Biostratigraphic table for Orkney indicating the range for arthrodira:

Hm Homostius milleri CC Coccosteus cuspidatus Dt Dickosteus threiplandi Mm Millerosteus minor Wf Watsonosteus fletti       Bibliography

Placoderms

Homostius milleri Coccosteus cuspidatus Dickosteus threiplandi Millerosteus minor Watsonosteus fletti  

Ranging from a few centimeters to several meters in length, these fishes were very important inhabitants of the lake system and the rivers. They are all “armoured” with big overlapping bony plates often with many tiny tubercles on the outside.

Kwikwi: modern armoured catfish from South America

The head and body were connected with a joint like structure. This probably made movement of the head more independent from the body. Almost the same structure can be found today in recent armoured (actinopterygii) catfish. This is a case of a structure reinvented in evolution by a completely different group of fishes.

Only about one third of the body was heavily armoured. The rest of the body, the skin of the tail, was occasionally covered with very tiny scales. Only the antiarchi specimens found on Orkney have scales.

It is not possible to generalize about placoderm feeding mechanisms because there are many. However most of the placoderms found on Orkney have well developed jaws often with tiny teeth-like structures. They may have fed on tiny fish or invertebrates.