GraptolitesGraptolites 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: ClasificationGraptolites: 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 FossilsZone

FossilsWhat 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 AttitudeStipe Attitude There is a particular terminology used in order

to describe the attitude of the stipes.

Graptolite Morphological Terms

aperture opening at the end of a theca

dissepiment skeletal crossbar joining two stipes

nema thin tube connecting a stipe to its float 

rhabdosome graptolite colony

sicula cup associated with the initial zooid

stipe individual branch within a rhabdosome

theca cup associated with all but the first zooid

virgulla spine at the end of a siculum

zooids individual within a graptolite colony

COMMON GENERA TO KNOW

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 HistoryGeological 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.