PLANT EVOLUTION
3 major revolutions
- all result of innovations directed towards life on land
iefreeing plants from over-dependence on water in which they originated
3 great heydays in plant evolution
1
2
3
4
5
Late Silurian - plants developed vascularity + began to colonize land
Devonian
- diversification of major plant groups
climax was tropical rainforests of Upper Carboniferous
Permian
- massive extinctions
Permo-Trias - new plant groups evolved
climax in Jurassic
Cretaceous - origin of flowering plants
colonized nearly all habitats
grasses
- drastically reduced amount of erosion
- allowed evolution of grazing lifestyle
major consequences for terrestrial vertebrates
- 'Third blooming' of Cretaceous extends to present day
DEVONIAN PLANTS OF THE RHYNIE CHERT
Devonian plant fossils very rare
Found at Rhynie, Aberdeenshire, Scotland
Petrified in silicified peat - chert
Exceptionally good preservation - even down to cellular level
Simplest plant = RHYNIA cf modern Psilotum - tropical plant - green tufts 30cm high
Little differentiation between root + stem (2mm thick); no leaves
Grew by extension of underground stem + RHIZOME
SPORANGIA = spore capsules on end of aerial shoots
Shoots branched simply by splitting in two = DICHOTOMY
(Process mainly seen in non-vascular plants)
CARBONIFEROUS PLANTS AND THE COAL-MEASURES
Equatorial forests in very swampy deltaic regions bordering land masses
Environment of gentle continuous subsidence
- balanced by constant sediment input from land
Migration of channels across delta led to cyclic deposition
- each cyclothem represents sequence of sedimentary facies
Variety of plants - trees 10s of m high + scrambling/climbing/shrubby plants
eg
Lepidodendron:
Stigmaria:
Lepidophylloides:
Lepidostrobus:
dominant tree - 1m thick trunk up to 35m high
branching root system (very common fossil)
leaves
cones
Calamites:
cf modern horsetail (Equisetum)
Much bigger - tens of metres
Hollow stems commonly preserved as pith-casts
Leaves arranged in radial clusters
Annularia:
Ferns:
Cordaites:
Carbonifreous = 'Age of the ferns'
Hundreds of genera
Some low growing like modern ferns
eg Pecopteris, Sphenopteris
Some massive tree ferns
Some actually not true ferns at all = SEED FERNS
eg Mariopteris 1m thick trunk - fronds only 50cm long
Grew in drier condiotions further inland
Usually found as driftwood remains
Coniferous tree 35m high, 1m long strap leaves
JURASSIC PLANTS
Post-Permian expansion of plants preceded by evolution of the SEED
eg in seed fferns, cordaites + other conifers
Seed = dispersal mechanism
Jurassic - development of many woody plants with seeds - most species still alive
FLOWERING PLANTS: ANGIOSPERMS
Explosive radiation in Cretaceous
Flower = colourful cone
Produces seed protected by thick leathery coat
Inhabit huge range of habitats
Remain dominant plants today
ANGIOSPERMS
- 2 groups
MONOCOTYLEDONS - leaves with parallel veins; grasses etc
DICOTYLEDONS
-net-like veins