A New Approach to Earth History » Recolonisation and plant fossil order

Recolonisation and plant fossil order

Recolonisation theory proposes that:

all organisms derive from one-off creations at the beginning of time, i.e. they had multiple ancestries
the order in which they appear in the fossil record had to do with the order in which pre-existing organism types recolonised the earth after a global cataclysm
owing to higher (but declining) levels of radioactivity the planet continued to be geologically unstable until long after the cataclysm
habitats stabilised only gradually
as a consequence of more intense tectonics vast quantities of water issued from areas of mountain-building, so that terrestrial habitats became drier only gradually.

These being the conditions, the order in which plants hit the fossil record would have been controlled by:

degree of substrate transience (frequency of erosion or deposition, and rate of deposition)
declining wetness: plants suited to watery environments would appear before plants suited to dry environments
atmospheric gas composition, especially levels of CO₂ and O₂, levels of which would have been affected initially by the cataclysm and subsequently by plant growth.

The first plants to appear would be:

fast-growing and/or small
biologically simple (because plants with complex biology take longer to mature)
colonisers and pioneers
capable of growing in watery substrates.

Over time, plants added to the fossil record would be:

larger and slower to reach maturity
biologically more complex
ecologically more dependent on other organisms preceding or co-existing with them
capable of growing in drier environments.

Here is a summary of what the record actually shows (see time chart for where the Ordovician, Silurian and Devonian periods fit in the sequence):

Plant type	Environment	Comments	Examples	Period
Bryophytes	Extreme and/or wet	Poikilohydric, i.e. cannot store water. Short life cycle. Tolerant of high CO₂ levels.	Mosses and liverworts	Ordovician onwards
Small, structurally simple vascular plants	Unstable wetlands	Homoiohydric? Short life cycle. Limited water- conduction but high storage, compatible with high CO₂.¹	Cooksonia	Mid Silurian to Early Devonian
Larger, structurally more complex vascular plants (height up to 3 m)	Wetlands, ‘water- ways regularly devastated by flooding’²	Homoiohydric. Larger size allows tapping of more stable water sources via rhizoids.	Rhyniophytes zosterophytes trimerophytes lycopods	Late Silurian onwards
Stigmarian plants: arborescent lycopods	More stable wetlands (swamps)	Short generation times (less than 10 yrs). Less tolerant of high CO₂ levels?	Wuxia	Late Devonian, Carboniferous
Rooted sporing trees: progymnosperms	Soils close to water, e.g. floodplains	Long generation times (up to 50 yrs)	Aneurophyton Archaeopteris	Mid Devonian to Early Carboniferous
Rooted seed plants: earliest gymnosperms (all sizes)	Full range of soils, from moist to dry	Diverse generation times. Wind-pollinated.	Elkinsia	Late Devonian onwards

References (full details in Further reading section)
1. Sperry 2003.
2. Edwards & Richardson 2004, describing the inferred environment of rhyniophytes.