DEVONIAN PERIOD - Zdenek Burian, 1956
The Devonian period experienced the first significant
adaptive radiation of terrestrial life. Free-sporing vascular plants began to
spread across dry land, forming extensive forests which covered the continents.
By the middle of the Devonian, several groups of plants had evolved leaves and
true roots, and by the end of the period the first seed-bearing plants
appeared. Various terrestrial arthropods also became well-established. Fish
reached substantial diversity during this time, leading the Devonian to often
be dubbed the "Age of Fish". The first ray-finned and lobe-finned
bony fish appeared, while the placoderms began dominating almost every known aquatic
environment.
Early Devonian plants did not have roots or leaves like the
plants most common today and many had no vascular tissue at all. They probably
spread largely by vegetative growth, and did not grow much more than a few
centimeters tall. By far the largest land organism was Prototaxites, the
fruiting body of an enormous fungus that stood more than 8 meters tall,
towering over the low, carpet-like vegetation. By the Middle Devonian,
shrub-like forests of primitive plants existed: lycophytes, horsetails, ferns,
and progymnosperms had evolved. Most of these plants had true roots and leaves,
and many were quite tall. The earliest known trees, from the genus Wattieza,
appeared in the Late Devonian around 385 Ma. In the Late Devonian, the tree-like
ancestral Progymnosperm Archaeopteris which had conifer-like true wood and
fern-like foliage and the cladoxylopsids grew. (See also: lignin.) These are
the oldest known trees of the world's first forests. By the end of the
Devonian, the first seed-forming plants had appeared. This rapid appearance of
so many plant groups and growth forms has been called the "Devonian
Explosion".
The 'greening' of the continents acted as a carbon dioxide
sink, and atmospheric levels of this greenhouse gas may have dropped. This may
have cooled the climate and led to a massive extinction event. See Late
Devonian extinction.
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