Placoderms

Major subgroups in the Class Placodermi

3. Order Ptyctodontida


The ptyctodontshave much reduced armour plating and are characterised by the trabecular structure of their gnathal plates. Like in the petalichthyids the sensory lines are enclosed in canals and the supraorbital sensory-line canal and posterior pit line meet in the centre of the enlongate nuchal plate to form an X-shaped pattern.

The surface of the dermal armour was spongy and covered by skin in all but the earliest groups, which retained dermal tubercles. While the thoracic armour was reduced to a mere 'ring' of plates posterior to the head.

The pelvic fins bore large claspers in the males, similar to those of extant holocephalans such as Chimaera. Anterior to the pelvic fins were the prepelvic scute, which again my have played a similar role to the prepelvic claspers of holocephalans.

The ptyctodontid Ctenurella demonstrating the much reduced dermal armour of this group and their general physical resemblence to the modern chimaeras. Modified from Janvier (1996).

Reconstructions of the sexual diamorphism present in the pelvic fins of the ptyctodontid Rhamphodopsis, showing the pelvic claspers and enlarged prepelvic scutes of the male which would be used during copulation. Modified from Janvier (1996).

 

4. Order Antiarcha


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