Placoderms

Major subgroups in the Class Placodermi

5. Order Rhenanida


The rhenanid placoderm Gemuendina in dorsal view. Modified from Janvier (1996).

The Rhenanida are characterised by their reduction of the thoracic armour, dorso-ventrally flattened shape and enlarged pectoral fins, which are similar to those of the extant rays. The body is covered in small tesserae, which fill in the spaces between the larger plates of the dermal armour, and even cover some of them entirely. The gnathal plates are flattened, but are covered with minute oral denticles to provide a renewable grinding surface.

Rhenanids are known from marine environments and probably persued a ray-like benthic lifestyle, perhaps even swimming using wave-like undulations of their enlarged pectoral fins, just as rays and skates do today.

 

6. Order Acanthothoraci


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