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 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, 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, 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 | Short life cycle. Limited water-conduction but high storage, compa- tible 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.
