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.

Graptolites: Clasification  Phylum: Hemichordata.  Class: Graptolithina  Orders: Graptoloidea Dendroidea

 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.

 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.

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.

 Copy the diagram on page 194 McLeish that shows the general structure of a graptolite.  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).

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

Stipe Attitude  There is a particular terminology used in order to describe the attitude of the stipes.

Graptolite Morphological Terms apertureopening at the end of a theca dissepimentskeletal crossbar joining two stipes nemathin tube connecting a stipe to its float rhabdosomegraptolite colony siculacup associated with the initial zooid stipeindividual branch within a rhabdosome theca cup associated with all but the first zooid virgullaspine at the end of a siculum zooidsindividual within a graptolite colony

COMMON GENERA TO KNOW Didymograptus:  Look at page 211 Draw. Diplograptus:  Look at page 211 Draw. Monograptus:  Look at page 211 Draw.

 Evolve from Upper Cambrian to Lower Devonian Extinction.  They evolve from the Dendroids.  Changes include: Decrease in number of stipes. Change in attitude of stipes. Increase in complexity of the thecae. Change in the positioning of the thecae.

 Draw a diagram of a dendrite page 219 Fig. 140 a. Black.  There are some significant differences in that the RHABDOSOME (colony) always has many stipes.  In places short bars called DISSEPIMENTS join the stipes.  The role of the dissepiments is to give extra rigidity and strength to the structure.

 The thecae, however, vary in size being a mixture of large PRINCIPAL THECAE and smaller BITHECAE.  Most dendrites are attached to the sea floor by the central STOLON.  However, some floated.

 Most dendrites are sessile and attached to the sea floor by the sicula end using a kind of root system.  This therefore allows them to live in shallower water (the dissepiments also give added strength).  Therefore they are often preserved in sandstones, siltstones and limestones.

 They tend not to appear over huge areas (unlike the graptolites) because they are confined.  They also did not evolve as rapidly as graptolites and are therefore not particularly good zone fossils.  Some forms though did have had a pelagic life style.

Geological History  First evolved in the Mid Cambrian and became extinct in either the U. Carboniferous or L. Permian.  By the late Cambrian they were common.  The sessile types appeared first i.e. Dictyonema.  The only major development was L. Ordovician pelagic forms and it is thought that these led to the development of the graptolites.  They continued without further development until their extinction.