Plants & Soil Animals

Devonian Times says "The development of soils is an often overlooked aspect of the Devonian
Early Devonian
A progressive increase in soil depth and geographic extent during the Devonian and Early Carboniferous is strongly associated with the development of plant rooting systems. Root traces become more frequent and extensive during the Middle Devonian, but most are still relatively shallow (<20 cm). 

The situation changes dramatically in the Late Devonian with the appearance of Archaeopteris. These trees had extensive root systems that reached depths in excess of 1 m.
Volcanic ash and soil

From Essential Elements and Amendments to Plants

Earth crust in Famennian times
Prof of Geology Cincinnati T Algeo says: "One of the most important consequences of the spread of vascular land plants during the Devonian was a major increase in the depth and geographic extent of mature soil development (left, adapted from Algeo and Scheckler, 1998). Root systems developed rapidly during the Middle and early Late Devonian as progymnosperms and other arborescent plants appeared"

From Algeo @ The Royal Society clearest description of what went on. 
"The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were 
(1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and 
(2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. 
These developments resulted in a transient intensification of pedogenesis (soil formation) and to large increases in the thickness and a real extent of soils."

1. This does not realise and recognise the significance of volcanic ash in providing the essential elements for this plant evolution.
2. Nor is there any mention of soil arthropods, yet this 'pedogenesis' fits exactly with my prediction of when soil animals first appeared. 

Devonian Times also supports 'my theory':"Fossil evidence of detrivorous invertebrates in the Late Devonian is extremely sparce, but investigations of several Late Silurian (e.g., Ludford Lane), Early Devonian (e.g., Rhynie) and Middle Devonian (e.g., Gilboa) sites suggest that a variety of detritivores were already well established by the Late Devonian. These include 
millipedes (Diplopoda),
Devonian Millipede
(Devonian Milipede)arthropleurids (many-segmented arthropods that superficially resemble millipedes), 
oribatid mites (Acari)
and springtails (Collembola)
Springtail in AmberSpringtail in amber (difficult to preserve as exoskeleton breaks down). 
With the exception of the extinct arthropleurids, these arthropods are abundant components in modern leaf litter faunae. Also...these early detritivores were small (typically < 2 cm)."
Late Devonian

The oribatids are the main 'detrivores' found in their thousands in soils around the world. They can withstand dry conditions by curling up to protect themselves, an important defence for the new terrestrial conditions. 
Dried Oribatid
They are essential in breaking down detritus, skeletonising dead leaves.

Other soil creatures - the springtails, also help by bringing the fungal spores to the roots and encouraging the development of mycorrhizal fungi. Springtails called Onychuriuds  
are said to carry the fungal spores to the roots accidentally - like bees pollinating flowers accidentally while trying to find nectar.
The mycorrizal fungi (see Glossary) bring essential elements (from the volcanic ash) to the plants in exchange for energy. The oldest fossil Collembolan, Rhyniella praecursor, was taken from Lower Devonian beds of the Rhynie chert in EnglandThe Rhynie chert "contains exceptionally preserved plant, fungus, lichen and animal material preserved in place by an overlying volcanic deposit. Fungal hyphae can be seen entering plant material, acting as decomposers and mycorrhizal symbionts."

It is interesting to note that the black shale that accumulates in the oceans is the result of organic matter being carried there from the land, so perhaps the role of animals in breaking down dead vegetation and recycling nutrients back to plants was yet to evolve more fully. 

So we have roots, soil beginning to form with its own biota, and volcanic ash.
All is ready for...