wait until 1865, when the task fell to Hitchcock's son, Edward Jr. At the time, it was one of the few dinosaur specimens of any completeness known, and was part of the first wave of North American dinosaur body fossils along with Hadrosaurus, "Laelaps", Leidy's Judith River scraps, and other bits and pieces. In one of those strange comedies that happen from time to time, it went through two genus names (Megadactylus and Amphisaurus) before finally coming upon one which wasn't already in use (Anchisaurus). ACM 41109 was sufficient to define Anchisaurus polyzelus for many years. Now this function has been passed to YPM 1883, previously known as Anchisaurus colurus or Yaleosaurus, both names which now must be considered rejected. YPM 1883 was somewhat more fortunate in terms of how it was preserved and discovered, but has no particularly colorful stories behind it. The quarry that produced it is now occupied by a shopping center, and by the time it was found dozens of good dinosaur specimens had already been recovered. Perhaps ACM 41109 will now be forgotten, a handful of bones in a drawer. Yet though it no longer suits the changing needs of taxonomy, it somehow reached out beyond 200 million years and briefly become the exemplar of untold forever-vanished animals.

Further reading:

International Commission on Zoological Nomenclature (ICZN). 2015. Opinion 2361 (Case 3561): Anchisaurus Marsh, 1885 (Dinosauria, Sauropodomorpha): usage conserved by designation of a neotype for its type species Megadactylus polyzelus Hitchcock, 1865. Bulletin of Zoological Nomenclature 72(2):176–177.

Tweet, J. S., and V. L. Santucci. 2011. Anchisaurus from Springfield Armory. Brigham Young University Geology Studies 49(A):75–82.

A tale of two packrat species

This is Neotoma cinerea, the bushy-tailed packrat:

From Oregon Caves via Wikimedia Commons.

The Decorah Shale

So we've gotten through the St. Peter, Glenwood, and Platteville. It's high time that the collection is completed with the one and only Decorah Shale. The Decorah is a bit of a shy bird in the Twin Cities because it's not the most resistant unit in the world and is pretty well covered otherwise. It's often more easily seen by what it erodes into: a gray-green clay with bits of buff-colored heavily fossiliferous limestone. In the metro, it is best known from western St. Paul and just south of the Mississippi River in the Mendota Heights/Lilydale area, but I suspect it is more widely distributed than it is usually mapped. Along the Minneapolis side of the gorge, I've encountered characteristic gray-green clay and buff fossiliferous limestone fragments near or at the top of the bluffs in a few places, perhaps representing erosional outliers.

The slope photo strikes again!

Crystal ball for 2016

Welcome to 2016! I trust you have arrived safely; this darn travel through time only goes one way. 2015 was all right, as years go. I crossed a couple of fossils off the "see in the wild" list (tabulate corals and conulariids, plus I finally got the hang of horn corals), plus for my job I got to sleep under the stars at Death Valley and see part of the Grand Canyon by small plane. All in a day's work, I say. Having had some good luck in the Platteville this year, I plan to pay more attention to it this year. My "white whale" would be finding an equivalent to the old "Johnson Street Quarry" echinoderm locality; I've been looking out for Hidden Falls Member exposures lately. I'd love to see an edrioastroid or a "cystoid".

I continue to update the Compact Thescelosaurus; if you have something you'd like me to correct, just let me know. Here's a few things I expect for dinosaurs in 2016 (nothing too serious, of course):