|
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.
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. 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. 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. 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. Clarkson, ENK. 1998. Invertebrate
Palaeontology and Evolution (4th 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. Theo Engesers' Nautiloid pages: http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm General Information on Prehistoric Nautiloids: Press release on Rayonnoceras
discovery: Post-mortem transport of the Tertiary
Nautilus Aturia: A Broad Brush History of the Cephalopoda by Dr Neale
Monks: http://is.dal.ca/~ceph/TCP/evolution.html 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/) 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. |
Nautiloids: The First Cephalopods
By Phil Eyden
- Published:
- Dec 29, 2013
- Page Views:
- 36,326
