graptolites

• Graptolites belong to the hemichordates. Modern hemichordates are worm like marine animals. They are planktonic that first appeared in early Ordovician, they are evolved from dendroid that lived on the sea floor. 
• They became dominant marine fauna into the early Silurian. They declined at the middle of the Silurian and were extinct by the Devonian. 
• Graptolites are zone fossils which can be used to date rocks. 
• Graptolites (zone fossil characteristics):

  • Composed of schleroprotein which resists decay. 
  • Planktonic marine organisms were distributed worldwide
  • Facies free so not restricted to a particular environment 
  •  evolve quickly with identifiable stages 
  • Very abundant, chances of fossilisation are high - deep water environments where black shale accumulated. 
• one trend in graptolites is there are changes in the number of stipes from lots in ordovician to only one in silurian
• a second trend in graptolites is a change in the attitude of stipes from downwards then outwards and then reclined to scandent. 
• a third trend in graptolites is Changes to the shape of the thecae from simple to elaborate curved and hooked ends
• a fourth trend in graptolites is Changes in the arrangement of the thecae on the stipes, early forms were uniserial and the biserial later on. Thecae became more widely spaced along the stipes.
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  When graptolites die they sink and line up with the current. 
• graptolite diagram

  A) rhabdosome

  B) stipe

  C) theca

  D) nema

  
• evolution of graptolites

  A) lower Ordovician

  B) upper ordovician

  C) silurian