Graptolites, its general characteristics, morphology, geological range, preservation, environmental significance and occurence

 

GENERAL CHARACTERISTICS: 

PHYLUM:

The Phylum of Graptolites is a class in the animal phylum Hemichordata.

SUB-CLASS:

The Graptolites belongs to Sub-class Graptolithia.

ORDER

        i. Order Dendroidea (Cambrian-Pennslyvianian).

      ii. Order Graptoloidea (Ordovician-Devonian).

    iii. Order Conodontomorphia (Cambrian-Triassic).

    iv. Order Hyolithia (Cambrian-Permain).

      v. Order Tentaculidita (Ordovician-Devonian).

    vi. Order Conularia (Cambrian-Triassic).

GEOLOGICAL RANGE:

The geological range of the Graptolites is from the Upper Cambrian through the Lower Carboniferous i.e., (Mississippian).

EARLY GRAPTOLITE:

The early Graptolite known is Chaunograptus, from the Middle Cambrian.

KINGDOM:

The kingdom of Graptolites is Animalia.

FIRST APPEARANCE:

They first became apparent during the Cambrian period.

Cryptograptus, early known graptolite, Silurian Age, South America.

CLASS:

The class of graptolites is Pterobranchia.

SYMMETRY:

The Graptolite is bilaterally symmetrical and tentacled.

GENERA:

The seven genera of Graptolites are well-known, these are Climacograptus, Clonograptus, Didymograptus, Diplograptus, Monograptus, Phyllograptus and Tetragraptus.

ANIMAL TYPE:

They are Invertebrate, small aquatic colonial animals.

INTRODUCTION:

Graptolite comes from the Greek graptos, meaning “written”, and lithos, meaning “rock.” Graptolites are the fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous (Mississippian). Graptolites are tiny, extint animals that lived together in groups or colonies and share the same skeleton, which was like an apartment building. Each animal build his own apartment or living chamber and thus stuck together to form a colonies. 

WHAT DID THEY EAT?

Graptolites were probably suspension feeders. They would have fed by straining plankton and other pieces of food from the water. They probably used tiny hairs (Cilia) attached to a tentacle to grab food.

 WHERE DID THEY LIVE?

Some Graptolites lived on the bottom of the ocean, where they would stick to the surface with a special structure. They grew upwards just like the plants, adding more living chambers as the colony got older.  Other Graptolites floated in the seawater, perhaps they drifted with the ocean currents like seaweed.  Graptolites that lived on the ocean floor appear in the fossil record first and became extinct later than floating Graptolites.

BRANCHES OF GRAPTOLITES:

Some colonies grew like branches of a tree, with many living chambers on each branch. Different kinds of graptolite colonies had branches with different shapes. They could be straight, curved or even spiral-shaped.

CLASSES OF GRAPTOLITES:

There are two main classes of Graptolites on the basis of their branch:

i. Graptolithina.

ii. Dendritic Graptolites.

GRAPTOLITHINA:

The few branched Graptolites belonging to order Graptoloidea that were good swimmers (Pelagic). The Graptolithina were present up to the Devonian period.

DENDRITIC GRAPTOLITES:

Branched graptolites of order Dendroidea. They are benthic marine animals. They lived on sea floor and attached to sea bottom. They were present till Carboniferous period.

MORPHOLOGY:

 The morphology of graptolites are as follow:

STIPES:

The many number of branches of Graptolites are called as stipes. Stipes are originated from sicula.

SICULA:

It is the individual organism of graptolites joins to make the tube like structure. It is initial growth point of the colony of graptolites.

RHABDOSOME:

It is the single or one-branched colony of Graptolites.

THECA:

The tree or tube like structure of the micro-organism hosted in the graptolites. Arrangement of theca is very important and useful criteria to determine the fossil of graptolites.

NEMA:

This is thin tube at larva stage of Graptolite that originates from the apex of sicula.

APERTURE:

The end point in the graptolites at which the theca opens is called as aperture.

VERGULLA:

It is the spine at the end of sicula.

DISSEPIMENT:

It is that bar which connects the adjacent stipes into the rhabdosomes of dendrites.

ZOOID:

Soft-bodied individual inhabiting theca.

PROSICULA:

Proximal, initially formed part of sicula, apparently selected as single conical unit with faintly marked spiral thread.

SOFT PART MORPHOLOGY OF GRAPTOLITE ZOOID:

A mature zooid has three important regions, the preoral disc or cephalic shield, the collar and the trunk. In the collar, the mouth and anus (U-shaped digestive system) and arms are found. Graptholitina has a single pair of arms with several paired tentacles. As a nervous system, graptolites have a simple layer of fibers between the epidermis and the basal lamina, also have a collar ganglion that gives rise to several nerve branches, similar to the neural tube of chordates. All this information was inferred by the extant of the Rhabdopleura, however, it is very likely that fossil zooids had the same morphology. An important feature in the tubarium is the fusellum, which looks like lines of growth along the tube observed as semicircular rings in a zig-zag pattern.

HARD PART MORPHOLOGY OF GRAPTOLITES:

The hard part morphology of graptolites are as follow:

i.  Dendroids.

ii. Graptoloids.

DENDROIDS:

The Dendroids which range from Middle Cambrian to Carboniferous, are characterized by numerous stripes, and by thecae differentiated into two types, one as male theca and other is female theca. v They are exemplified by Dendrograptos and Dictyonema.

GRAPTOLOIDS:

The Graptoloids (Ordovician-Early Devonian) are characterized by less numerous stripes, and in some cases only one and by un-differentiated thecae, which led to having in been interrupted as hermaphroditic. The evolutionary trend observed in graptoloids is towards a reduction in number of stipes.

THECAL STRUCTURE & ITS TYPES:

Thecae or Thecal structure are small cups joined together, which house the zooids that are joined together by a common nervous system. These may be present on one side (UNISERIAL) or both sides (BISERIAL) or even four sided (QUADRISERIAL) that gives an X- cross section.

GEOLOGIC RANGE

PRESERVATION:

Graptolite fossils are often found in shales and mudrocks where sea-bed fossils are rare, this type of rock having formed from sediment deposited in relatively deep water that had poor bottom circulation, was deficient in oxygen, and had no scavengers. The dead planktic graptolites, having sunk to the sea-floor, would eventually become entombed in the sediment and are thus well preserved.

Graptolites are normally preserved as a black carbon film on the rock's surface or as light grey clay films in tectonically distorted rocks. The fossil can also appear stretched or distorted.

ENVIRONMENTAL SIGNIFICANCE:

Graptolites are useful in biostratigraphy to date and correlate Paleozoic rocks. The occurrence of graptolite-rich rock is an important record of shelf-margin development and oceanographic conditions in sea above shelf margin. They are key to plate tectonics reconstructions and their interpretations. Graptolites provide a promising tool for recognizing the maximum flood surfaces.

OCCURENCES IN PAKISTAN:

The Graptolites occurrences in Pakistan in the following formations i.e, Broghil Formation, Toi Formation, Wargal Formation, Shogram Formation and Lun Shale (Chitral).