EOE 3104 Evolution of Higher or Vascular Plants

EOE 3104  Evolution of Higher or Vascular Plants

Prior to the Silurian the principal organisms that lived in terrestrial habitats were algae and bacteria in soil.

One of the most perceptive papers to be published in the late 21121i816v 1960's was by Schumm in 1968. His perceptive speculations on the impact on fluvial systems of the evolution of vascular plants and their colonisation of the land surface of the Earth have yet to be fully appreciated by many sedimentologists.

Before the Silurian unconsolidated or unlithified sediment could only exist in areas of deposition or where slopes were negligible. This meant that much of the land surface consisted of bare rock. Soils, as recognised, today are a biological phenomenon - a consequence of the evolution of plants with capacity to bind soil particles with their roots.

Macroscopic algae (seaweeds) are known from at least 1000 Ma but these were restricted to aquatic environments. The earliest sign of more advanced plants is found in the form of tetrads of spores (organic-walled microfossils) in the Late Ordovician.

Cuticles and vascular plant megafossils appeared in greater numbers and diversity through the Silurian. The Devonian witnessed the transition from small moss-like plants to substantial trees 10-20m tall.

Important evolutionary features of these plants included:

• erect stems with cells strengthened by new biopolymers - lignin
• roots to anchor the plant and provide water and nutrients
• cells to transport water and nutrients to the sites of photosynthesis (leaves)
• cells to prevent water loss - cuticle
• specialised cells to allow gas (CO₂) transfer without water loss
• an effective reproductive system

The 'greening' of the land was accompanied by the evolution of insects and by the late Devonian early amphibians were venturing onto land.

The Devonian and Carboniferous floras increased in size, diversity and probably overall biomass. The debris of these plants gave rise to increasingly extensive coal deposits.

During the Carboniferous the Gymnosperms (conifers) evolved.

At some stage in the Early Cretaceous the first Angiosperms (flowering plants) appeared and through the Cretaceous and Early Tertiary came to dominate the terrestrial flora.

As in the Devonian the evolution of the angiosperms was accompanied by co-evolutionary developments in insects.

The most important group of angiosperms - the Grasses (Graminae) - appeared in the early Tertiary but their major impact on terrestrial habitats occurred in the Late Oligocene-Early Miocene when grassland ecology became distinctive and true grassland soils developed. A significant change in the dentition of horses occurred at around this time reflecting the development of a grazing strategy taking advantage of the increasingly food source.

Grasslands cover 30% of the land surface and are the habitat for 10,000 species. Grasses provide 50% of the calories consumed by animals each day.

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