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Abstract

The earliest steps of seed plant evolution have been extensively studied during the past 25 years. There is a growing body of evidence indicating that the first major spermatophyte radiation occurred during the Late Devonian. At least fourteen Late Devonian species are now recognized, and our knowledge of the diversity of those early seed plants has dramatically increased. Five morphotypes of seeds have been defined, mostly based on cupule morphology and on the number and degree of fusion of the integumentary lobes. In this paper, we critically discuss the abundant environmental information in order to characterize the environment in which this radiation occurred. Sedimentological information indicates that seed plants evolved in disturbed environments. It is suggested that early seed plants thrived in the shade of the dominating Archaeopteris, and that their evolution was canalized by this strong biotic pressure. We also confirm the previous suggestion that the variability of seed morphotypes can be explained by the weak abiotic selective pressure that existed in the Archaeopteris understory.

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