cephalopods

cephalopods Cephalopods are the most highly evolved members of the phylum Mollusca. They are marine animals that are mostly fast-moving predators and they are named for the close union of the head and foot (Greek kephale, head and Latin pes, foot), the latter organ having been elaborated into a series of tentacles that surround the mouth. Modern members of the class include the squids, octopus, and Nautilus, while belemnoids and ammonites are important fossil groups. Modern cephalopods include within their numbers the largest, fastest, and most intelligent marine invertebrates; the giant squid Architeuthis princeps, for instance, reaches lengths of 22m; they are, however, relatively insignificant when numbers of species are considered. Only 730 species exist in our oceans today compared to the approximately 10 500 extinct species that are known. Cephalopods have a long geological history, ranging from the Cambrian to the present day. They were particularly abundant during the Mesozoic, and the ammonites of that period are particularly valuable as zonal indicators. The molluscan body is characteristically divided into four components; the head, foot, visceral mass, and mantle. As in the other classes of molluscs the cephalopods have a mantle cavity that lies behind the head. In this class the mantle cavity has, however, been adapted as a propulsive organ. Water enters around the edge of the mantle but is ejected only through a flexible tube, the hyponome, producing a strong propulsive jet. This system is particularly well developed in the squids, which with their long streamlined bodies are the only invertebrates capable of competing with fish as fast efficient predators in the oceans. All cephalopods have tentacles surrounding the head. In Nautilus, which is a primitive form, they are small and numerous; higher cephalopods have eight to ten muscular arms with suckers. Squids have eight short arms and two long ones, whereas octopus have eight arms of equal length; both use the arms for catching active prey such as fish and crustaceans. Modern cephalopods have very well-developed brains and sense organs, particularly the eyes (which show a similar design to the vertebrate eye). The main feature that enabled cephalopods to become capable of sustained rapid swimming was the development of a buoyant shell. This is an important feature of the ammonites and Nautilus, where it is a large coiled and chambered structure; it has, however, been reduced and adapted in the modern squids and is lost entirely in the octopus. One feature of major taxonomic importance in the shell is the suture, which is the line along which the walls between the chambers meet the main shell wall.

Taxonomy

The numerous shells of fossil cephalopods present in rocks as far back as the Late Cambrian attest to the importance of this group in the past. Using this abundant material as a basis, it has been possible to erect a classification that recognized a division into three broad groups: the Nautiloidea, extending back to the Late Cambrian and characterized by an external shell that may be straight, curved, or coiled and with a simple suture; the Ammonoidea, also with an external shell but with a complex suture, and found from the Early Devonian to the Late Cretaceous; and the Coleoidea, which have an internal shell and are known from the Early Carboniferous.

Nautiloidea

The modern cephalopod Nautilus (Fig. 1a) appears to be very similar in design to the shelled cephalopods of the past and has been studied extensively for this reason. It consists of two parts, the body and the shell that encloses it. The soft body is surrounded by the mantle and can be completely accommodated within the final chamber of the shell. Surrounding the mouth are about ninety tentacles. These do not have suckers but are very adhesive and are used to catch and hold prey and draw it towards the mouth, where a beak, like that of a parrot, is used to cut it up. On the lower part of the animal is the hyponome, a funnel through which it can eject water from its mantle cavity to provide a propulsive force. Sideways movements of this funnel can be used to change direction. The shell is a smooth, thin, and light planar-spiral. It is held above the animal and opens to the front. Internally the shell is divided into about thirty chambers, increasing in size towards the most recent, and separated by thin walls or septa, which meet the main shell walls along a gently curving line termed the suture. Most recent work on Nautilus has been carried out by Peter Ward of the University of Washington, who has shown that the rate of chamber formation diminishes with age. Early chambers are formed at two-week intervals, but in the adult the formation of a new chamber can take up to three months. The chambers are filled with gas, which makes the shell buoyant, and are connected with each other and the body of the animal by a fleshy tube, the siphuncle. Ward has shown that Nautilus strives to maintain steady non-fluctuating neutral buoyancy during its life by secreting liquid into the chambers or withdrawing it, using the siphuncle, to compensate for changes in body weight. These processes are too slow to power vertical movements, which the animal accomplishes by active swimming.

Nautilus is restricted to the Indo-Pacific area and is a predator that is active at night. During the day the animals rest on the bottom at depths up to 500 m. As the light starts to fail they move up to the reef, a journey that may take several hours. Predators such as fish that might otherwise be a threat are asleep at this time and the Nautilus are able to feed through the night on crustaceans and their moulted exoskeletons, apparently a favourite food. As morning approaches they move back over the reef edge and descend to the bottom, where they will spend the day. They are slow swimmers and appear to find their prey by touch; their sight is poor.

Nautiloids are the first known cephalopods, represented by small curving shells in the Late Cambrian of China. An explosive radiation within the group during the Early Ordovician produced the many straight and curved shell forms that were important into the Silurian. Many of these were of large size, some of the straight forms reaching lengths of 9–10 m. These endoceratoids gave rise to several groups, most notably in the Late Silurian or Early Devonian to the Nautilida, which includes the modern Nautilus. Most of the Palaeozoic forms became extinct by the end of the Permian, only the Nautilida surviving until the present day, although their relatives the ammonoids were extremely successful during the Mesozoic.

Ammonoidea

Ammonoids, and particularly the Mesozoic forms known as ammonites, are abundant and well-known fossils frequently collected because of the beauty of their shells. They were once known as ‘Ammon's Horns’ from a fancied resemblance of the coiled shells to the coiled horns of the Egyptian god Ammon, who was represented by a ram's head. The common name ‘ammonite’ is derived from this. Ammonoids evolved from straight-shelled forms during the Palaeozoic and developed the typical planar-spiral coiled form during the Early Devonian.

Ammonoid shells differ from those of other cephalopods in having a complex suture (Fig. 2a). This is due to the fact that the septa became increasingly frilled towards the point of attachment with the shell. Early ammonoids or goniatites may have simple zig-zag sutures, but they become increasingly complex from the Triassic to the Cretaceous. These sutures are often represented graphically, one side being drawn starting from the venter, or outer edge of the shell, and proceeding round to the dorsum or inner edge. The direction of the shell opening or aperture is indicated in the diagram by an arrow, and inflections in the suture that point in that direction are termed saddles; those that point in the opposite direction are termed lobes. As the sutures are important in classifying ammonoids, these diagrams are useful tools in developing an understanding of relationships. It is generally considered that ammonoids achieved buoyancy in much the same way as the modern Nautilus, and that the increased complexity of the septa might have developed to improve the strength of the shell so that it would resist implosion at depth. Calculations of shell strength suggest that many ammonoids had shells that were similar in strength to that of the modern Nautilus, although the shell material was thinner, while others had weaker shells and must have been restricted to shallow waters.

Many modern cephalopods show size differences between the sexes or sexual dimorphism, and this has been demonstrated in ammonites also. It was noted by the Polish palaeontologist H. Makowski in 1963 that Jurassic ammonites of the genus Quenstedtoceras from one locality consistently showed two adult forms, one of which was larger and had more whorls or turns to the shell. The earlier whorls of both forms were exactly the same but the later whorls of the larger form showed a different ornament. This same relationship has since been shown to occur in many ammonites, and it is thought that, by analogy with modern forms, the microconchs, or smaller shells, were the males while the macroconchs, or larger shells, were the females.

Little is known about predation on modern Nautilus beyond the fact that turtles and sea perch will feed on them. It is clear that the shell provides little defence from the attacks of powerful vertebrate predators. Evidence of predation on ammonoids comes from shell damage or the presence of shell material in the stomach contents of other organisms. Healed damage to shells of Early Jurassic ammonites has been attributed to attacks by fish; shells of various ammonites have been found in pellets derived from plesiosaurs, which suggests that they were preyed on by these marine reptiles. Erle Kauffmann of the University of Colorado published in 1960 a study of a large ammonite of the genus Placenticeras that had been bitten a number of times by a mosasaur, a large marine lizard. The mosasaur had clearly moved the ammonite around in its mouth several times before devouring it, apparently indicating that the mosasaurs hunted ammonites methodically and knew exactly how to handle them.

The earliest ammonoids appeared during the Early Devonian and were derived from a group of straight-shelled nautiloids. The sutures rapidly became more complex leading to the ‘goniatitic’ condition, which characterized a large group of ammonoids that were important during the Carboniferous and the Permian. This group almost became extinct at the end of the Devonian, only one genus surviving to give rise to the Carboniferous radiation. Almost all the goniatites became extinct at the Permo-Triassic boundary. The few survivors again gave rise to an explosive radiation during the Triassic, in this case of a group known as the ceratites in which the basic goniatitic suture had increased in complexity. From the ceratites the basic ammonite stock arose during the Triassic; these ammonites, of the Order Phylloceratida, gave rise to the very diverse ammonites that occur throughout the Jurassic and Cretaceous. This pattern of short-lived groups diversifying rapidly and then being replaced by offshoots from an ancestral stock is termed iterative evolution.

Ammonites became extinct at the end of the Cretaceous after a slow decline that started in the early Late Cretaceous and was probably due to adverse environmental conditions related to a series of marine regressions. During the Late Cretaceous a group of ammonites developed with shells that deviated from the normal planar-spiral pattern. These are termed heteromorphs (Fig. 2b). At one time their sometimes bizarre shapes were thought to indicate racial senility of the lineage. More recent study has shown that this view is incorrect and that they were in fact highly specialized forms adapted to a variety of environments. Their asymmetric shapes would have made locomotion by jet propulsion impossible and they would have led a predominantly benthonic life as scavengers, or even filter-feeders.

Coleoidea

The coleoids include modern squids, cuttlefish, octopuses, and the argonauts or paper-nautilus, together with extinct groups such as the belemnites. In these animals the body structure is broadly comparable to that seen in Nautilus; the shell, however, is internal or, in the case of the octopus, has been lost entirely. In the cuttlefish Sepia the internal shell is oval and consists of a series of closely spaced chalky partitions (the cuttlefish-bone). The animal is able to control its buoyancy by pumping liquid in and out of the chambers between the partitions, a system very similar to that found in Nautilus. In addition many squids use a system in which ammonium chloride is stored in the tissues to provide buoyancy. This system freed them from the depth limitations imposed by buoyant shells, and to this day the deeper parts of the ocean are extensively colonized by squid.

The earliest coleoids are known from the Mississippian (Early Carboniferous) of North America and appear to have been derived from straight-shelled nautiloids. By the Triassic the well-known belemnites had appeared (Fig. 3a). These are characterized by a solid calcite rod or guard, which has developed over the initial part of the chambered shell or phragmocone, which has itself been reduced to a small conical structure (Fig. 3b). A flat expanded tongue extended forward (the pro-ostracum) and presumably protected the anterior part of the body. Like the ammonites, the belemnites expanded and diversified through the Jurassic and Cretaceous and then dwindled, although the belemnites continued into the early Tertiary before becoming extinct.

David K. Elliott

Bibliography

Lehmann, U. (1981) The ammonites: their life and their world. Cambridge University Press.
Morton, J. E. (1967) Molluscs. Hutchinson, London.
Ward, P. D. (1988) In search of the Nautilus. Simon and Schuster, New York.