Plant Life

PALEOZOIC PLANTS

• PRECAMBRIAN THROUGH CAMBRIAN
  • bacteria (Monerans) , algae (Protoctistans) & fungi only
    • MOSTLY MARINE, ALTHOUGH SOME FRESHWATER VARIETIES, PARTICULARLY GREEN ALGAE (THOUGHT TO BE ANCESTAL TO LAND PLANTS)
• LAND PLANTS (See Figure 12-15, p. 310)
  • mostly vascular (have tubes to carry water & nutrients)
  • Oldest remains - spores in Middle-Upper Ordovician African rocks
  • Psilophytes - Oldest body remains from the Middle Silurian
    • HORIZONTAL STALKS UNDER THE GROUND
    • VERTICAL STEMS BEARING BRANCHES & SPORE SACS
    • VERY PRIMITIVE - NO TRUE LEAVES OR ROOTS, & SEEDLESS (REPRODUCED WITH SPORES)
    • DIVERSIFIED IN THE LATE SILURIAN & WENT EXTINCT DURING THE DEVONIAN
  • Scale trees (leaves grew directly from the trunk) [LYCOPSIDS], Rushes & horsetails [SPHENOPSIDS] and True ferns [FILICINOPHYTES] evolved from Psilophytes during the Early & Middle Devonian
    • ALL HAD WELL-DEVELOPED LEAVES, BUT ONLY FERNS HAVE TRUE ROOTS
    • ALL WERE STILL SEEDLESS
    • SOME SPECIES OF ALL WERE TREE-SIZED (10'S OF METERS TALL)
  • Seed ferns [PTERIDOSPERMALES] evolved from true ferns during the Late Devonian
    • SEED FERNS HAD TYPICAL FERN LEAVES, BUT REPRODUCED USING SEEDS RATHER THAN SPORES
    • HOWEVER, THEY HAD NO FLOWERS & WERE THE FIRST GYMNOSPERMS
    • THE GLOSSOPTERIS FLORA TYPIFYING GONDWANA WAS A SEED FERN
  • Carboniferous coal forests were dominated by these 4 plant groups that evolved during the Devonian
    • ALL 4 GROUPS DECLINED IN IMPORTANCE AT THE END OF THE PALEOZOIC
    • SEED FERNS BECAME EXTINCT DURING JURASSIC; THE OTHER 3 GROUPS HAVE LIVING DESCENDANTS
  • The other 3 gymnosperm groups evolved from seed ferns during the Late Paleozoic and Early Mesozoic -> Conifers evolved during the Mississippian, and Cycads and Ginkos evolved during the Triassic
    • CONIFERS WERE IMPORTANT AWAY FROM CARBONIFEROUS COAL SWAMPS
  • Note: Flowering, seed-bearing plants [ANGIOSPERMS] evolved from seed ferns or conifers during the Late Jurassic or Early Cretaceous

MESOZOIC PLANTS

• MARINE PLANTS
  • Groups of phytoplankton (floating photosynthesing protists) that are important members of modern ocean ecosytems (most marine plants are protists) evolved during the Mesozoic
    • Dinoflagellates (cellulose) - Triassic
    • Coccolithophores (calcium carbonate) - Jurassic
    • Silicoflagellates (silica) & Diatoms (silica) - Cretaceous
• TERRESTRIAL PLANTS
  • Cycads, along with other gymnosperms, were the dominant land plants during most of the Mesozoic
  • Angiosperms (flowering seed plants) evolved in the Late Jurassic or Early Cretaceous & became the dominant land plants starting in the Late Cretaceous
    • ANGIOSPERM SEEDS ARE CARRIED IN FRUIT
  • Seed Ferns went extinct in the Jurassic; True Ferns, Microphyllophytes & Anthrophytes became greatly diminished in importance compared to the Paleozoic

CENOZOIC PLANTS

• MARINE PLANTS
  • Phytoplankton recovered from terminal Cretaceous mass extinction
    • PEAKS IN DIVERSITY IN the EOCENE & MIOCENE
      • Diatoms particularly abundant in the Miocene
    • DECREASE IN DIVERSITY IN the OLIGOCENE
• TERRESTRIAL PLANTS
  • Angiosperms continued to diversify at expense of Gymnosperms
  • Grasses evolved in the Oligocene & became widespread in the Miocene
    • FORESTS DECREASED IN SIZE; GRASSY STEPPES & SAVANNAHS DEVELOPED
    • EVOLUTION OF GRASSES (TOUGH, ABRASIVE & COATED WITH DUST) & DEVELOPMENT OF OPEN GRASSLAND HAD A PROFOUND EFFECT ON GRAZING MAMMALS
      • Evolution of high-crowned teeth with folded enamel that continued to grow at the roots & elongation of face to hold bigger teeth
      • Evolution of 4-chambered stomach
      • Elongation of limb & toe bones, elevation of ankle & reduction in number of toes, & evolution of hoofs
      • Carnivorous mammals also evolved & diversified in response

Study Questions

1. When & what groups of land plants evolved during the Paleozoic?

2. Summarize the details of Paleozoic land plant evolution (the sequence of different divisions that evolved, the divisions that are extinct & the divisions that are reduced in importance).

3. Summarize the differences in reproductive strategies used by the various divisions & the significance of the evolution of the seed.

4. What were the divisions of land plants that dominated during the Mesozoic?

5. What division of land plants evolved during the Mesozoic?

6. What was the effect on mammalian evolution of the evolution of grasses?

7. What might have been the cause of the evolution of grasses at the expense of forest plants?

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