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
Magic mushrooms, phallic fungi
Prototaxites, a bizarre cylindrical fungus
a mass of interwoven fungal filaments
1 m diameter x 8 m high
Early Devonian floras
close to water monospecific stands ?seasonal lot of effort into reproduction fast to colonize, like ‘weeds’ (ruderals)
A complex ecosystem
coprolites (poo) in sporangia
something eating spores – high quality food
fungi – breakdown of plant tissues
A bug’s life…
chert also preserves arthropods earliest mites springtails myriapods (millipedes) Freshwater zooplankton
Eight-legged crawlers
arachnids
harvestmen
trigonotarbids
air-breathing ‘book lungs’
Soiling the planet
before plants: only physical/chemical weathering
plant roots penetrate deeper and deeper: soil processes develop
enzymatic and fungal / bacterial breakdown
palaeosols in geological record
radically changes global carbon cycle
The complexities of life on land
terrestrial ecosystem diversifies through Devonian
plants progressively fill terrestrial ecospace, “up-river” and “uphill”
terrestrial ecosystem becomes progressively more complex and interconnected
The problems: reproduction
terrestrial reproduction:
In air sex cells must develop desiccation-resistant walls (spores)
produced inside desiccation resistant structures called sporangia
located at ends of plant axes to aid distribution in air
reproduce via homospory (♂ and ♀ spores the same)
spores need moist conditions in which to germinate and reproduce
