Early Land Plants and the Greening of the Globe

Palaeobotany

  • Palaeobotany is the study of fossil plants and other photoautotrophs, including red/green algae and cyanobacteria.
  • These groups are the basis of almost all animal food webs.
  • They have symbiotic relationships with most other branches of life, including animals, fungi, protists, and bacteria.
  • They are atmosphere manufacturers and regulators.
  • Their evolutionary story has affected the evolution of all life.

Colonizing the Land

  • Land was the next logical step in the struggle for more light and nutrients.
  • Note where the rivers meet the oceans, as marine algae (kelp) were among the first to move landward.
  • The problems faced included:
    • Reproduction (dispersal & desiccation)
    • Tissue desiccation
    • Gas exchange
    • UV radiation
    • Water and nutrient uptake and distribution
    • Support against gravity
  • Water is 1000x denser than air; organisms are 1000x times ‘heavier’ in air.

Challenges and Adaptations

  • Reproduction (dispersal & desiccation):
    • Earliest evidence of land plants: spores with sporopollenin, a very tough biopolymer (taphonomic bias!).
    • Ordovician (~460–470 Ma).
    • Initially, water was used for dispersal.
    • Later, plants rode the wind with air sacs.
  • Tissue desiccation:
    • Cuticle: 'skin' for water retention + UV-B protection.
    • Ordovician (~450 Ma).
  • Gas exchange:
    • Stomata: regulate gases.
    • Late Silurian (~420 Ma).
    • Guard cells: minimize water loss.
    • Early Devonian (~410 Ma).
    • Not in liverworts!
  • UV radiation:
    • Sinapic acid & derivatives: Secondary metabolites.
    • A natural ‘sunscreen’.
    • Protects against UV-B (ionizing) radiation.
    • Evolutionary history: ???
    • Reproductive cells protected by a closely related compound: sporopollenin.
  • Water and nutrient uptake and distribution:
    • Rhizoids (?mid-Silurian; ~425 Ma).
    • Later: true roots and vascularity (Early Devonian; ~415 Ma).
  • Support against gravity:
    • Lignin: Rigid biopolymer, primary component of wood.
    • Late Silurian (~425Ma).
    • Algal precursor or convergent evolution?
    • Then… plants could grow UP!

First True (Land) Plants

  • Late Silurian: the way was paved for large vascular plants
  • Devonian: the first TRUE forests ~420–360 Ma
    • Early Devonian: ~ 30 cm
    • Late Devonian: ~ 30 m!

Plant Pioneers

  • Land plants are monophyletic (= Embryophytes).
  • “Bryophytes” - liverworts, mosses, hornworts.
    • All non-vascular à no ‘plant plumbing’ à need proximity to water …but not as much as algal ancestors (e.g., cuticles)
  • Almost entirely cellulose… poor preservation potential
  • Fossils can fill the gap
  • Ordovician (450 Ma) liverwort ancestor(??) spores
  • What do the fossils show?
    • Cooksonia, mid-Silurian (~ 430 Ma)
      • Branching photosynthetic stems
      • Sporangia (reproductive structures)
      • Cooksonia stoma… with guard cells!
    • Baragwanathia, late Silurian (~425 Ma)
      • The oldest lycopsids: Advanced tissue differentiation

Early Leaf Evolution

  • Two types of leaves:
    • Microphylls (single veined)
    • Megaphylls (many veins)
  • Evolution of microphyllous leaves:
    • Enation theory
      • elaboration vascularization
  • Evolution of megaphyllous leaves:
    • Telome theory
      • overtopping
      • planation
      • webbing
  • Euphyllophytes = ‘true leaf plants’
  • Convergent evolution of leaves

The First Forests (Devonian–Carboniferous)

  • Eco-niches filled
  • Animals
    • (Mega-)myriapods
    • Amphibians
    • Amniotes (AKA: snacks)
  • Plants: Canopy
    • First “true trees” (e.g., Archaeopteris / Callixylon)
      • Middle Devonian (~390 Ma) to Early Carboniferous (~320 Ma)
      • Probable ancestors of all seed plants… but not seed plants!
    • Pro-gymnosperms (extinct)
    • Sphenophytes (horsetails)
    • Lycopsids
  • Plants: Understorey
    • Ferns
    • Gymnosperms (seed plants)
  • Animal-plant interactions
  • Fungi…?

The Carboniferous Climate Crisis

  • Most rapid coal formation in Earth’s History
  • Carboniferous Period: 359–299 Ma
  • Why so much coal from that time?
  • Effects on the global carbon cycle?
  • The consequences for life + environments?
  • Enormous tropical peatlands
  • High O2
  • Very low CO2CO_2
  • Longest icehouse for the last 600 Myrs
  • Peat = accumulated plant matter deposits
  • Peat + burial + time = coal
  • Plant productivity > biological consumption
  • Low: O, mineral input, pH
  • Where in the world?
    • Land, plants (x lots), moist, low elevation, low relief
    • Southeast Asian tropical rainforests

Carboniferous Cycle

  • Carb. Tropical peatland area: <5% of Earth surface today; ~ 25% in late Carbonif.
  • Global temperature
  • Marine sediment (incl. soil minerals)
  • Major temperature changes lead to sea level fluctuations
  • Broad, long-term basin formation (subsidence)
  • Carbon cycle short-circuited …and re-peat!

The Lasting Evolutionary Effects

  • Supercontinent-wide, long-term C sink leading to Very low CO2CO_2 à Longest global glaciation in ~600 Myrs!
  • Global intermittent drying
  • Highest ever O2O_2: ~27–32% of atmos. volume
    • Fires common and intense, even in wetlands
    • Lots of charcoal!
  • These enviro factors favour preferential survival of seed plants à dominant forest-builders ever since
  • Carbon(iferous) cycle incomplete… humans completing the cycle 300 million years later!
  • Global warming in reverse… reversed!