THE DEVONIAN AS A TIME OF MAJOR EVOLUTIONARY INNOVATION AND DIVERSIFICATION IN PLANTS- NEW DEVELOPMENTS

Accumulating evidence suggests that plants originated much earlier than the Devonian, but it remains a time of major diversification of plant structures and lineages. Both euphyllophyte (trimerophyte-grade) and lycophyte clades dominated early Devonian landscapes, along with several plant types which currently don’t fit any established categories, such as Chaleuria, Loganophyton, and as yet undescribed taxa. New data shows increased diversity of form among zosterophylls; smaller early lycophytes are less well known with exception of Early-Middle Devonian protolepidodendrids. The possibility that basal euphyllophytes encompass more diversity than presently recognized, being comparatively static morphologically but anatomically variable is supported by recent discoveries of 1) secondary xylem in Psilophyton-size stems and 2) variable anatomy among larger Pertica- grade plants. Rooting structures in attachment with aerial parts are poorly known.

Plants considered major components of Middle Devonian floras, and attributable to a revised concept of cladoxyls, are shown to be small to large trees, forming forests. A possible earlier advent of laminate leaves is documented by several Early and Middle Devonian plants from China, but laminate foliage among Laurussian plants still is known from Mid-Late Devonian on. Upright lycopsids first occur in Middle Devonian, diversifying and exhibiting a variety of rooting structures in Middle and late Devonian times. Growth habits of Mid-late Devonian plants, such as iridopterids, progymnosperms, and others are either reinterpreted or suspected of being differently organized than previously known. The occurrence of at least 14 different types of ovules/pre-ovules in Late Devonian, often from marginal marine sediments, may suggest an earlier origin and a major diversification of the seed habit. Recent discoveries of plants buried in situ or parautochthonously have vastly improved understanding certain paleoenvironments and vegetation associations, particularly in Middle Devonian; similar approaches or discoveries are needed from Lower and Upper Devonian strata to expand our knowledge of the composition and variation among early vegetation associations and to tie structural innovations to growth strategies and architectures in this context.