Wednesday, February 3, 2016

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.


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.

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