Cephalopods

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Cephalopods
ammonites
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Cephalopds

• Phylum Mollusca
• Class Cephalopoda
• Sub Classes
• Nautiloids (U. Cambrian - Recent)
• Ammonites (L. Devonian - Cretaceous)
• Order Belemnites (L. Carboniferous - Eocene)

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GENERAL COMMENTS ON CEPHALOPODS

• The Cephalopods basically consists of a straight
  or coiled cone.
• Belemnites are straight, ammonites and nautiloids
  are coiled.
• They are pelagic and nektonic using a mechanism
  of jet propulsion to move and trapped gas/air to
  give them buoyancy.
• They are marine and live at varying depths due to
  their swimming and buoyancy changes.

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GENERAL COMMENTS ON CEPHALOPODS

• Therefore they are common in shelf areas but can
  be found on abyssal plains.
• Modern forms include squids, cuttlefish and
  octopuses.
• Nautilus is the only living form with external
  hard parts and soft parts protected inside.
• Fossil cephalopods were able to withdraw their
  soft parts which often contained 8 or 10
  tentacles.

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NAUTILOIDEA

• Usually coiled.
• If the shell is cut in half there are a number of
  separate divisions chambers which have a wall
  dividing the chambers septa.
• These septa vary in shape and are important in
  identifying genera.
• Each septa has a type of perforation roughly in
  the centre siphuncle.

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NAUTILOIDEA

• The shell is usually made of aragonite with some
  calcite and other organic matter.
• If the shell was uncoiled it forms a cone which
  is closed at the narrow end and gradually widened
  as the animal grew.
• The widest end is open aperture.
• A straight shell is called an orthocone.
• Some nautiloids only have a loosely coiled shell,
  others are tight.

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NAUTILOIDEA

• In modern nautilus some of the inner chambers
  contain a mixture of nitrogen and fluids which
  control buoyancy.
• When the animal moves forwards the extended soft
  body will be at the back allowing jet propulsion.

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NAUTILOIDEA

• If an internal mould preserves a nautiloid then
  the shape of the septa will be preserved.
• These lines are called suture lines.
• In coiled shells the suture lines will undulate.
• In modern nautiloids in the final chamber the
  aperture is always very large.
• The aperture is usually rounded or oval in shape.
• This large aperture helps identify nautiloids.

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NAUTILOIDEA

• The siphuncle forms a neck as it goes through the
  septa.
• These are called septal necks.
• In a nautiloid these septal necks point away from
  the aperture and towards the protoconch.
• This is called retrosiphonate.

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AMMONITES

• Now extinct and although there is no fossil
  evidence about the soft parts the similarity with
  the hard parts of Nautilus suggests that
  ammonites had similar soft parts.
• Nautilus does differ slightly in its shell
  morphology and so there might be a slight
  difference in soft parts.

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MORPHOLOGY

• They normally have a Plani (flat)-spiralled shell
  but sometimes it is fully or partly uncurled.
• The original shell is called the protoconch and
  occurs in the centre of the shell and is small.
• The protoconch is not chambered and is cut off
  from the chambered phragmacone by the first
  septum.

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MORPHOLOGY

• As the septa reach the shell wall they get folded
  and contorted.
• Therefore they produce a complex zig zag line
  called suture lines.
• The sections of the suture line pointing forwards
  are called saddles the backward folds are called
  lobes.
• Early ammonites have fairly simple suture lines
  but as the class evolved so these lines become
  complex.

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MORPHOLOGY

• A thought! Complex suture lines give strength to
  the shell possibly to help withstand pressure of
  water.
• Ammonites had a simple suture line but as they
  evolved these became more complicated
• Nautiloids
• Goniatites
• Ceratites
• Ammonites

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MORPHOLOGY

• Many early ammonites had smooth shells.
• The Mesozoic forms tend to be more decorated and
  vary from fine striations to heavily ribbed,
  tubercles and spines.
• The ornament can follow the pattern of coiling or
  cut transversely across it.
• Some genera have a groove running along the edge
  of the coils sulcus, others have a ridge keel.

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Differences between ammonites and nautiloids

• Ammonites
• More complex suture line as they evolved.
• Siphuncle starts central and then moves to the
  outer edge venter.
• Septal neck starts retrosiphionate, becomes both
  and then prosiphonate.
• Last chamber small.
• More ornamentation as they evolved.

• Nautiloids
• Simple suture line.
• Siphincle is central.
• Septal necks retrosiphonate
• Last chamber large.
• Smooth shell