Have you ever come across an example of something that looks like an incredibly good idea, but for some unknown reason it just never catches on? In the oceans of the Silurian, the Ascocerida must have seemed to be the ultimate cephalopod. Furnish & Glenister (1964b) described the ascocerids as "almost perfectly adapted for an active nektonic [swimming] mode of life", which, in the normally dry and dusty context of a Treatise on Invertebrate Paleontology volume, comes across as high praise indeed. Despite this, ascocerids never attained a high degree of diversity or wide distribution, and by the end of the Silurian they had disappeared forever.
In some ways, the Ordovician and Silurian were the high points of cephalopod diversity. While the total number of cephalopod species may not have been as high as in later periods, this was more than made up for by the diversity of body forms. Cephalopod shells (internally shelled or shell-less cephalopods were not to appear until much later) ran the gamut from coiled forms similar to later nautilids and ammonoids (also both not yet on the scene) through to completely straight orthocones and everything in between. In older classifications, this diversity is lumped under the heading of "nautiloids", a particularly unfortunate imposition of obscurity that hides just how different the early "nautiloids" were from the modern Nautilus.
One way in which Palaeozoic cephalopods probably did resemble modern taxa is that the majority of cephalopods have probably all been active predators. In actively-swimming modern cephalopods, the main means of propulsion is through the expulsion of water through the hyponome, a muscular tube close to the mouth and tentacles, which shoots the animal through the water. For shelled cephalopods, jet propulsion adds a particular challenge, as the shell must be buoyant to allow the animal to swim freely, and finely balanced to allow the animal to move horizontally. To achieve and control bouyancy, the cephalopod shell is divided into a series of hollow chambers, with the bulk of the animal occupying the anterior body chamber and only a long cord-like extension, the siphuncle, extending into the posterior chambers. The only two cephalopod lineages with external shells to survive into the Jurassic, ammonoids and nautilids, had both independently developed a tightly-coiled form as the most effective way to maintain balance. Straighter-shelled forms appear to have developed complex arrangements of valves, diaphragms and mineral deposits to keep the shell effectively counter-balanced.
Ascocerids, on the other hand, took a different approach. Ascocerids were small for cephalopods - the largest known examples were about fifteen centimetres in length, but some species reached maturity at less than two centimetres. Their shells were lightweight marvels - even the largest examples rarely had shells more than one millimetre thick, while the paper-thin septa dividing chambers within the shell were generally around 0.1 millimetre thick. Ascocerids started their life as fairly ordinary cyrtocones - that is, rather than being perfectly straight their shells were curved like a crescent, but not so curved as to be coiled - with fairly ordinary evenly-spaced concave septa. As an ascocerid reached maturity, though, the mode of growth changed significantly. The shell become inflated and bulbous. The septae became tightly-spaced posteriorly, and sigmoid and extended forward dorsally, so that the gas chambers contained by the septa lay above rather than behind the body chamber. The posterior juvenile part of the shell was then shed, leaving the flask-shaped mature shell only. It is quite likely that there was a concurrent change in lifestyle from more benthic juvenile to fully nektic adult. With their lightweight construction and well-placed dorsal chambers, ascocerids would have been among the most mobile of Palaeozoic cephalopods, probably rivalling modern-day cuttlefish.
It might be thought that such marvels of construction would be well-placed to take over the world, but ascocerid fossils are spectacularly rare. Though specimens have been found in a number of localities in Europe and North America, they have generally only been found in very small numbers, and only in localities that have been intensely collected. Holland (1999) noted that he had seen many thousands of cephalopods from the Silurian of Britain, but only eight ascocerids. What is more, most of those specimens that are known from these localities are in very poor condition. The only two localities from which ascocerids have been found in any sort of numbers are in Gotland in Sweden and Bohemia. Gotland is also the only locality from which juvenile ascocerids have been recovered. Modern chambered Nautilus shells are capable of significant floating dispersal after the death of the animal inhabiting them - for instance, specimens have been washed up on the coast of New Zealand roughly a thousand miles south of the limits of their living distribution. Furnish & Glenister (1964a) suggested that was not impossible that the Gotland locality was in fact the only locality that had been inhabited by living ascocerids, and that all other specimens had floated to their eventual point of deposition post mortem.
I am completely at a loss as to why ascocerids remained as restricted as they did. All I can do for now is just write it off as another one of life's little mysteries.
Furnish, W. M., & B. F. Glenister. 1964a. Paleoecology. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K114-K124. The Geological Society of America and The University of Kansas Press.
Furnish, W. M., & B. F. Glenister. 1964b. Nautiloidea – Ascocerida. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K261-K277. The Geological Society of America and The University of Kansas Press.
Holland, C. H. 1999. The nautiloid cephalopod order Ascocerida in the British Silurian. Palaeontology 42 (4): 683-689.
Kesling, R. V. 1961. A new species of Billingsites, an ascoceratid cephalopod, from the Upper Ordovician Ogontz formation of Michigan. Contributions from the Museum of Paleontology, the University of Michigan 17 (3): 77-121.