L2- The Invasion of Land

LOs:

  • evidence for the invasion of land by plants

  • nature of the earliest flora

  • a hint of terrestrial animals

There is a long history of Precambrian life but it is microbial:

Major macroevolutionary event→ land plants appear:

  • evolution→ a monophyletic multicellular group appears- land plants (embryophytes)

  • ecology→ invade land, create soil, create multicellular ecosystem for others to come onto land too

  • environment→ are primary producers, creating another carbon cycle, greener landscape, create soil

Evidence for the origin of land plants:

  • extant plants

    • ancestors→ phylogenetic analysis, sister group are multicellular green algae that lived in freshwater, crawled out to become land plants

    • evolutionary relationships→ there are 2 major groups of living plants

      • bryophytes→ non-vascular plants, mosses

      • tracheophytes→ vascular plants

      • most basal plants are in between these two groups

    • physiological comparisons→ comparing the physiology of multicellular green algae in freshwater and basal land plants to see the jump (evo-devo)

    • molecular clocks→ not very good for major events though

  • fossils

    • dispersed spores and megafossils→

      • earliest land plants reproduced through spores in sporopollenin walls

      • 2 fossil records

      • studying spores method→ take siltstone rock, dissolve up in hydrofluoric acid to take out inorganic, leaves organic content, put onto slide or embed in resin, study under a microscope

      • found weird spores from the mid-ordivican called cryptospores (Jane Grey)→ evidence that land plants reproducing, are spores from the earliest plants- evidence of this:

        • are modern plant spore analogues→ are the same size as modern plant spores, different in morphology though, some liverworts are tetrads w/ envelopes, form in tetrads→ evidence of meiosis, have a tough sporopollenin wall

        • are found in non-marine sediments→ lived in or around freshwater

        • old phylogeny suggested basal plants were the liverworts where these spores came from

        • walls of spores are made of parallel lamellae- same as modern plants

        • found Eophyte fossils from the Silurian that were producing cryptospores

        • found entire sporangia (used to make and store spores) in Oman from the Ordivican

        • spore wall chemical composition is identical to sporopollenin in modern plants

        → all evidence shows that these cryptospores derived from land plants in the mid-Ordivician

    • dispersed fragments→ find cuticles and tubular fragments that were suggested to be bits of plants

  • palaeoenvironments

    • palaeogeography→

      • Ordovician world→ Gondwana supercontinent (equator to the pole), find cryptospores all over the world (red dashes)

      → plants invaded the planet, spread quickly, live everywhere as generalists as spores are found everywhere and are the same types

      • get identical assemblages around world for 30million years

    • palaeoclimate→

      • huge ice age at the end of the ordovician, ice cap over gondwana, mass extinctions, get same plant spores above and below the ice age→ generalists

      • climatically sensitive sediment fossils→ cold area in the upper ordovician

    • palaeoenvironment→

      • when plants invaded the land there was high co2 levels and low o2 levels→ doesn’t make sense with the ice age

      • reconstruction of earliest plant communities:

      • before→ microbial soil crusts and mats→ bind surface together, release acids, build up soil

      • after plants evolve→ are mm in size, ground huggers, plants create more organics in soil, more acid dissolving up the rock, increases soil thickness

      • soils are cm thick, on top are eophytes (predate liverwort/vascular plant split), sit on surface, create soil, photosynthesise, every now and then produces a sporophyte, releases cryptospores, have tetrads that can produce a new gametophyte, lands on damp soil and can reproduce into male and female parts, can self-reproduce

      • also may have had animals→ find fossil soils called palaeosols that have burrows- similar to millipeed, but are overlayed by marine sediments so others think these are marine worms

      • Jane Grey found more organic content when dissolving up rock- arthropod (made of chitin)- survives acid digestion, unsure if these lived on land or freshwater

        → there are aquatic arthropods at this time

    • in Australia there is are non-marine series of sediment (tumblagooda sandstones)- have ripples and arthropod traces

      → large animals have come out onto this surface when it was dry

  • spores and pollen- sporopollenin

  • cutan- plant cuticles

  • lignin- wood, conducting tissues

  • chitin remains- fungi, animals

  • sporophyte→ diploid multicellular generation, produces spores- haploid reproductive cells produced by meiosis in sporangia, released in the air

  • gametophyte→ haploid multicellular generation, spore undergoes meiosis to become a gametophyte

→ plants invade land, go worldwide, animals are possibly there/thinking of coming onto land, is like this for 30million years