Placoderms

Major subgroups in the Class Placodermi

4. Order Antiarcha


The antiarch Pterichthyodes showing shme of the external distinguishing characters of this order, many of which are probably a consequence of their benthic mode of life. Image modified from Benton (2000).

The Antiarcha have a peculiar morphology, probably linked to their benthic or burrowing habits in the marginal marine and deltaic environments where they lived. The trunk shield is enlongate and has two median dorsal plates rater than the one of other placoderm orders. The eyes, pineal opening and nostrils are located atop the head in an oval orbitonasal fenestra in the dermal armour. The superognathal is absent and it's place is taken by the edge of the suborbital plate, which bites against the inferognathal. The cranio-thoracic articulation is the reverse of that found in arthrodires, petalichthyids and ptyctodontids in that the condyle in on the paranuchal plate of the head while the fossa is on the anterior dorso-lateral plate of the thoracic shield. The nuchal plate bears an X-shaped commisure between the posterior pit line and the central sensory line.

The rest of the body is either covered in large scales, as in Pterichthyodes, or almost naked, as in Bothriolepis. There is no evidence for the presence of an anal or pelvic fins. The most distinctive feature of all but the earliest antiarchs is the arthropod-like jointed pectoral appendage, which are covered by dermal armour. This highly modified pectoral appendage was moveable by a complex joint at its base and a secondary ball-and-socket joint approximately half way along its length and may have been used in life to help the animal bury itself in the substrate.

The antiarch Genus Bothriolepis is the most specious genus of the Placodermi known, containing more than 100 known species. Image modified from Benton (2000).

An adult Asterolepis in dorsal view showing the oval orbitonasal fenestra in the centre of the skull roof and the joint layout of the pectoral appendages. Modified from Janvier (1996).

 

5. Order Rhenanida


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