Although the modern nautiloids are known for their coiled shells, more often than not Paleozoic nautiloids had straight shells (known as "orthoconic"), and the farther back you go, the more straight shells you see. The nautiloids of the Late Ordovician of Minnesota hadn't really gotten into coiling yet. Some were curved (especially Cyrtoceras), but coiling wasn't a big thing. Well, except for this one guy, Plectoceras, but there you go. Nautiloids are not the only cephalopods to have used a straight shell; the common Late Cretaceous ammonite Baculites possessed a straight to gently curved shell (and various other heteromorph ammonites took on all sorts of wacky Dr. Seussian shell curves). The internal shells of belemnites gave them a linear form as well.
|Plectoceras, in all of its nonconformist forward-thinking glory.|
The major advantage of the coiled shell over the non-coiled model seems to be related to buoyancy, which requires a bit of anatomical exposition. A nautiloid's shell is divided into a number of chambers (camerae), and the nautiloid itself dwells only in the largest, most recent chamber. The rest of the shell, with its partitions (septa, singular septum, a generic word for partition which turns up a lot in anatomic contexts), is called the phragmocone. (Incidentally, where a septum meets the outer shell, you get a line called a suture, which is important for taxonomy.) The living chamber and the chambers of the phragmocone are connected by a tube called the siphuncle. The chambers of the phragmocone are filled with gas, making the animal more buoyant. If you just have a plain old chambered shell with a nautiloid living at one end, you tend to have a problem wherein the center of gravity is within the critter part, and the center of buoyancy is within the phragmocone part, giving your nautiloid a delightfully awkward orientation. Evolution produced two general solutions to this issue: adding more weight to the phragmocone, via mineral deposits in the chambers or in the siphuncle; or coiling the shell over the living chamber, which produces a sort of underwater shell balloon. (See Meyer and Davis 2009 for a longer nontechnical discussion of the buoyancy issue.)
|Somewhat more typical than Plectoceras: an assortment of small orthoconic nautiloid fragments from the Decorah.|
If you've been following this series on Minnesota's Ordovician menagerie, you're probably familiar with two characteristics: animals being a few cm or smaller, and animals fixed firmly in place (at least as adults). Nautiloids get to break both rules: the largest reached sizes around 10 ft (3 m) long, and all were mobile, inferred to have used the same form of water jet propulsion used by modern cephalopods. Granted, most of the length was gas-filled phragmocone and not tentacle beast, but they were still quite a bit larger than just about all of their contemporaries. The next largest individual animals (take that, bryozoan colonies!) appear to have been certain trilobites, which got up to around 12 in (30 cm) of trilobite goodness (just don't think about all of the legs—oops, you're thinking about them now, aren't you?).
Nautiloids are classically restored in the act of grabbing some poor sap of a trilobite with malicious intent, or just kind of hanging out flat on the seafloor. The trilobite thing is doubtless because trilobites and nautiloids were among the rare Ordovician animals that could move under their own power and had some size, so there's more scope for visual drama. (Well, snails moved too, but *you* try making a gripping illustration of a snail ravaging a tabulate coral or something.) The seafloor thing is both because some nautiloids probably spent most of their time on the seafloor, and because if you're making a diorama, it's convenient to be able to put things on the ground. At any rate, it is probably safe to picture the largest nautiloids as the top dogs of the Ordovician. When practically everything else is smaller than a few centimeters and glued to something else, and you're a meter-long or greater tentacle beast with the ability to move freely, the art of subtlety is not high on your list of priorities, and the only things you have to worry about are other nautiloids.
As noted with the bivalves, nautiloids, as mollusks, have some issues with shell preservation. Meyer and Davis (2009) note that Cincinnatian nautiloids are often preserved only as internal molds of phragmocones, which is not uncommon here, either. Something else to watch for is that small nautiloids and crinoid stems can look an awful lot alike if you've got the right chunk. Ideally, you'll have a clear end-on view which can show you typical crinoid columnal features and surface textures (radial ridges, a five-lobed central opening, other five-fold features), or the cylinder in question will have knobs on the side or appear to be made of stacked discs of varying diameter, which are also crinoid features. Incidentally, if you're getting into younger Paleozoic rocks, where there are also ammonites, there are a couple of ways to tell coiled ammonites from coiled nautiloids. Nautiloids have straight sutures, whereas ammonites have complex zig-zag sutures, and nautiloid siphuncles pass through the center of chambers, while ammonite siphuncles are typically right up beneath the outer wall of the shell.
Stauffer and Thiel (1941) came up with a list of 78 species in 33 or 34 genera for the St. Peter Sandstone, Platteville Formation, Carimona Limestone, Decorah Shale, and other "Galena" of Minnesota. Again, the limestone-dominated units, especially the Platteville and "Galena", come out well. Huh. This leads to a question of interest: did mollusks prefer the conditions that led to the limestone units, compared to the shaly Decorah? This is just idle thinking, though; it's a bit difficult to see what could be drawn from this inference, given the radically different lifestyles of bivalves and nautiloids. I suppose the extinction following the Deicke K-bentonite could always be thrown at it, too. Anyway, the genera:
Actinoceras (Pl, De, Gp)
Beloitoceras (Pl, Ca)
Cameroceras (Pl, Gp)
Cameroceras? multicameratum (Pl, Ca, De, Gp)
Cycloceras (Pl, Ca, Gp)
Cyrtoceras (Pl, De, Gp)
Endoceras (Pl, Ca, De, Gp)
Gonioceras (Pl, Ca)
Manitoulinoceras? (Pl, De, Gp)
Metaspyroceras (Pl, Ca)
Oncoceras (Pl, De, Gp)
Orthoceras (Sp, Pl, De, Gp)
Plectoceras (Pl, Gp)
Scofieldoceras (Pl, Ca, De)
Spyroceras (Pl, Ca, De, Gp)
Tripteroceras (Pl, Ca, De, Gp)
Whitfieldoceras (Pl, De, Gp)
Zitelloceras (Pl, De)
Nine forms are broadly distributed, if you permit some leeway with "sp." (shorthand for "the author is happy with the genus identification, but is unable to determine a species"):
Actinoceras bigsbyi (Pl, De, Gp)
Cameroceras? multicameratum (Pl, Ca, De, Gp)
Cyrtoceras sp. (Pl, De, Gp)
Endoceras proteiforme (Pl, Ca, De, Gp)
Orthoceras junceum (Pl, De, Gp)
Orthoceras sp. (Sp, Pl, De, Gp)
Spyroceras bilineatum (fun with taxonomy!) (Pl, Ca, De, Gp)
Tripteroceras planoconvexum (Pl, Ca, De, Gp)
Tripteroceras sp. (Pl, Ca, Gp)
Catalani (1987) has a more recent list, but it's a bit more difficult to interpret, being in the form of a diagram which could stand to be larger. It does, however, corroborate that significantly more species have been identified from the Platteville and its equivalents than the Decorah and its equivalents.
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.
Meyer, D. L., and R. A. Davis. 2009. A sea without fish: life in the Ordovician sea of the Cincinnati region. Indiana University Press, Bloomington and Indianapolis, Indiana.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.