Tuesday, February 16, 2016

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):

The basic shape and the striations are apparent. The striations appear to delineate almost "scale"-like sheets of material. The striations are not perfectly straight, but are somewhat wavy. Note the tip overlapping a small round object, and how the fossil as a whole seems to slightly "float" above the matrix. I was quite surprised that these photos turned out at all!

Between "profile" and overhead.

More or less directly overhead. Part of the lower side is overlapped by matrix. The slight asymmetry of the structure is apparent.

This is somewhat blurry, not surprising given the depth of field issue, but it should give an idea of the "apertural" end.

What is this fossil? I have asked around and put it up on The Fossil Forum and Facebook, and there has been no definite conclusion. There are a few things that it's not. The first-order negatives, things with morphologies that are completely incompatible, include receptaculitids, algae, sponges, tabulate corals, bryozoans, brachiopods, monoplacophorans, rostroconchs, bivalves, gastropods, scolecodont elements, ostracodes, graptolites, conodonts, obscure microfossils, and trace fossils. Horn corals, cornulitids, and nautiloids? Well, if somebody asked me to come up with small triangular versions of these fossils, they would probably look something like this, but I am not aware of the existence of such beasts. This leaves conulariids, hyoliths, trilobites, echinoderms, and uncommon exotic fossils.

  • Conulariids: conulariids can have similar surficial structures and textures, and although they are generally four-sided, some are three-sided. The slightly convex "apertural" edges are also consistent. However, conulariid structures are composed of rods, and this looks more like sheets of material, unless we're looking at a superficial outer layer here.
  • Hyoliths: hyoliths are the best fit for basic shape and size, as small triangular objects with triangular cross-sections. The major issue in my mind is whether or not they exhibit this type of surficial structures. Briefly speaking, either the little buggers don't preserve quite like this very often, or nobody who has one like this has posted a clear photo.
  • Trilobites: some structures on trilobites can look similar to this object, such as spiny projections and parts of the hypostome in some species. They tend to have different surficial texturing, though.
  • Echinoderms: the sheer abundance of crinoid columnals in the Decorah tends to make us forget about the rest of the animals, but crinoids and the other echinoderms of the area would have had many small body plates. Most of these plates tend to be fairly equant, though, and/or do not have this kind of surficial texturing.
  • Miscellaneous exotica: a couple of people have suggested machaeridian worms, or polyplacophorans (chitons and friends). I am not not very familiar with fossils of these groups and have not heard of them being present in the Decorah Shale of Minnesota, although they (or things like them) are certainly present in comparable faunas. The major difficulties with these are dissimilar surface textures and the segmented nature of their fossils, as opposed to this thing being a single block.

I am leaning to hyoliths being most compatible with the object, although I cannot rule out the other options, especially given my level of experience with conulariids, hyoliths, trilobite fossils that are not small pygidia, echinoderm fossils that are not columnals, and (more or less by definition) fossils I've never seen in person. I'd love more opinions!

Item Two: And another thing

A couple of people have recently alerted me to the fact that Thescelosaurus.com is still operating, in a fashion. The front page is still at the old location, but the internal links go to the corresponding pages on Internet Archive (at least they did when I checked it out). I will state right off the bat that it isn't me. Honestly, I have no idea who is doing it, although it is a nice thing to do (albeit a bit misleading because I'm not involved with it). [Note: it hasn't done this for a long time. Don't click on any thescelosaurus.com links that are not to Internet Archive addresses predating April 2015.]

1 comment:

  1. It looks like it may be a conulariid to me, but it's only an educated guess.