Graptolites

1
Graptolites

• The specification states that you need to be able
  to recognise and know the function of the
  following morphological features
• Thecae (including the different shapes).
• Shape of rhabdosome.
• Position and number of stipes.
• In addition you need to be able to
• describe and explain the morphological changes
  and evolution of graptolites in the Lower
  Palaeozoic.
• Also you need to be able to
• describe and explain the factors which make a
  good zone fossil.
• Outline the principles of using graptolites as
  zone fossils .

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Graptolites Classification

• Phylum
• Hemichordata.
• Class
• Graptolithina
• Orders
• Graptoloidea
• Dendroidea

3
INTRODUCTION

• The exoskeleton is made of organic protein
  material secreted by the soft parts of the
  animal.
• Both orders consisted of small animals, which
  were mainly pelagic (planktonic) living in
  colonies.
• The one skeleton is actually a colony containing
  several soft-bodied animals (zooids) which could
  extend out of the cups.

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GRAPTOLOIDEA (GRAPTOLITES)

• Found in Palaeozoic rocks only (middle Cambrian
  to the Carboniferous).
• They are pelagic and planktonic.
• Graptolites are most commonly found in deep water
  black shales, but do extend into shallow
  conditions.
• They are nearly always carbonised. 
• The process of carbonisation combined with the
  highly compressible nature of shales made most
  graptolite fossils extremely flat and therefore
  difficult to study.
• They are useful zone fossils.

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Zone Fossils

• What is a zone fossil?
• A fossil species characteristic of a certain rock
  horizon and is restricted to this time span e.g.
  certain Monograptus, ammonites, goniatites and
  ceratites. Micraster...
• What features make a good zone fossil?
• Abundant.
• Short lived (evolved rapidly).
• Easily Identifiable.
• Widespread Geographically.
• Hard parts.
• Graptolites did evolve quickly between L.
  Ordovician and U. Silurian and the planktonic
  mode of life allowed them to be widely
  distributed.

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MORPHOLOGY

• ? The colony originates as a single conical cup
  SICULA and from this the colony develops.
• Consist of hollow tubes (STIPES) single or
  multiple.
• THECAE are small cups joined together, which
  house the zooids (which are joined together by a
  common nervous system).

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MORPHOLOGY 2

•     THECAE form as overlapping cups along the
  length of the skeleton (STIPE).
• These may be present on one side (UNISERIAL) or
  both sides (BISERIAL) or even four (QUADRISERIAL)
  giving an X cross section.
•  
•  The colony can contain a varying number of
  stipes commonly 1, 2, 4, 8 etc.

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Thecal shape variation
9
Stipe Attitude

• There is a particular terminology used in order
  to describe the attitude of the stipes.

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Graptolite Morphological Terms

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COMMON GENERA TO KNOW

•  Didymograptus
•  Look at page 211 Draw.
•  Diplograptus
•  Look at page 211 Draw.
•  Monograptus
•  Look at page 211 Draw.