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Half a Billion Years of Floating Slugs and Racing Snails
<< Cephalopod Species | By , Department of Paleontology, Natural History Museum, London1. When did cephalopods appear and how are they characterized? 2. Is the nautilus a living fossil? 3. What are ammonites and how did they get their name? 4. How did the ammonites go extinct? 5. What were belemnites like and what was the guard for? 6. What did fossil cephalopods eat,and what ate them? 7. Did fossil cephalopods reproduce in the same way as modern squids and octopuses? 8. Is Spirula an ammonite? 1. When did cephalopods appear and how are they characterized?  Cephalopods—like the Late Cretaceous Scaphites in the image to the right—evolved from primitive molluscs during the Late Cambrian, approximately 500 million years ago. Unlike the other molluscan groups, cephalopod evolution has concentrated not on a energetically conservative, defensive lifestyle but on mobility, dynamism, and intelligence. The key evolutionary development was the acquisition of the chambered shell—the phragmocone. As the cephalopod grew, it partitioned off the apical part of the shell into a series of chambers. These chambers are connected by a channel, the siphuncle, through which a strand of tissue well-supplied with blood runs all the way from the tip of the shell to the body of the cephalopod itself. By building up the salt concentration in the blood around a new chamber, an osmotic gradient is built up that draws water from the chamber into the blood, and gases from the blood diffuse into the emptying chamber. This process results in the overall density of the cephalopod decreasing. With enough of the shell divided into chambers and emptied, the cephalopod can reduce its density until it is the same as that of sea water. This condition is called neutral buoyancy.
In primitive cephalopods like the pearly nautilus, this shell is external and superficially like that of a gastropod snail. Such a shell serves two functions: defence and buoyancy. More advanced cephalopods have internal shells. The cuttlefish is a cephalopod with an internal shell. While still providing buoyancy, it is no longer protective. These more active cephalopods rely on camouflage or speed to evade predators. The most advanced cephalopods, such as the squids and octopuses do not have chambered shells at all. In squid the shell is a simple, often feather-like, strut while in the octopus it is reduced to small plates around the brain. When swimming, these cephalopods do not rely on buoyancy from the shell. The chambered shell with its connecting siphuncle is a unique characteristic of the Cephalopoda. No other mollusc shell has these features. Although there are many variations, the three main groups of cephalopods—the subclasses Nautiloidea, Ammonoidea and Coleoidea—can be easily distinguished in most cases. The table below lists the main ways of telling fossil cephalopod shells apart. The way the shell coils can be important. Primitive nautiloids all had long, straight shells, but later ones, including those alive today, have shells which coil in a regular, "ram's horn" spiral. Most ammonoids had coiled shells of a similar form and can look very nautilus-like. The coleoids almost all have more or less straight shells (though the familiar cuttlebone curves very gently). While the siphuncle is runs through the centre of the shell in nautiloids, in both ammonoids and coleoids the siphuncle is ventral. The suture line is a very important feature in determining the identity of many cephalopods, especially ammonoids. The suture line is the pattern the dividing walls of each chamber, called septa, make with the perimeter of the shell. In both nautiloids and coleoids these walls are basically shallow, saucer-like discs and the suture line is simple. In the ammonoid, however, it is folded along the edges resulting a much more complex pattern (see section on ammonites, below). Some cephalopods had counterweights to maintain a correct orientation. In the nautiloids these weights took the form of solid, calcareous deposits within some of the chambers, usually the ones near the apical end. Among the coleoids, the belemnites had solid counter weights as well, but they surrounded the outside of the shell (but underneath the skin). These counter weights, or guards, are common fossils (see section on belemnites, below).
2. Is the nautilus a living fossil? The living nautiluses are limited to a few species confined to the tropical Indo-Pacific which inhabit moderately deep water, between about 100 and 300 metres. Authorities argue whether there are one or two genera, the Linnaean genus Nautilus and a new genus, Allonautilus proposed by Bruce Saunders and Peter Ward recently for the angular forms with an opening in the centre of the spiral, as well as differences in the soft body parts. Regardless, nautiluses barely distinguishable from the ones alive today are known from the Cretaceous Period, and the subclass is by far the oldest cephalopod group, going back about half a billion years. In this sense, the living nautiluses are living fossils: animals which have changed very little over the course of around 100 million years at least. However, the living nautiluses are very odd in some respects. They are much longer lived than the other cephalopods, probably living over 10 years. They also lay a few, very large eggs, and both males and females reproduce many times over their lives. Upon hatching, the babies are replicas of their parents, and live on the sea floor like adult nautiluses. There is no planktonic stage. It is simply not known whether these features of nautilus ecology are peculiar to the extant forms or typical of nautiluses over the ages. Perhaps these are new features, explaining why these nautiluses are alive today while their legion relatives are extinct. 3. What are ammonites and how did they get their name? Ammonites appeared in the Late Palaeozoic and were the dominant cephalopods throughout the Mesozoic. They very largely replaced the nautiloids, with which they can be confused on account of the external shells. They seem to have been more like the coleoids in most other respects. They have nine teeth on the radula like squid and octopus, rather than the thirteen typical of nautiloids. Baby ammonites were small, and presumably produced in large numbers, and the adults seem to have been relatively fast growing. Sexual dimorphism is common, the large "macroconchs" (presumed to be females) many times larger than the small (male?) "microconchs" in many species. On the other hand, no soft body parts of any ammonite are known, and the biology of ammonites is largely based on guesswork and deduction. 
The suture line of Rossalites superbum is clearly visible in the fragment above. It is this zig zag pattern which characterizes the ammonites on the whole, and each species has its own pattern. Like fingerprints, ammonite experts use this suture line pattern to identify species of ammonites, especially where only fragments are available. Ammonites were named after Amun (or Ammon), an ancient Egyptian deity equated with the Greek Zeus or Roman Jupiter. One of his characteristics were ram's horns behind each ear. Most ammonites are more or less spiral, and some indeed resemble ram's horns. Many ammonite names end in the suffix-ceras, which means "horn"; for example Sciponoceras ("walking-stick horn") and Worthoceras ("horn from Fort Worth"). 4. How did the ammonites go extinct? Ammonite extinction is a hotly debated topic. The basic fact is that no ammonites are known beyond the end of the Cretaceous, while the other cephalopod groups, the coleoids and nautiloids, survive to the present day. This has been tied with other extinctions which occurred at the end of the Cretaceous (known as the Cretaceous-Tertiary, or K/T, boundary). The best known victims were the dinosaurs, but other groups vanished too, including many forms of plankton, pterodactyls, mosasaurs, rudist and inoceramid bivalves. The ammonites were not the only cephalopod victims, though; the belemnites became extinct as well. Meteoritic impact is currently the theory favoured by many geologists for explaining all these extinctions. A buried crater in Yucatan, Mexico appears to have been formed at the right time, and such an impact, the proponents say, would have had a profound affect on the world's climate. One effect would be cutting off sunlight for months, maybe years, killing off many of the planktonic plants upon which the economy of the sea is based. Ammonites and belemnites are guessed to have had small, planktonic babies which depended upon these planktonic plants, or the zooplankton they supported, for food. Possibly the nautiluses, with their large, opportunistic or scavenging babies, were better able to find food during this plankton famine. How the squids and octopuses, which lived in the Mesozoic seas along with the ammonites, belemnites and nautiloids, survived is unclear. Modern squid and octopus are generally pass through a planktonic stage, when they feed on zooplankton. The main problem with ammonites and belemnites as victims of the K/Tmass extinction is that only a few examples of either were around at that time. Both groups had been in decline for the 30 million years preceding the K/T boundary. By the time the meteoritic impact occurred, there were very few of either left. So far as we can say at this stage with confidence, ammonites and belemnites were going extinct anyway; the meteor impact may have been the coup-de-grace that dispatched the last survivors. 5. What were belemnites like and what was the guard for?  In many ways belemnites were very like squids, as shown in this reconstruction of the Jurassic belemnite Cylindroteuthis puzosiana in the sketch to the right. They were streamlined, torpedo-shaped animals with an internal shell. Like squid they had a ring of short, muscular arms equipped with sharp hooks for catching prey (they don't seem to have had the pair of long tentacles, though). Belemnites also had an ink sac. However, belemnites had a very different kind of shell. Where the squid shell is a long, thin strut, the shell of a belemnite was robust and divided into chambers. The apical end is surrounded by a bullet-shaped solid guard, which acts as a counterweight to the head and arms. This allows the belemnite to maintain the right orientation.
There are some other possible functions for the guard. Many palaeontologists believe that belemnites had muscular fins attached to the guard, as shown in the picture here. The guard may also have had a defensive function, or perhaps was some sort of "wave cutter" when the belemnite was swimming close to surface. Some writers have questioned whether the density of the guard is an artefact of preservation, and that it was originally less dense, perhaps organic in composition and only mineralized after death. Belemnites are very common in Jurassic and Cretaceous sediments. Though the phragmocone is rather delicate and often crushed, the guard is very robust. It is often reworked into younger sediments, and can be transported many miles by rivers, glaciers, etc. Belemnite guards have some use in biostratigraphy. 6. What did fossil cephalopods eat, and what ate them? All living cephalopods are predators, and fossil belemnites appear to have been predatory as well. Fossilized gut contents of ammonites are quite well known, and are good evidence for this, and include mostly small invertebrate fragments. Crustaceans such as shrimps and ostracods, pieces of echinoderms, and foramaniferans suggest that ammonites tended to eat small or inactive animals, perhaps scooping in mouthfuls of sediment and sorting the food from the sand in the buccal cavity. Open water, pelagic ammonites may have eaten small zooplankton. The hooks on the arms of belemnites and the various fossil squids indicate a more active predatory lifestyle, probably based on catching smaller fish and other cephalopods. As in modern seas, fossil cephalopods were important parts of the marine food web. Ammonites in particular are frequently found with damaged shells, showing that they were frequently attacked. Marine reptiles, large fishes and other cephalopods were probably the main predators. Tooth marks on shells of Placenticeras have been identified with belonging to a giant marine mosasaur lizard. Other ammonites have elongate scars. Similar damage can be seen in snails today following attack by crustaceans such as spiny lobsters or crabs. Belemnite guards and squid hooks have been found in huge numbers in the guts of ichthyosaurs and sharks. 7. Did fossil cephalopods reproduce in the same way as modern squids and octopuses? This is unclear. Cephalopods today are divided into the nautiluses, which reproduces many times, and the coleoids which reproduce only once (usually). It is generally assumed that the fossil nautiloids were very much like the modern species, with a long lifespan and numerous reproductive periods, but the evidence is lacking. Hatching size of the Mesozoic and Tertiary nautiloids is broadly similar to the living species. Ammonites seem to be different, with a hatching size much more like that of the coleoids. Sexual dimorphism is pronounced. Some writers have suggested that ammonites grew quite quickly—perhaps in as little as a year for the small species—and died after spawning. In some ammonites the shell eventually coils back on itself, almost sealing off the aperture. It has been suggested that like the modern octopus, the female ammonite brooded the eggs within the security of the shell, but didn't feed, and eventually died after the hatchlings swam away. On the other hand, some estimates of ammonite growth rates produce quite different results. Epiobionts, like serpulid worms, encrusting the shells of ammonites have been used to estimate lifespans of over 12 years. It is difficult to imagine animals taking 12 years to reach maturity and then reproducing only once (but some animals do, such as cicadas). 8. Is Spirula an ammonite? No. Spirula spirula is a coleoid, and probably related most closelyto the cuttlefish (though that is not certain). Fossil spirulids are known from the Pleistocene of the Canary Islands, the Pliocene of New Zealand, and the Miocene of Japan. The probable ancestors of Spirula are known as far back as the Eocene, which begin coiled like Spirula but straighten out later. A key difference between Spirula and ammonites is the way they coil. In Spirula the coiling is endogastric—i.e., under the animal. Ammonites, in contrast, have shells which are exogastrically coiled—i.e., the shell coils over the back of the animal. Also see Ammonoidea and Neal's Ammonite Gallery. There is a nice collection of online images of ammonites on the Ammonites at the Black Hills Institute web page.
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