Lecture 6 Cephalopods

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Lecture 6 Cephalopods
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Classification Phylum Mollusca Class
Bivalvia (Lamellibranchia Pelecypoda) (Clams,
eg cockles, mussels etc) Ordovician-Recent) Cl
ass Gastropoda (Snails, slugs) (Cambrian-Recent
) Class Cephalopoda (Nautiloids, ammonites,
belemnites, octopus, squid etc) (U.
Cambrian-Recent) Class Scaphopoda (Tusk
shells) (Ordovician-Recent)
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Cephalopods belong to the phylum Mollusca, and
are related to gastropods and bivalves. They are
entirely marine and possibly the most advanced
group within the invertebrates. The vast majority
are highly mobile carnivores and actively stalk
and capture their prey. They have well developed
nervous systems, reasonably complex brains,
prominent eyes and excellent vision. Several,
such as octopus and cuttlefish can very rapidly
change colour to suit their surroundings (or even
their mood). All of these adaptations ideally
suit their predatorial lifestyles. Cephalopods
may secrete an external calcareous shell which is
commonly fossilised (e.g. ammonites), they may
possess a reduced internalised skeletal structure
(e.g. squid and cuttlefish) or they may have lost
the shell altogether (e.g. octopus) - making them
much rarer in the fossil record. Shelled
cephalopods are not very common today, however
they were abundant in the Palaeozoic and Mesozoic
when they were very important in the marine realm
as carnivores. In fact before the arrival of
jawed fish in the Silurian and, in particular,
the appearance of sharks in the late Devonian
they were the top predators in the oceans.
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Internally shelled Cephalopods
Fins are used as stabilisers during swimming and
to power low velocity movement. Squid are the
best free swimming invertebrates in the ocean.
fin
(a)
This is an internal support structure and in
squid is composed of horny material. Cuttlefish
posses a calcareous internal support (often
referred to as cuttlebone)
pen
ink gland
gill
tentacles
hyponome
eye
Cephalopods have remarkably well developed eyes
and Octopus and squid have the most advanced eyes
in the invertebrate world they possess lenses
and can see in colour.
(a) Internal anatomy of the modern squid Loligo.
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(b)
(b) Loligo pedlei in life position.
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(c) Median longitudinal section of a present day
Nautilus highlighting the soft part internal
morphology. (d) Nautilus in life position
Externally shelled Cephalopods
The Pearly Nautilus, which today inhabits the
Indo-West Pacific, is the only known modern
example of an externally shelled cephalopod. It
is often described as a living fossil and
knowledge of its biology is invaluable when
considering the mode of life of fossil forms
The hood is pulled down to cover the aperture
(opening to the shell) and protect the nautilus.
This feature is only found in modern nautiloids
and it is debatable whether fossil ammonites
possessed a similar protective cover.
The tentacles seize prey and pull it towards the
mouth where the parrot-like jaws shred it alive.
The radula, positioned behind the jaws further
processes the prey.
hood
(c)
siphuncle
(d)
radula
stomach
jaws
tentacles
ovary
eye
heart
crop
Hyponome
The siphuncle is a tube which connects to the
older gas and fluid filled chambers (camerae).
This allows the Nautilus to regulate fluid levels
in the camerae, which affects the buoyancy of the
shell. It has been found to be quite a slow
process.
pharynx
The brains of cephalopods are amongst the most
advanced of all invertebrates. They have the
capacity for problem solving. Octopus have been
known learn how to unscrew glass jars to get at
food inside!
Nautilus eyes are actually quite simple in their
construction a cup shaped cavity without a lens
(like a pinhole camera). It appears that they
rely more on smell and touch for hunting.
gills
cerebral ganglion (nerve centre)
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(e) Cross section through an empty Nautilus shell.
DORSUM
Septal neck (siphuncle passes through)
Chamber
septa
ANTERIOR
POSTERIOR
(e)
The shell serves two very important functions 1)
Protection from attack and support for soft
tissues 2) The chambered construction of the
shell traps gas and fluid which are used to
enable the creature to remain buoyant and float
(like a submarine)
body or living chamber
The last formed chamber houses the animal and is
the largest.
VENTRUM
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Note the phragmocone (the embryonic chamber) and
the concave septa of this fossil nautiloid
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Cephalopods - Form Function
Growth
Orthoceras sp.
Vector of growth
Cephalopods with straight or coiled external
shells grow incrementally. They produce a new
septum which seals off a new fluid and gas filled
chamber immediately behind the body chamber.
Vector of growth
Locomotion
Movement is achieved through a form of jet
propulsion. Water is drawn into the mantle cavity
via an inhalent slit. It passes over the gills
(facilitating respiration) and is then directed
out of the animal at pressure through the
hyponome. The end of this tubelike structure is
flexible and can be pointed many directions to
steer the creature.
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Buoyancy Orientation
Suggested positions for various ammonoid genera
when at rest (see Nautilus pompilius below left
for explanation of symbols)
Two factors are important when considering the
buoyancy of cephalopods with chambered shells
2) This is counterbalanced by the positively
buoyant gas filled chambers - often with an
internal gas pressure of less than one atmosphere
- which cause the animal to float upwards.
Normannites sp.
Dactylioceras commune
Promicroceras marstonense
Nautilus pompilius
Caloceras sp.
1) The negatively buoyant (i.e. heavier than sea
water) effect of the calcareous shell and the
weight of the fleshy parts
Sigaloceras micans
Ludwigia sp.
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Attitude in life of Nautilus pompilius
It is generally believed that in life cephalopods
with chambered shells orientated themselves with
the centre of gravity positioned directly beneath
the centre of buoyancy.
Gas filled chambers
Centre of buoyancy (X)
Centre of gravity (?)
Some cephalopods, in particular forms with
straight shells, laid down calcareous (cameral)
deposits inside the chambers of their shells to
counterbalance the shell (like a see-saw)
X
?
Living chamber (occupied in life by
animal)
cameral deposits (black)
?
X
gas filled chambers (white)
Orthoceras sp.
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Terminology often used to describe the shape of
cephalopod shells
Cephalopod - Shell Morphology

• Straight or slightly curved forms

• Several physical characteristics are important
  when trying to classify fossil shelled
  cephalopods. These include
• Overall shape of shell - straight, tightly
  coiled etc.
• Shape of septa
• External ornamentation

Breviconic
Orthoconic
Cyrtoconic

• Helically coiled forms

• Loose or open coiled forms

• Terminology applied to cross sectional profile
  of shells

Gyrocone
Lituicone
Torticone

• Tightly coiled planispiral forms

Evolute
Convolute
Involute
Oxycone
Cadicone
Sphaerocone
Platycone
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Tightly coiled involute shell (left) and slightly
curved cyrtoconic shell (right)
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Suture lines are often visible on the surface of
fossil cephalopod shells. They mark the position
where the internal chamber dividing walls (septa)
join to the inner side of the external shell and
may form intricate patterns
Sutures
Lioceras concavum
When sutures are visible it is important to
realise that the external shell has not been
preserved or has been removed and that you are
looking at an internal mould.
External shell partially removed
saddles
Recording the Suture line
Finished product
Ventral margin
Dorsal margin
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2
lobes
2
3
1
1
Begin along the edge of the shell. The arrow
marks the plane of symmetry and points in the
direction of growth
ventral lobe
Continue around towards the inner edge of the
whorl
Arietites bisulcatus
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Cephalopod - Classification
There are three important subclasses or groupings
of cephalopods to remember
Coleoidea

• Range ?Carb-Recent
• Shell is internalised may be straight or
  coiled
• Siphuncle is lacking (except in the belemnites -
  opposite)
• Some forms have lost the internal shell
  altogether
• Two gills present

Belemnites
ventral
lateral
dorsal

• Extinct group within the coleoids
• Good fossil record
• Calcareous bullet shaped structure which was
  internal support for a squid like creature
• Range Jur-Cret.

Group includes modern octopus, squid and
cuttlefish (above), which generally have a poor
fossil record.
Acrocoelites sp.

• Range U. Cam-Recent
• External shell which may be straight, curved or
  coiled
• Chambered shell construction
• Simple sutures
• Siphuncle present
• Four gills present in recent forms

Nautiloidea
Vestinautilus cariniferus (Carboniferous)
Nautilus (Recent)
Orthoceras (Carboniferous)
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Ammonoidea

• Range L. Dev.- Upper Cret. (EXTINCT)

Placenticeras sp. (Cretaceous) reconstruction
Goniatites crenistria (Carboniferous)

• External chambered shell which is generally
  coiled
• Outer surface often ribbed with complex sutures
• Siphuncle present
• Gill number unknown
• Wiped out at K/T boundary

Binatisphinctes comptoni (Jurassic)
Nautiloid
Morphological comparison between Nautiloids
Ammonoids
Ammonoid
Simple sutures
Complex sutures
Thick siphuncle in centre of whorl
Thin siphuncle on outer region of whorl
Septa concave towards body chamber
Body chamber
Body chamber
Shell wall generally thicker
Septa convex towards body chamber
Shell wall generally quite thin
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Further subdivision of the ammonoids is based on
the type of suture. There are 3 different types,
increasing in complexity

• 1. Goniatitic suture (as in the goniatites)

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2. Ceratitic suture (as in the ceratites)
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3. Ammonitic suture (as in the true ammonites)
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