1. Orogony (there is such a word..)
The Variscon Orogony is the name given to the collisions (over 100 million years from about 350-250 mya) that bought two continents together to form – Pangea. Most of the  main soil animals groups are the same and found all over the world - including Artic and Antarctic. This would fit with the evolution of soil animals coinciding with the two continents becoming one - Pangea. 

Devonian Landscape
Devonian world when two continents start to become one, and more terrestrial rather than just marine.
The Geological Society says “The Variscon Orogony created the Variscan mountain belt, which includes the mountains of Portugal and western Spain, southwest Ireland, Cornwall, Devon, Pembrokeshire, the Gower Peninsula and the Vale of Glamorgan. The orogeny resulted in intrusions and volcanics in Devon and Cornwall …
Enhanced burial of organic carbon is documented by positive carbon isotope excursions of inorganic and organic carbon during the anoxic episodes leading to a drawdown of atmospheric and oceanic carbon dioxide levels and culminating in in significant climate cooling.” Does this mean carbon is being captured – by plants and animals in the soil – or rather volcanic dust?

You can see here that soil animals would of great service to the plants by holding on to the minerals and recycling them rather than them going out into the oceans (as Geological Society says) and causing eutrification. and making a lot of algae in the ocean –the process of eutrophication (for more see, Role of Soil Animals)

2. Extinctions
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota
 Period Series Epoch Stage  Age Age (Ma
 Carboniferous  Mississippian  
   Tournaisian   
 Devonian Late  Famennian  372.2–358.9
   Frasnian  382.7–372.2
 Middle  Givetian  387.7–382.7
   Eifelian  393.3–387.7
 Early    Emsian  407.6–393.3
   Pragian   410.8–407.6
   Lochkovian  419.2–410.8
 Silurian   Pridoli  no faunal stages defined older

Subdivision of the Devonian system according to the ICS.[1] Geologists are looking at what disappeared - many sea forms, rather han look at what was evolving - as it is harder to see what was going on in the land. The major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, theFamennian faunal stage (the Frasnian-Famennian boundary), about 375–360 million years ago.Overall, 19% of all families and 50% of all genera became extinct.[3] Tetrapods (aquatic) first appeared around this time.
Extinction

From Volcanism, Impacts & Mass Extinction (Geological Society) Note The Volcano picture in Famenian Period!

A second, distinct mass extinction, the Hangenberg Event, closed this stage and the Devonian Period as a whole. While most of the tale is about the extinctions, (and the only records I could find of the fauna) did this period also give rise to new populations that survived - on land? Perhaps in a newly forming substrate? The soil.
So could the Famenian faunal period be key? The fauna then included some spectacular sea creatures, but can’t find any record of terrestrial fauna. Is that becuase there arent any or not looking in the soil? This time period would fit with the evolution of plants into trees which came to dominate the land in the next Period – Carboniferous.

3. Oceans 
According to the Geological Society Volcanism, Impacts and Mass Extinctions‘Most prominent are the black shalefacies that developed in the world’s ocean with the Upper Kellwasser Event…. Several elements contributed to increased nutrient influx and eutrophication of the oceans, including enhanced weathering due to uplift during Variscon Orogony (see below) the evolution of plants and phosphorus from volcanic ashes’ Some big creatures - Tetrapods, came out of the oceans and lived on the land, but then disappeared.

SO BOTH HIGHER PLANTS AND VOLCANIC DUST WERE TOGETHER AT SAME TIME AND PLACE, BEING WASHED TO THE SEA, AS SOIL ANIMALS APPEARING!!

Devonian Gases


400 mill years ago, CO2 levels dropped steeply throughout the Devonian period as the burial of the newly-evolved forests drew carbon out of the atmosphere into sediments. The burial and development of forests go hand in hand. As this – Devonian period warmed, there is no corresponding increase in CO2 concentrations, implying the biological function of the soil is developing and coping with the increase in vegetation. there was an increas in oxygen from the newly colonising plants.


How was this remarkable carbon holding power  developed?  it must be in large part thanks to the soil animals and their capacity to move carbon around. (see Overview for details) 

Once established in the soil, these creatures are not going travel very far very easily. So while there are only a few taxa dominating soil animals, there many distinctive species as they are easily 'geographically isolated' ever after..

Now lets see in detail how the volcanic ash, created in the great collision made such a key contribution - See Essential Elements
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