Vertebrae




The vetebrae were the original defining characteristics of the temnospondyls. There were originally two different classes of temnospondyls. There were the Rachitomi, with complex vetebrae, and the Stereospondyli, with more simple vetebrae.

Rachitomi:

The Rachitomi represent the earlier forms of temnospondyl. They are vetebrae characteristic of weight supporting animals. The vetebrae interlocked extensively and provided a strong backbone for a terrestrial animal. The Rachitomi were almost certainly terrestrial or at least semi-aquatic. Eryops is an example of a temnospondyl with rachitomous vetebrae. These vetebrae did not grow from one single piece of bone but three. These three different elements grew together to create each individual vetebra. These three elements are called the hypocentrum, pleurocentrum and the neural arch.


A comparison of Rachitomous and normal vertebrae

The above picture shows a Rachitomous vetebrae and its three elements in lateral and longitudinal views. It also shows a modern amniotic vertebra (stereospondylous) for comparison.

Key
  • Hatched area -   Hypocentrum
  • Red area       -  Pleurocentrum
  • White area    -    Neural Arch
The Rachitomi ar no longer regarded as a sub group of the temnospondyls. Some individual temnospondyls have rachitomous, semi-rachitomous and stereospondylous vertebrae at different point of their spine. Some also have intermediate vetebrae that dont appear to fit in anywhere. As such the Rachitomi are no longer classed as a true sub-order of Temnospondyli.

Stereospondyli:

The stereospondylous vetebrae are classic vertebrae found in the later more aquatic forms of the temnospondyls. These vetebrae are not equipped for load bearing means of transport. These vertebrae are completely fused. There is only one piece of bone as the neural arch has fused with the pleuro centrum without leaving a suture or any evidence of being two different elements.  This is the condition seen in all modern amniotes (see picture above). Some experts believe that the Stereospondyli can still be considered a sub order of the Temnspondyli though not only because of this shared character.




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