- 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.
- 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.
- 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.
- 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):
|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
|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
|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.