When did heterospory evolve?

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It is said that the heterospory has been evolved independently through several lineage and it is regarded as the precursor of seed reproduction. Fossils of the seed bearing seed ferns shows variety of seed and seed like structure. During the process of evolution the seed must have evolved one or many times. The three major trends that have produced the transition from the seed ferns to the gymnosperms are:

  1. The evolution from the homosporous to the heterosporous ferns which is also linked with the formation of the single large functional mega sporangium from many small sporangium.
  2. The evolution of integument which protects the ovule; it later forms the seed coat.
  3. The evolution of the pollen receiving structures and also the transition to a water independent phase of reproduction.
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