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Lecture 11 Reading


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1 THE INVASION OF LAND KEY QUESTIONS ADDRESSED 1) What are the difficulties of moving from water to land for a) plants and b) animals? 2) What does the fossil record indicate about when and where the transition was made? 3) What is the origin of vertebrate lungs? 4) How did the vertebrate limb evolve from a fin? I. PLANTS : PROBLEMS OF TERRESTRIALITY A. Water is a supportive medium that protects organisms from dessication and extreme temperature fluctuations, and contains dissolved nutrients and gases that are necessary to life. B. Problems to be overcome in making the transition include: 1) Lack of support 2) Dessication 3) Near absence of dissolved nutrients and gases 4) No liquid medium for transport of gametes B. Solutions 1) Support : evolution of lignin (an organic compound that strengthens plants) and wood 2) Dessication : evolution of waxy outer covering known as the cuticle 3) Respiration : Evolution of stomata , specialized cells on plant surface that can open and shut allowing gas exchange 4) Feeding : evolution of roots and internal transport system , xylem 5) Reproduction : spores that could be wind dispersed, and then seeds that did not require water for fertilization to occur. C. Fossil record indicates that these were not acquired all at once but in stages, as plants evolved greater and greater independence from an aqueous environment. Over the course of the Devonian, plants became increasingly complex, evolving stems, roots and leaves to serve separate functions. Undoubtedly – they moved into drier and drier environments as their adaptations to life on land improved.
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2 II. FOSSIL RECORD OF PLANTS A. First invaders of the land were probably cyanobacteria and green algae that lived at the water’s edge and were routinely exposed to dessication and sunlight. These would have moved on to the land as mats. No fossil evidence – but there are living cyanobacteria that form mats in very inhospitable environments such as desert soils – and in many places, algae that join with fungi to become lichens. Lichens represent a symbiotic relationship between fungi and algae – the fungi dissolve the rock and extract nutrients that are used by the algae, and the fungi utilize the energy produced by the algae via photosynthesis. The ability of the fungi to break down rock allows the algae to exist where they otherwise could not. B. First evidence of terrestrial plants is late Ordovician, about 450-470 ma. Includes microscopic spores that appear to have belonged to primitive, non-vascular plants, such as mosses, and fragments of cuticle and tube-like structures appear suggesting increasing adaptation to land. C. First megafossils of plants are Silurian, about 420 million years old. The earliest landscapes would have been concentrated in moist environments, and would have been very low in stature with tiny plants, some erect and many prostrate.
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  • Winter '10
  • fossil record, Tetrapod, acanthostega, Tetrapod Limb, typical tetrapod limb

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