Devonian Plants

The Devonian Plantscape 
Devonian plantscape

In the oldest fossil forest - - Brown Mountain Bilboa, around 380mya, there are big fern like plants Wattieza(pro-gymnosperms – see later) and the first recorded centipedes and oribatid mites. The First Forests

University of California Museum of Paleontology says:

"By the Devonian Period, colonization of the land was well underway. Before this time, there was no organic accumulation in the soils, resulting in soils with a reddish color. This is indicative of the underdeveloped landscape, probably colonized only by bacterial and algal mats.

By the start of the Devonian, early terrestrial vegetation had begun to spread. These plants did not have roots or leaves like most plants today, and many had no vascular tissue at all. They probably spread vegetatively, rather than by spores or seeds, and did not grow much more than a few centimeters tall. These plants included the now extinct zosterophylls and trimerophytes. The early fauna living among these plants were primarily arthropods: mites, trigonotarbids, wingless insects, and myriapods, though these early faunas are not well known.

By the Late Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many grew quite tall. The progymnosperm Archaeopteris (meaning 'ancient wing' 
was a large tree with true wood. It was the oldest known tree until the 2007 identification of Wattieza. By the end of the Devonian, the first seed plants had appeared. 

This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion." Along with this diversification in terrestrial vegetation structure, came a diversification of the arthropods."

It it this final period we are concentrating on. Most of the commentaries on this 'Late Devonian' period talk about the great extinction of marine animals, some fantastic enough like Dunkleostus to be featured in Sea Monsters.

But the biggest losses were those that lived near the shores, particularly the reef communities. There is no talk about terrestrial losses. Is that because the reverse was the case..there were terrestrial gains? Perhaps from the oceans? And there is this..
"The Devonian Plant Hypothesis" (from Devonian Times)
"A variety of causes have been proposed for the Devonian mass extinctions. These include asteroid impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes and climatic change. One of the more interesting of these is the "Devonian Plant Hypothesis". This theory, first proposed by Thomas Algeo, Robert Berner, J. Barry Manard and Stephen Scheckler in 1995, credits the expansion of terrestrial plants as the ultimate cause for mass extinctions in the tropical oceans.
They say 
"We hypothesize that rapidly increasing root mass led to transient intensification of the rate of soil formation and to permanent gains in the thickness and areal extent of deeply weathered soil profiles"

"Large root traces and remains of leaf litter are indications that the late Devonian palaeosol supported a low diversity, streamside gallery fores" Royal Society B

Back to (Devonian Times) and the Devonian Plant Hypothesis""Devonian marine deposits are notable in part for the widespread occurrence of black shales (see below 3. Oceans) in the shallow inland seas of North America and Eurasia. These organic-rich sediments, which indicate anoxic (oxygen-deprived) bottomwaters, occur at about the same time as the multiple extinction events in the Middle and Late Devonian. Algeo et. al. argue that these deposits were the results on organic matter and nutrient imports from increasingly vegetated landscapes. In addition to causing widespread eutrophication in shallow seas, terrestrial plants —particularly with the spread of Archaeopteris forests—contributed to accelerated pedogenesis (soil formation) which in turn resulted in accelerated silicate weathering. This process, which creates calcium and magnesium carbonates, removes CO2 from the atmosphere. These carbonates enter the rivers and are exported to the oceans where they precipitate and become buried in marine sediments".