Australia: The Land Where Time Began

A biography of the Australian continent 

Chondrichthyans - Basic Structure of Primitive forms 

Sharks, rays and rabbitfish are members of the class Chondrichthyes - having an internal skelton of cartilage. The cartilage forming he brain case, jaws, gill arches, vertebrae and fin supports is of a special type. The only hard bony tissues that is present is in parts of the chondrichthyans, such as defensive fin spines, scales and teeth.

The cartilaginous skeletons of sharks was previously thought to be a primitive condition from a time before the ossification of bony skeletons evolved, now it is regarded as a specialised condition that sharks have adopted because of its advantages in their lifestyle. Calcite needles can form in the cartilage, adding strength but not much weight. Weight would be a problem for sharks as they have no swim bladder, depending on large livers containing oil for buoyancy, and the lift generated by their broad fins as they move through the water. 

The jaws of sharks are of a simple type, the primary upper and lower jaw cartilages known as Meckel's cartilage and palatoquadrate. There are multiple rows of teeth that grow throughout life, the back rows slowly moving forward, replacing the forward rows as they are lost, through damage or shedding.

The teeth are composed of a bony base pierced by canals and covered with a layer of dentine. They are set into the skin, and do not overlap each other. The scales can be simple blades or complex with several generations of growth on each base.

Chondrichthyans reproduce by internal fertilisation. The males have organs called claspers attached at the base of the pelvic fins that they insert into the female's cloaca during mating. Claspers have been found in well-preserved fossil forms from the Early Carboniferous. In some fossils, as in fossils from the Bear Gulch Limestone from Montana, there are distinct shape differences and fin differences between the sexes. As well as claspers, males can have elaborate fin-spines with brush-like structures. Examples where these occur are Falcatus, Damocles and Stethacanthus.

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995

 

 

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Last Updated 25/02/2011 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading