Fun with nautiloids: an essay in futility?

Or: "What good is a seven-foot nautiloid?"

In most reconstructions of marine Paleozoic life, large orthoconic (straight-shelled nautiloids) get two jobs: they are either large menacing objects in the water, usually seizing some unfortunate trilobite; or they are parked on the seafloor, again often engaged with a trilobite. The general takeaway is that a giant nautiloid was a voracious predator. Trilobites presumably are selected as the prey because so few other animals that lived during the heyday of the giant nautiloids both moved under their own power and were bigger than a few inches, making them about the only group worthy of the honor of artistic predation (although we can guess that a not-insignificant part of a giant predatory nautiloid's diet would be smaller nautiloids). Another, less scientific reason for having a nautiloid capture a trilobite is that otherwise Paleozoic marine reconstructions would be a whole lot of small filter-feeding animals not going anywhere, which doesn't make for dramatic art.

Nautiloids and trilobites: natural enemies forever locked in combat. Also pictured: many snails.

Super-predator giant nautiloids appeal to the imagination, and we certainly have voracious predatory cephalopods today. Inconveniently, though, giant nautiloids have left us very little in the way of data on their actual habits. We don't have any kind of remains except the hard parts of the shells. We know that orthoconic nautiloids lived in gently tapered chambered shells that looked kind of like exaggerated princess hats on the outside. For all the evidence we have, nautiloid bodies might as well have looked like princess heads, engulfing trilobites, smaller nautiloids, conodonts, etc. with their long flowing golden tresses. (Admittedly, this is rather unlikely.) (Also, if you dislike the princess comparison, feel free to substitute your own. Wizard hats and gnome hats would fit about as well.)

Or perhaps delicious ice cream

All of this is a long way of noting that giant nautiloids are blank slates in a lot of ways. About a year ago, I was communicating with Kat Cantner of EARTH Magazine about various aspects of the Twin Cities Ordovician, and the lifestyle(s) of giant nautiloids came up. I was surprised on checking to find how little firm information there actually was, and we kicked around the possibility of their being planktivores. I filed the idea away as a possible blog topic, which is where it stayed until the recent announcement of Mironenko (2018), which considers the possibility of endocerid nautiloids as suspension feeders.

Endocerids belong to the group Endocerida, one of several groups under the nautiloid banner, Nautiloidea itself being more of a descriptive term than a natural group. Endocerids lived only during the Ordovician and included the largest known nautiloids, with unconfirmed rumors of specimens up to 9 m (30 ft) long. In Minnesota, we don't have anything quite that extravagant, but we do have several species of Endoceras and fellow endocerid heavyweight Cameroceras (Catalani 1987). Clarke (1897) reported concerning Endoceras proteiforme (Cameroceras proteiforme of his usage, the large nautiloid of the Platteville and Decorah) that "entire shells referable to this species have been found with a length of ten to fifteen feet, though all the material before me is of a smaller size." Of course, after 120 years we don't necessarily know where these 10-to-15-foot specimens came from, nor whether they would be assigned to E. proteiforme as we now know it. For the record, Klug et al. (2015) give the longest nautiloid of the Sandbian (the part of the Ordovician including the Platteville and Decorah) as a 1.7 m (5.6 ft) specimen of the non-endocerid Ormoceras from Estonia and the longest from the Katian (covering the rest of the local Ordovician) as a 5.7 m (18.7 ft) Endoceras from New York. Based on my own experience in museum collections and miscellaneous other references, such as a photo of an in situ nautiloid in Ojakangas (2009) and Winchell (1888)'s brief reference to Twin Cities quarry workers' tales of large "petrified snakes", my guess is that a reasonable range for local big endocerids is approximately 2–3 m (7–10 ft). A 3-m-long nautiloid would be very convenient for comparative purposes, because now you're talking about a squid plus the proverbial ten-foot-pole, but for fairness I will go on with a 2-meter nautiloid; this keeps it within Clarke's limits of smaller than 10 feet, after all. However, the "pole" analogy is helpful because of the very gradual taper of endocerid shells.

Aside from size, endocerids are noted for the internal anatomy of their shells: the internal passage known as the siphuncle was proportionally wide and not centered within the shell, and mineral deposits formed stacked cone-like structures called endocones near the apical (tip) end of the siphuncle. The animal itself probably lived partially within the wide siphuncle (Moore et al. 1952). If you've ever seen nautiloid specimens or photos that look something like a drumstick or bottle, with a wide end followed by a "handle" or "neck", this was probably a partially preserved endocerid, with the "handle" being the siphuncle.

So, like this. I got this from an estate, without locality information. There's some Decorah-looking matrix attached, and I wouldn't be surprised if it's Twin Cities, but it could be from a different unit and/or elsewhere in the state or neighboring states as well.

Returning to the intermediate issue at hand, Mironenko (2018) challenged the "apex predator" model on various grounds of anatomy, ecology, and fossil distribution. I do not intend to re-argue Mironenko's grounds, but instead to riff off of some of the ideas from my own knowledge as it pertains to one familiar small area of marine deposition over one familiar slice of the early Late Ordovician. I recognize that there is much that is speculative in the article, but I think it is completely fair to wonder what in the world giant endocerids were doing. I will divide this discussion into anatomy and ecology.

Anatomy

In the absence of soft anatomy, we have shells. Thanks to its shell, our idealized 2-m endocerid can be considered as perhaps not the most graceful of marine animals. In the absence of any heretofore-unknown soft-tissue structures that could have been used as fins or flippers (and which also somehow left no evidence on the shell), an endocerid was left with its own tentacles (of unknown configuration) and a hyponome, an organ by which living nautiloids and other cephalopods can expel water in a jet. We can infer the presence of a hyponome in endocerids by features of the shell (Mironenko 2018). We can hope that the endocerid's hyponome could be maneuvered to some extent, as modern nautiloids can do; otherwise, the animal would have been permanently stuck in reverse. (Aside from the maneuvering problems a fixed hyponome would pose, it wouldn't have helped a large jetting endocerid on the hunt, either: everywhere it went it would have been heralded by its entire shell, a subtle clue which just might draw the attention of an observant trilobite.) Mironenko (2018) includes a restoration with a very flexible hyponome pointed full aft, but I don't know how feasible that is.

So, our 2-m endocerid is moving about by hyponome jetting. The thrusting power of the hyponome is limited by the size of the hyponome, which is limited by the size of the wide end of the shell, which proportionally is not all that wide and also has to hold the rest of the business end of the endocerid. Our 2-m endocerid is also hauling around 2 meters of mineralized shell, weighted at the far end to guarantee a tendency to fishtail during maneuvers. This shell contributes absolutely nothing in terms of propulsion and maneuverability except the feeling of satisfaction at overcoming the challenge of inertia, and we have no evidence on whether endocerids had sufficient mental development to experience satisfaction.

The difficult acceleration is also something of a problem for "parked giant nautiloid" reconstructions, where the nautiloid is on the seafloor and presumably has to either wait for a trilobite to amble into range, lug itself along on its arms, or explosively raise itself (and its shell) when it sees something of interest. There is also the absence of trace fossils that would indicate a 2-m endocerid sitting or moving on the seafloor. If we're looking for "demersal" nautiloids, we're much better off with things like Gonioceras, which was much smaller as well as dorso-ventrally flattened.

I confess that it's a bit difficult for me to see this endocerid as a particularly effective predator, ambush or otherwise, unless large trilobites and other prospective prey were similarly poor at acceleration, speed, and agility, as well as being unable to tell when the largest living thing in their world was nearby. It would seem to me that giant endocerids were doing something that did not require a lot of quick acceleration or precise rapid turning, and suspension feeding certainly fits those requirements, although it is not the only lifestyle that does.

Paleoecology

If you look around today, the largest animals are feeding in bulk on low trophic levels: they're herbivores, like elephants, or filter feeders, like baleen whales and whale sharks. Giant nautiloids feeding at the base of the trophic pyramid rather than from the apex to the next level down admittedly appeals to me, although it's dangerous to be lulled by personal biases.

However, the trophic structure of Minnesota's Ordovician seas is kind of stunted. For example, in the Platteville, haven of nautiloid diversity per Catalani (1987), we're looking at our familiar 2-m endocerid, smaller nautiloids, the trilobite Isotelus (in the overlying Decorah reaching as much as about a foot long, or about 25 cm, but usually smaller), some snails on the order of 10 cm across (~4 inches), and then a whole lot of things a couple of cm or less across, most of which were stationary and required no particular predatory innovations to catch (I'm lumping crinoids with this group). Granted, the actual endocerid in the 2-m shell was not 2 m long, but the point still holds that there wasn't a great diversity of things for our apex predator endocerid to act apex-predatorily toward. Nor does there appear to be an obvious reason for apex predator Endoceras to need a shell 2 m long. A nautiloid half that length could probably catch an Isotelus just as well.

Thoughts

If we're not looking at the stereotypical apex predator, what kinds of roles are there for something that is the largest animal of its time, has known carnivorous relatives, is not an obvious herbivore, almost certainly does not move very well, and lives alongside a number of smaller relatives that would be faster and more maneuverable? Or, more glibly, what might a 2-meter endocerid do that cannot be done just as well by a 1-meter nautiloid, in a world where the largest non-nautiloid prey is a trilobite that sometimes reaches 25 cm long?

• Over-grown predator: 2-meter endocerids were not born 2 meters long, after all; perhaps the giant examples had been more active predators when younger, but lost that niche at a certain size and turned to something less challenging like scavenging, with the large size perhaps advantageous for something like reproduction or its new ecological role. We could look at this with a suitable sample of juvenile to adult endocerid specimens, checking their hydrodynamic characteristics.
• Hunter of other large unwieldy nautiloids: Although this would certainly be in the cards for the occasional meal, as a primary lifestyle it ends up being hard to sustain, because the largest species would tend to end up cannibalizing themselves.
• Opportunistic scavenger/predator: An extension of "over-grown predator" to the whole lifespan. We may opt to reject the "parked" endocerid for the lack of mobility and trace fossils, and the "apex predator" endocerid as too much of a struggle against inertia, but that doesn't mean we have to reject predatory endocerids. What if we envision the giants as being opportunistic scavengers and predators, drifting through the water in patient search of material to scavenge as well as opportunities to pick off easy prey?
• Suspension feeder: The endocerid is somehow using its arms to gather up and eat large quantities of tiny food, like ostracodes, other tiny arthropods, planktonic larvae of other larger animals, and so forth. I certainly like this idea, particularly on ecological grounds, and the endocerid doesn't need to move all that well to do it. Large size can also be advantageous here, by allowing a larger area for collection. As Mironenko (2018) noted, there are living cephalopods that pursue a similar strategy, and it is likely that many ammonites, including the orthoconic Baculites, fed on plankton (Kruta et al. 2011). (Would that we had the same kind of "dental" and digestive evidence with orthoconic nautiloids!) The "how" is a question, given the absence of soft-tissue fossils; Mironenko (2018) included a speculative restoration of endocerids with membranes stretched between the arms, which as others have noted online is a bit difficult to work if the hyponome was more limited in movement than the reconstruction assumes (the endocerid might blow away its food if it had to move). (I do have one crazy idea that might overcome this problem: if the weighted apex was "over-weighted", it could have allowed a vertically oriented endocerid with upward-facing aperture and arms, and gravity bringing the food, but I digress. As usual.)

Whatever the exact lifestyle(s) of the giant endocerids, they burned out relatively quickly. Perhaps they had gotten locked into niches that were unstable. If they were hunting large trilobites, trilobite size crashed in the latest Ordovician (Klug et al. 2015), and if they were suspension feeders, the mass extinction at the end of the Ordovician would have done a number on the plankton crop.

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 E. Ulrich, 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.

Klug, C., K. De Baets, B. Kröger, M. A. Bell, D. Korn, and J. L. Payne. 2015. Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia 48(2):267–288.

Kruta, I., N. Landman, I. Rouget, F. Cecca, and P. Tafforeau. 2011. The role of ammonites in the Mesozoic marine food web revealed by jaw preservation. Science 311(6013):70–72.

Mironenko, A. A. 2018. Endocerids: suspension feeding nautiloids. Historical Biology in press.

Moore, R. C., C. G. Lalicker, and A. G. Fischer.1952. Invertebrate fossils. McGraw-Hill Book Company, Inc., New York, New York.

Ojakangas, R. W. 2009. Roadside geology of Minnesota. Mountain Press Publishing Company, Missoula, Montana.

Winchell, N. H. 1888c. The geology of Ramsey County. Pages 345–374 in N. H. Winchell and W. Upham. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 2. Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.