Lecture 8: Invasion! The greening of the Earth Flashcards
What was the land like in the Cambrian
Land surface completely barren
a very different hydrological cycle from today
more run-off and erosion
like Mars around 3.5 Ga when it probably still had oceans
The greening of the earth
Tiny mosses and liverworts had a profound effect on the planet
They turned the barren Earth green, created the first soils and pumped oxygen into the atmosphere, laying the foundations for animals to evolve in the Cambrian explosion that started 542 million years ago.
Aquatic ancestors
Green algae (e.g., seaweed): chlorophylls for photosynthesis
H2O easily diffuses into cells
CO2 dissolved in water easily absorbed by osmosis
Water full of dissolved nutrients
No need for rigid support as H2O buoyant
The aquatic-terrestrial transition
CO2 more easily extracted from air than H2O
light levels higher in air than H2O, more efficient photosynthesis
empty ecospace
fewer competitors
The problems: water loss
Led to…
how to retain H2O in a harsh desiccating environment? develop resistant (waterproof) outer cuticle
The problems:
gas exchange for photosynthesis
Can’t take in CO2 dissolved in water so need to take it in from atmosphere
Cuticle develops stomata
but sun warming plants and stomata taking in CO2 accelerate water loss
The problems: nutrient supply and transport
How to absorb nutrients and H2O?
cuticle increases diffusion distance
develop branching axes
photosynthesis by aerial shoots
other axes specialise as anchoring and H2O/nutrient absorption system: roots
The problems: nutrient supply and support
plants later develop more effective system for upwards
xylem and phloem
The problems: reproduction
aquatic reproduction:
Cross fertilization and dispersion easy
What produced them?
bryophyte-like plants modern mosses liverworts simple conductive strand for nutrients/water need damp conditions
A Lilliputian world
When were the first vascular plants?
Silurian Cooksonia few mm tall, no leaves simple branching axes simple vascular bundle
Where is the best known early terrestrial ecosystem?
Rhynie Chert, Aberdeenshire, Early Devonian
cellular preservation
3D preservation in life position
Rhynia: a simple plant
only shoots
no true ‘roots’
no leaves
vascular tissue for support and transport of H2O/nutrients
terminal sporangia, spores
cuticle, stomata
may have lived partially submerged in water
Asteroxylon: more complex
<40cm high, 20cm deep roots complex vascular bundle scale like ‘leaves’ larger photosynthetic area greater density of stomata could live in drier conditions
Asteroxylon’s living relatives?
small scale like leaves similar to today’s club mosses lycophytes e-like leaves many sporangia
