Colonization of the land:

• Algal mats in intertidal zone back in Proterozoic
• Possible short excursion by mollusks and arthopods in Cambrian onward
• However, long-term settlement on land doesn't happen until latest Ordovician

Why colonize the land?

• For plants, new untapped nutrient resources
• For animals, plants & other animals for food
• Protection from predators
• Ability to survive desiccation as lakes dry up

Land dwellers need to combat:

• Gravity: no longer buoyant
• Desiccation: no longer surrounded in liquid
• Lack of dissolved oxygen/carbon dioxide: need to process gaseous air
• Lack of water to aid in reproduction
• Lack of dissolved nutrients in surrounding medium: need other ways to get food
• For animals: different senses (sound moves slower and more weakly, pressure and electric senses don't work, air has different refractive index than water, etc.)

Different groups of organisms combat these features in different ways.

Plants (Plantae):

• Terrestrial chlorophytes (green "algae")
• First evidence of possible terrestrial plants from vascular tissue and spores of the Ordovician
• Early vascular plants of the Silurian: use tubes in body transport nutrients & water upwards and manufactured food throughout body
• Spores to transmit genetic material:
  • In primitive plants, spores land and develop into sexual stages, which transmit sex cells via surface water
  • Therefore, still need wet surfaces to breed
• Plants have a waxy surface to prevent dessication, and openings (stomata) through the tissue to allow them to breath
• A famous early land plant is Cooksonia, which has spore-bearing organs, vascular tissue, waxy surface, stomata, but NO leaves
• Largest known Silurian plant is 30 cm tall Baragwanathia, a lycopod (clubmoss), which has very small leaves (increasing surface area for getting sunlight and for breathing)

Presence of plants modified the surface of the Earth (at least around lakes and streams), because ground cover would retard erosion. Retention of sediment on land, and incorporation of decaying plant matter in that sediment: development of first biological soils.

Arthropods (Arthropoda):

• Permanently invaded terrestrial realm at least three times (chelicerates, uniramians, isopod crustatceans)
• Already had many features making them good prospects for life on land:
  • Chitinous skeleton to support mass
  • Jointed limbs to move them around
  • Complex mouthparts to feed on plants or each other
  • Box gills (and later box lungs &/or trachea) to breath air
• By the Late Silurian, several groups had already invaded land:
  • Arachnids
    • A clade of chelicerates, close relatives of the eurypterids
    • Silurian arachnids included scorpions, which were still partially aquatic and still had gills, and most likely mites and other small forms
    • Spiders evolved later in Paleozoic
  • Myriapods
    • A group containing millipedes (herbivores and detritivores) and centipedes (carnivores)
    • Present in the Silurian onward
    • Contraversial trace fossils in the Late Ordovician
  • Hexapods
    • The group containing insects and their flightless relatives
    • Oldest definite fossils in the Devonian; may have actually appeared in the Silurian
    • Once thought to be close to the myriapods, but now considered closer to (or within!) Crustacea

Other early colonists (with no fossil record) probably included fungi and various "worms" (nematodes, earthworms, etc.).

Very simple early terrestrial community with simple plant producers, millipede herbivores, centipede and arachnid carnivores, worm and myriapod detritivores, and fungi decomposers.

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Last modified: 10 January 2008