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, 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
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.

Plant succession through the Devonian
See also: