The Devonian period was a time of great change for life on the planet. The colonisation of land by plants gathered pace during the early part of the Devonian and by the end of the period great forests covered the land. Arthropods are thought to have been present on land margins by as early as the Cambrian period but now insects consolidated deep inland and then the vertebrate tetrapods emerged from the water margins to begin their colonisation of the land. 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 - and the most dominant fish group were the armoured placoderms.
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 known to have been plentiful as their collective remains have been found right around the globe from Northern Europe to Southern Australia.
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.
The position of placoderms within the evolutionary tree is 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 reframing of the paleozoic evolutionary tree.
Sarcopterygian Mandageria fairfaxi
The middle to late Devonian period also saw the emergence of the lobe-finned fish - the sarcopterygians. With an ability to hold a position in the water column sarcopterygians were great ambush predators and were among the largest of the fish species. The fore-limbs in particular had bones that correlate to the bones of later land vertebrates and it is thought that the fist tetrapods evolved from the sarcopterygian line.
Stem-tetrapod Acanthostega gunnari
It is still unclear when and how vertebrates 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 Tiktaalik roseae
It could be that the placoderms had become so successful and ferocious that likely prey - such as the softer bodied 'fishapod' Tiktaalik - were forced up into 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.
The end of the Devonian period is identified as one of the five great global extinction event in the history of the planet. What caused this mass extinction event is still under debate - as are so many other questions in paleontology - but there is a growing school of thought that believes the great emergent forests may have played a part. The hypothesis is that these forests acted as great carbon sinks causing the oxygen levels of the planet to rise rapidly, which in turn caused global temperatures to drop catastrophically. This is the opposite of global warming caused by rising carbon dioxide levels but equally problematic when running out of control. This hypothesis posits that the Earth has cycled throughout its history between oxygen driven ice ages and carbon dioxide driven greenhouse events.