The Devonian period is also known as the Age of Fishes as this was the time that fish became the dominant lifeform throughout the oceans of the world. 

Placoderms are an extinct class of armoured Palaeozoic fish that were amongst the most successful and interesting creatures of this time. They are considered indirectly related to all living jawed vertebrates and are presumed to have been plentiful because their collective remains have been found right around the globe from Northern Europe to Southern Australia.

Dunkleosteus

The most obvious defining feature of placoderms would be the distinctive armour plating of head and thoracic shield - sometimes fused together as a single carapace, sometimes separate with a jointed 'neck' section.  However there were numerous other defining features of note that include articulated lower jaw bones, pelvic fins, complex nasal and inner ear structures, male and female dimorphic anatomy, and the first known example of placental birth. There are even indications that some placoderms may had developed rudimentary lungs.  New finding are continually coming to light, consequently placoderms are of renewed interest to palaeontologists worldwide.

Bothriolepis

Groenlandaspis

The actual position of placoderms within the evolutionary tree is also a subject of ongoing debate.  Although the entire class is believed to have become extinct before the end of the Paleozoic era many of their anatomical features are the first examples we have of such things in the fossil record.  It may be the case that modern lifeforms are more directly related to placoderms than first thought and if so it will require a rewriting of the early evolutionary tree.

Mcnamaraspis

Remigolepis

There is ongoing debate about when and how life moved from water onto land but by the end of the Devonian period it is clear that various creatures had made the transition. The great placoderm predators such as Dunkleosteus that dominated the oceans by this stage may well have played their part in this evolutionary transition.

Sarcopterygian

It could be that the placoderms had become so successful and ferocious that likely prey - such as the 'fishapod' Tiktaalik - were forced up onto the tidal margins for protection - from there to eventually move out of water altogether. This is unlikely to have been the sole evolutionary mechanism at play but it could well have been a compelling factor.

Tiktaalik

Paleozoo replicas are museum grade models, based on accurate data.  They are available for sale here via the store link below.