Nautiloids: The First Cephalopods

Introduction

Imagine yourself standing on a bleak windswept Ordovician shore. It is 470 million years ago and you are standing on a rocky coastline staring out to sea. As you turn and pan the landscape behind you, all you can see are barren rocks, with no trees, plants or any form of animal life. You feel the salty wind on your face and the near total silence is interrupted only by the soft sound of the lapping tide at your feet. You feel something tickling your toes, look down, and a trilobite a couple of inches long scuttles across your foot. You pick it up, see its legs pulsate rhythmically and its feeble antennae wave. You place it down in the rock pool where it quickly scurries away under a crevice. You see another shell of a trilobite in the water, but this looks different, it has been torn to shreds with puncture marks and you wonder what creature could have possibly caused this. As you stare out to sea something disturbs the breakers and the tip of a huge horn, metres in length, mottled a dull red and cream and gleaming in the sunlight briefly breaks the surface before sinking from view.

This is the Ordovician period, when gigantic cephalopods ruled the seas. The world span faster on its axis than it does today, a day would have lasted just twenty-one hours, and no fewer than 417 such days such days were crammed into a single Ordovician year. The Moon would have appeared much larger than it does now causing vast tidal ranges, much more extreme than the present day. The air would have been harder to breathe, with 15% oxygen and higher levels of carbon dioxide. The continents were unrecognisable to our modern eyes and were mostly grouped in the southern latitudes.

Origins

Life is estimated to have been present on earth from a date of around 3000 million years ago, but until a date of approximately 1000 million it had existed in simple prokaryotic cell forms. The first eukaryotic cells containing a nucleus and other structures appeared around this date and slowly began to form colonies and live cooperatively. It is roughly at a date of 600 million that we start to find evidence of complex organised life though the fauna would appear very strange to our eyes; strange quilted animals that are hard to interpret, jellyfish, forms of sea pen and peculiar discoid creatures with a three-fold symmetry. Animal life that we would recognise, primitive molluscs, arthropods and chordates are known from deposits from China at a date of 540 million. These early molluscs were an insignificant group of animals in the Cambrian period, largely immobile and overshadowed by the much more spectacular arthropods. At the very end of the Cambrian an environmental event happened that had opened up the world to allow the molluscs to ascend to take the place of the arthropods as top predator.

At a date of approximately 510 million years at the end of the Cambrian, the world entered an ice age. Glacial sheets spread out from the poles and spread out from the continents, trapping seawater and causing the sea level to drop. The Cambrian animals, trapped on the warm continental shelves, were decimated as their habitats became colder, darker and much reduced in size. When the ice sheets finally retreated vast amounts of freshwater would have been deposited into the saline oceans and which would have acted as poison to many marine communities. The predatory arthropods such as the metre long anomalocarids were much reduced in number and many forms became extinct. Some of the trilobites made it through the extinction, but the position of top predator was now vacant, and the small-shelled molluscs now seized their chance to assume the role.

The first cephalopods had appeared in the Late Cambrian, about 515 million years ago. These were tiny creatures a few millimetres long with crude chambered shells but with a diagnostic siphuncle, or tube that connects the chambers of the shell already in place. It has been theorised that these small animals, such as Plectronoceras, used their chambers to drift along like a jellyfish seizing plankton with their arms, or were snail-like creatures crawling along the sea-bed using their arms to search of food. In just fifty or so million years these tiny insignificant animals would have evolved into eleven metre long giants such as the Cameroceras we have just witnessed breaking the surface of the waters.

Form

The most primitive cephalopods are the nautiloids characterized by their external shells with relatively simple septal walls. Nautiloids show all the basic structures that went on to be adapted transformed and, in some cases, lost by their descendants. Traces of nautiloid ancestry can be demonstrated in the later ammonites, belemnites, squid, and in their most derived form, the modern octopus. There are somewhere around 11,000 names/taxa of fossil nautiloid, though how many of these are valid is unknown.

The shell of the nautiloid or phragmocone, was divided into many chambers that would have contained gases provided by a connecting tube known as a siphuncle. The siphuncle works by drawing water out of the saturated chambers of the nautilus into the bloodstream and gas seeps out of the blood to fill the empty space in the chamber. When the nautilus completes a new chamber it is initially filled with water. The nautilus will then increase the salt content and acidity in its bloodstream flowing through the siphuncle. The increase in salt levels sets up an osmotic gradient causing the water in the chamber to move to the more concentrated blood. As the water levels drop in the chamber and the pressure reduces, gases are diffused from the siphuncle to fill the space creating buoyancy. Contrary to the widely held misconception that nautiloids (and other shelled cephalopods) work in the same way as submarines, there is no actual pumping action involved in this process at all.

In many species of nautiloid the siphuncle was quite wide in diameter compared to the later ammonoids and tended to run through the centre of the shell. These chambers were separated and sealed from each other by structure known as a septal wall, these divisions produce characteristic suture lines on the surface of the shell. As the animal grew more and more chambers were added; with the increase in size of the head, the animal took on an ever-expanding cone-shape in some of early forms. Some of the orthoconic nautiloids filled the chambers at the apex of the shell with calcite instead of gas, which would have acted as a counterweight to balance the shell. The animal itself inhabited the living chamber that could account for up to a third of the length of the animal.

It is believed that these animals had a whole series of grasping arms and a beak, and although no soft-bodied fossils have been found, guesses can still be made about their soft-bodied anatomy. Almost all fossil nautiloids have a notch cut into the underside of the aperture of the shell known as the hyponomic sinus. This is the hole through which the hyponome projects, this being the directional organ through which water is squeezed after it is drawn into the mantle. This demonstrates that the nautiloids had evolved a form of jet propulsion as early as the Ordovician, a feature that has been retained in all living cephalopods. The hyponome in the modern Nautilus is a much simpler structure than the coleoids and comprises of a rolled fleshy tube formed from two separable flaps of tissue, rather than a single tube. One can imagine the smaller nautiloids as being quite fast and efficient swimmers when swimming backwards at least, with the pointed end of the shell cutting through the water, but the larger species would have been much more cumbersome, the sheer size of the shell making steering difficult. As with modern nautiluses swimming forwards would have been accomplished by bending the siphon backwards under the shell through the hyponomic sinus. While an adequate solution, bending the "exhaust" of a jet like this reduces its power and efficiency greatly. Like the living nautiluses, the fossil species probably had two speeds: slow forwards swimming when looking for food, and a much faster escape speed backwards when threatened.

The form of the shell varied greatly between the nautiloid orders and by no means all were cone-shaped. Some were slightly curved, known as cyrtocone and the straight shells were known as orthocone. Other forms were loosely coiled, such as the Early Ordovician form Estonioceras. The orthocones are subdivided into two main forms known as longicone with very long tapering shells, and others are known as brevicone, which tend to have rather short and swollen shells. Still others, such as the Early Ordovician Lituites grew in a tightly coiled shape up before suddenly developing a long orthocone, these animals resembling a traditional shepherd's crook in form. The highly coiled genus Nautilus is a distant relative of the last nautiloid order to evolve, which was sometime in the Early Devonian, about 400 million years ago though some forms of nautiloid evolved a similar form of tightly coiled shell during the preceding Silurian period. It is worth remembering that there was no clear transition from straight to curved shells as many of these forms co-existed with each other, with some straight-shelled forms lasting well into the Triassic (Michelinoceras spp.).

There has been much controversy about the classification of nautiloids and many researchers have come to differing conclusions as to exactly how many orders and suborders there were of these animals. Even after more than a century of study there are several debated orders of taxonomic hierarchy and there is no single system that has gained widespread acceptance. It is however generally accepted that in the late Cambrian there was just one order of nautiloid, the Ellesmeroceratina that acted as a stem group for all the later orders. During the Early Ordovician the group radiated enormously and no less than five other orders appeared (some researchers believe more).

The most important group in terms of diversity, longevity and sheer numbers of these was the Orthoceratatida, (Early Ordovician-Late Triassic). These orthoconic nautiloids tend to have straight or very weakly curved shells and a graceful appearance. They are believed to be the ancestors of the Bactritida, from which the Ammonoidea and Coleoidea arose. The largest nautiloids such as the Early Ordovician Endoceras and the Middle Ordovician Cameroceras are believed to have been part of another order, the Endoceratida (Early Ordovician-Late Silurian). These are distinguished by the presence of a particularly wide siphuncle and endocones, or calcareous hollow conical deposits around the open mouth of the shell linked by a tube and were probably used to help counterbalance the animal. Imagine a stack of tapering polystyrene cups stacked together with the rims projecting outwards and one can begin to visualise the structure of these animals. No endocerid has calcite deposits in its shell so this must have been a separate approach to the problem of buoyancy and weight distribution. It is important to remember that these giant orthoconic nautiloids, such as Cameroceras, are exceptionally large and are rare in the fossil record. The overwhelming majority of nautiloids are much smaller, usually less than a meter long fully grown.

There is some evidence to suggest sexual dimorphism amongst some orthoconic nautiloids. A 325 million year long specimen of Rayonnoceras solidiforme measuring eight foot long was discovered at Fayetteville in Arkansas in January 2003. The largest example of this actinoceratoid nautiloid ever found by a foot, it was found to have an aperture a third smaller than the second largest, a seven-foot specimen. This may indicate that the males and females had differing sizes of apertures, as with the modern Nautilus. These specimens were also interesting, as it has been speculated by Walter Manger that these two examples were 'pathological giants'. Riddled with trematode parasites and rendered sterile, he believes that the energy that could have been diverted to reproduction was devoted to growth instead.

It is probable that there were three stages in the evolution of nautiloid buoyancy and orientation. In their most primitive forms nautiloids were small and because the body was heavier than the shell floated with the head pointing downwards. Some palaeontologists believe that they were planktonic organisms living a jellyfish-life existence; others envision them as buoyant snails, making short jet-propelled hops across the seabed in search of prey or safety. Either way, the later forms were much larger, and these seem to have been benthic predators, pulling their shells behind them as they crawled along the seafloor. Though the shell was somewhat buoyant still, it wasn't quite buoyant enough to lift the animal off the seafloor, and if these nautiloids could swim it was lethargically and only intermittently. Most of the later nautiloids added counterweights to their shells which meant that they could be both buoyant and oriented horizontally allowing them to swim in midwater effectively. The more streamlined forms may well have been pelagic, spending their entire lives well away from the seafloor feeding on plankton, swimming crustaceans and trilobites, and perhaps even some of our distant relatives among the early chordates such as

the conodont animals. Other forms had hook or horn shaped shells seemingly much less well suited to active swimming, and were probably nektobenthic, cruising over the sea bed picking off the benthic invertebrates they ate, and if they were anything like the modern nautiluses, feeding on carrion as well.

The proximity of the Moon has been argued to exert a direct effect on nautiloid development. Microscopic ribs, or laminations are secreted on a daily basis in relation to the lunar-tide cycle, forming a logarithmic spiral. The modern Nautilus has 29 or 30 of these growth laminations per chamber, indicative of the length of the current lunar-tidal month. Progressively further back in time the number of laminations decreases at a linear rate, thus the nautiloids in the Ordovician have just eight or nine laminations per chamber. This has been used as evidence that the Ordovician lunar month was eight or nine days long, though the theory does assume that these ancient nautiloids had the same growth rate as modern species.

Later History

The first major extinction of nautiloids occurred at the end of the Ordovician Period when there was another Ice Age, but much greater in size and effect than that that of the Cambrian-Ordovician transition. It is estimated this accounted for the loss of 25% of all animal species in existence at that time including the earliest nautiloid order, the Ellesmeroceratina. The Endoceratida, which contained the giants Endoceras and Cameroceras failed to survive the Silurian. The other nautiloid orders such as the, Tarphyceratida, Discosorida, Oncoceratida and Orthoceratida survived into the Devonian though as a group the nautiloids began to slowly decrease in size and number of species.

The reason for this is almost certainly interlinked with the development of bony fish and latterly the sharks in the Devonian. Whereas the nautiloids of the Ordovician were sluggish creatures and almost certainly unwieldy swimmers, this was not to their detriment as their prey animals were mainly bottom crawling arthropods, speed and dexterity would have not been a necessity. However, with the rise of the nimble jawed bony fish in the late Ordovician and Early Silurian, arthrodires such as Dunkleosteus in the Devonian and primitive hagfish and lampreys in the Late Carboniferous, all would have put environmental pressure on the nautiloids in a competition of resources. There is a clear decline in nautiloid numbers and diversity from the Silurian onwards with the final archaic orthoconic nautiloids becoming extinct at the end of the Triassic, roughly about 208 million years ago. Another factor worth considering is the rise of the ammonoids in the late Devonian, which evolved from an offshoot of the nautiloids known as the Bactritida in the early Devonian. The ammonoid goniatites were very successful cephalopods in the Carboniferous and Permian and were succeeded by the more complex ceratites in the Triassic. These other groups of cephalopods would have further increased pressure on the nautiloids. Obviously this is not the end of the story as we have the five species of modern Nautilus as testament.

Certainly the shell of some of the nautiloids became thicker implying they could reach increasingly deep-water habitats. In 1985 Gerd Westermann published a paper in which he examined various nautiloid shell shapes and has calculated the depths they could have reached before imploding. Late Silurian brevicones have been shown to be able to resist depths no greater than 150m and must have lived close to the surface. Some longicones with their thin, closely spaced septa, lived at a depth of no more than 300m and other longicones with thick, widely spaced septa have been demonstrated to have withstood depths of 1100m. The Carboniferous nautiloid Michelinoceras had a shell that could resist a depth of 1125m, making its maximum depth twice as deep as the modern Nautilus. Whether or not Michelinoceras actually frequented these depths is a controversial theory; food at these depths may not have been plentiful given the anoxic environment and may not have been enough to support populations of such nautiloids.

Nautilus

Perhaps it is due to the increased environmental pressure from other groups of vertebrates and cephalopods that forced the nautiloids to adopt increasingly deeper waters. Although the last orthoconic nautiloids became extinct 208 million years ago, one order, the Nautilida, continued to thrive and modern descendants can be found at depths of up to 600 meters in coral reefs across the Indian Ocean to Australia and from Samoa to the Philippine Islands. The animal never moves far from the reef face and undertakes a vertical migration at night to a depth of 100 meters or so to feed off a concoction of crab and lobster exoskeletons, crustaceans, nematodes, small fish or echinoderms (and occasionally even each other). Perhaps it was this opportunistic method of feeding that allowed the Nautilus to cling on, the creature avoided specialised food and became a scavenger, whilst retreating to a singular environment where food is abundant.

The modern Nautilus is a descendant of the most recent group of nautiloids to arise, the Nautilida, which is still an incredibly ancient group. The first Nautilida are known from the early Devonian, approximately 400 million years ago. Nonetheless, it would be a mistake to assume that Nautilus is a 'living fossil' and identical in most respects to its earliest ancestors. For example, the beak of the middle Triassic Germanonautilus has recently been reconstructed and has been shown to be significantly larger than the modern Nautilus. Nonetheless, Nautilus is certainly indicative of cephalopods at their most primitive state of development with approximately 90 tentacles, lens less pinhole type eyes that are open to the water, two pairs of gills, a chambered gas-filled shell, no suckers, no ink sac and the large external shell.

From the Devonian period onwards, the nautiloids, essentially a catch-all group for all the disparate primitive cephalopod lineages, began to be slowly replaced by newer more advanced forms of cephalopod. One can speculate as to how the nautiloids prior to the Devonian had more primitive features than Nautilus, after all, Nautilus has evolved from and is not the same animal as the earliest Nautilida in its order. However, the picture is not straightforward. It is generally accepted that the more advanced belemnites, squid and other coleoids stemmed from the Bactritida in the Early Carboniferous, about 360million years ago which itself as a group had only split from the nautiloids about 400 million years ago. The Bactritida are also believed to be the ancestor of the ammonoids. As the ammonoids are believed to have had radulas reflecting a closer affinity to the coleoids than the nautiloids, but probably not advanced eyes, hoods or ink sacs, it is likely that the Bactritida, bar the radula, contained few advanced features. The coleoid branch that led to the belemnites probably stemmed from the Aulacocerida, which contained a mixture of orthocone and belemnite anatomy but without guards.

Within those forty million years advanced coleoid features, including hooks, suckers, internal shells and ink sacs evolved. Perhaps the evolution of a short lifespan in early coleoids from the very slow reproduction rate with nautiloids (the modern Nautilus achieves sexual maturity at 15 years, yet most modern squid live one to two years) enabled these features to develop over rapidly succeeding generations. Perhaps then one can surmise that the ancient nautiloids were a disparate group by the Devonian with varying soft-bodied morphologies. One branch with ten arms may have led to the Bactritida and then onto the coleoids and ammonoids, and other forms may have led to Nautilus and the Nautilida. Until numbers of soft-bodied nautiloid fossils are found, one can only speculate.

The Nautilida was a very important group in the Early Carboniferous with about 75 genera in 16 families. After that date there was a gradual decline in diversity on both levels, with the group reduced to 35 genera in 8 families in the early Triassic. By the late Triassic, only one genus, Cenoceras, survived. There then followed a recovery that lasted to the Cretaceous, the group being widespread with 24 genera, followed by another decline before the end Cretaceous extinction. Following the Cretaceous extinction the Nautilida began to diversify again, perhaps filling some of the ecological niches vacated by the ammonites. It is interesting to note that it is estimated that whilst three families and five genera crossed this extinction transition 65 million years ago, the ammonites were completely wiped out following a lengthy decline in their diversity and numbers. The earliest known examples of Nautilus itself date from the Oligocene, roughly 35 million years ago. Prior to that, fossils of the Nautilida such as Aturia (Paleocene-Miocene) show slight morphological differences, notably in the flattening of the shell, length of the septal necks and positioning of the siphuncle. There are currently five species of Nautilus divided into two genera, Nautilus and Allonautilus.

Ammonites

Due to their outward resemblance to nautiloids, and especially Nautilus, and similar morphology it is worth mentioning a few points about ammonites. Ammonites worked on a similar system utilising gas filled chambers in a (normally) coiled shell to create neutral buoyancy, connected by a siphuncle and with the body of the animal inhabiting the living chamber. The shell of the ammonite is generally considered to be thinner than the nautiloid, but the strength is considered to have been maintained by the much more complex septal walls and not the thickness of the shell itself. There were notable differences between the two groups and is worth mentioning a few of the most important ones:

The suture line is generally much more complex in the ammonoids, except with the some of the goniatites which closely resemble their nautiloid ancestors. The siphuncle tends to be centrally placed with most nautiloids and is often wider than the ammonoids, and with the ammonoids the siphuncle is usually located near the outer margin of the shell, the venter. The overall shape of the septa is different, with ammonoids the septal necks are backward pointing, whereas with nautiloids they are reversed and are forward pointing. Another important difference is in decoration and ornamentation, whereas most coiled nautiloids have featureless or unornamented shells, some ammonites such as Collignoceras, Kosmoceras and Hoplites are covered in ribs, keels and spine bases. Lappets, projections from the aperture of the shell are never present with nautiloids, and the notch that would have held the hyponome, the hyponomic sinus, is not present in the ammonites. Other differences include the shape of the radula and the number of teeth per row is fewer in ammonites than in the nautiloids, 9 rather than 13, the ammonite radula shows a closer affinity with most octopuses which have a similar number and in some cases a very similar radual morphology.

Hatchling nautiluses are practically miniature versions of the adult animal, about 25-30 mm and are able to hunt and scavenge immediately. It seems highly likely that ammonite hatchlings may have been planktonic in size, probably around 10mm and probably grew from a paperclip shape quite rapidly, demonstrating a rapid change in shell shape to a coiled form. This was probably the key to survival of the nautiloids over the ammonites; when the asteroid impacted 65 million years ago, the ammonite hatchlings as part of the zooplankton would have suffered following the blanketing of the sky and reduced sunlight, whereas the nautiloid hatchlings would have been able to continue to feed on biological matter filtering down from the upper levels. In addition, living at a deeper water level than the ammonite, the nautilus may have been afforded a degree of protection from acid rain, doubtless a consequence of the vaporisation of millions of tons of limestone into the atmosphere following the impact.

It is important to remember that the ammonites were a group in severe decline by the time of the Cretaceous extinction. At the beginning of the Cretaceous they were represented by 20 families, by mid Cretaceous about ten, and by the end only three or four. One theory argues that oxygen levels were much lower in the Jurassic and Cretaceous as the polar caps were much smaller than today, the oceans being less oxygenated by melting glaciers and icebergs. Ammonites could have populated the vast areas of the oceans higher in the water column at maximum depths of no greater than 250 metres but below the higher oxygenated top 50 meters. There they would have had little competition from fish or belemnites, or marine reptiles that would have populated the upper levels of the ocean. Perhaps as the climate slowly cooled in the Cretaceous and the polar caps began to increase, oxygenating the oceans with melt water the available habitat for the ammonites began to slowly decrease in size. Unable to adapt to deeper waters due to implosion, and unable to rise to the surface due to high oxygen levels, the ammonites were slowly squeezed into an environmental trap.

As previously mentioned Nautilus undertakes a vertical migration along reef walls at night from depths of around 600 meters maximum to 100 meters or so in search of food. It seems likely that many of the ammonite forms would have done the same in the water column, following the plankton. Perhaps it was this dependence of the ammonite on plankton that eventually led to its extinction for the reasons mentioned above. A lifestyle that was closely locked with plankton would make the ammonite vulnerable to environmental changes within the planktonic system, Nautilus on the other hand, being a scavenger of larger pieces of animal and an active predator, would have a plentiful food source as long as the reef system in which it exits continues to be active. With the demise of so many marine creatures at the end of the Cretaceous, perhaps this would have actually been a boon to the scavenging nautilus?

This leads us on as to the question of why ammonite and nautiloid remains are often found in similar deposits if they had varying lifestyles? Deceased nautiluses do not tend to fill up with water and sink to the sea floor if their soft tissues remain in place. Gases arising from the decomposition of the carcass tend to expel water from the shell and the dead animal tends to rise to the surface, held buoyant by the gases retained in the chambers. After the body decays, the remaining water is expelled from the siphuncle into the living chamber and the shell is free to drift as to wherever the tide takes it. This helps to explain why nautilus fossils are common, if the shells drifted downwards then they would reach implosion depth and shatter and fossils would be rare things indeed. Floatation studies of modern nautilus shells have demonstrated that they can remain afloat for years after death has occurred. This is important as it implies that the location of the deposition of fossilised coiled nautiloids (and probably ammonoids) may be largely due to oceanic currents and not necessarily reflect the location of the population. This would not, of course, apply to mass mortalities.

Diet

It is believed that Nautilus hunts by using smell rather than using its poor vision and uses receptors that are located below each eye known as rhinopores, these are capable of detecting odours up to a distance of thirty feet or so. The tentacles also contain chemoreceptors, which allow them to detect prey in close proximity. The Nautilus samples the current sweeping across the reef face for chemical trails, then homes in and seizes small crustaceans using its sticky tentacles. As soon as it seizes prey items, it tends to swim downwards, presumably to reduce the chance of having its food stolen. It is thought that the calcium provided by arthropod exoskeletons is converted and used by Nautilus to create new shell material. It seems likely that ancient nautiloids would have operated on a similar system.

During the Ordovician period (443-510mya) the trilobites were by far the most prominent of the ancient marine arthropods. This period of maximum diversity and population sizes of the nautiloids and the trilobites appears to coincide and it seems highly improbable that this can be attributed to coincidence alone. Some orders of trilobites developed increasingly complex patterns of spines from just about any part of their exoskeletons and it seems certainly possible that these spines could have been used defensively, notable examples here are the Ordovician trilobite Cryptolithus which had an extremely large spine on each side of cephalon, or head shield, and Selenopeltis bucii, another Ordovician trilobite which had large swept back spines stemming from each body segment or pleuron. Such an animal if rolled up could have presented a difficult prey item for a nautiloid with its exposed soft parts on its head. Other trilobites such as Phacops in the Devonian developed heavier and more robust exoskeletons.

It would, however, be unwise to assume that defensive features adopted by some trilobites were purely a response to nautiloid predation. In some of the swimming forms, spines may have been used for counterbalance, and benthic bottom dwelling forms may have used them as stabilisation structures for crawling on loose substrate. In other words, increased spinosity may have been as a result of selective pressures on trilobites, of which nautiloids may have provided a factor, albeit, a major one. Other animals such as the eurypterids, or sea scorpions, also began to make an appearance in the Early Ordovician, and bony fish began to emerge as an important group and it seems likely that these were also predators of trilobites.

Physical damage on trilobite exoskeletons that can be attributed to nautiloids is rare and hard to prove. As mentioned, eurypterids and fish could cause damage; the problem exists in trying to attribute one particular culprit. Certainly at least one example of Phacops has been found with a damaged body segment which appears to have been damaged from an attack from above, but this is difficult to prove. Damaged trilobite exoskeletons are known to undergo repair often resulting in malformation, this makes wound marks even harder to identify. However, given the abundance of trilobites and nautiloids in the Ordovician, it seems unthinkable that the two groups did not exist in a predator/prey role, and it is quite possible that some of the larger forms of nautiloid could have attacked the smaller marine eurypterids as well. Certainly, modern Nautilus is an opportunistic feeder that will take live crustaceans and crustacean moults alongside whatever else it can find, and all the coleoids are fierce predators on crustaceans, fish and other molluscs; there is no reason to imagine Palaeozoic nautiloids were any different.

Finally...

Although the Nautilus is heavily harvested for the shell trade, the animal appears to be doing rather well and is very widespread around the Pacific. In fact, the animal may even be diversifying; the Papua New Guinean Allonautilus differs in its physiology so much from Nautilus, mainly in the creases and encrusting layer on its shell, size of its gills and variation in reproductive structures that in 1997 it was declared by Ward and Saunders to be a separate genus. Following DNA studies it is believed that Allonautilus scrobiculatus may be a recent offshoot of Nautilus. If so, this would indicate that the Nautilida is undergoing speciation again. If so, the Nautilus in one form or another may be around for a long time yet to come.

References

Clarkson, ENK. 1998. Invertebrate Palaeontology and Evolution (4^th ed). Blackwell.

Ellis, R. 2001. Aquagenesis. Viking.

Gon III, SM. 2003. A Pictorial Guide To The Orders of Trilobites.

Hanlon, RT.; Messenger. JB. 1996. Cephalopod Behaviour. Cambridge.

MacLeod, N. 2003. Paleobase Macrofossils pt.2: Mollusca. Blackwell.

Maisey, JG. 1996. Discovering Fossil Fishes. Henry Holt.

Monks, N; Palmer, P. 2002. Ammonites. The Natural History Museum.

Norman, D. 1994. Prehistoric Life. Boxtree.

Norman, M. 2000. Cephalopods, A World Guide. Conchbooks.

Parker, A. 2003. In the Blink of an Eye. Free Press.

Websites

Theo Engesers' Nautiloid pages:

http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm

General Information on Prehistoric Nautiloids:

source

Press release on Rayonnoceras discovery:

source

Post-mortem transport of the Tertiary Nautilus Aturia:

source

A Broad Brush History of the Cephalopoda by Dr Neale Monks:

http://is.dal.ca/~ceph/TCP/evolution.html

Photo credits

Kevin Bylund for the Rayonnoceras, Bactrites, Mitorthoceras, and Eutrephoceras photos.

Richard Fein for the Nautilus photo from the New York Aquarium. (Thank you, Tani!)

Tom Johnson of www.phacops.com for the Selenopeltis photograph.

Lituites and Smithsonian Article from author's personal collection.

Virtual Silurian Reef (Reef Banner image and Silurian nautiloids: permission granted)

University of Michigan (Dioramas. Public Access from http://seaborg.nmu.edu/earth/)

Acknowledgements

And a big thank you to Neale Monks for his guidance, contributions and input. Also, thank you to Kevin Bylund for providing the spectacular photographs from his personal collection and suggestions.