Early vascular land plants: proof and conjecture

Abstract

Megafossil evidence does not fill the 'evolutionary gap' between land plants and their hypothetical green algal ancestors. Rare Late Silurian vascular plant megafossils provide little information about the morphological, physiological, biochemical, and ecological steps that preceded their evolution. Dissociated trilete spores, spore tetrads, cuticle-and tracheid-like structures far exceed the abundance and diversity of Silurian vascular plant megafossils, and appear millions of years before them. In reference to whole-bodied organisms, these or analogous structures belong to land plants or emergent aquatics; they may represent plants evolutionarily intermediate between green algae and descendent vascular plants at the bryophyte or pre-bryophyte stages. Changes in the cellular biochemistry of pre-Devonian land plants in response to the selective pressures of terrestrial life may have led to the origin of lignin and cutin, neither of which has any counterpart among the algae, and to the evolutionary surge of the vascular plants in the Early Devonian represented by the plant megafossil record. Positive correlation between abundance and diversity of trilete spores and shallow-water, nearshore sites reinforces conclusions based on morphology that a terrestrial flora existed well prior to the appearance of vascular plant megafossils.

Boron, Lignification and the Origin of Vascular Plants‐a Unified Hypothesis

“…The evolutionary relationship between the two processes is in doubt. 'Whether the evolution of lignin as a chemical entity proceeded concurrently with, followed, or came before the evolution of cell types specialized for the conduction of water, is a question that may never be answered from direct evidence' (Gray and Boucot, 1977). However, from physiological and biochemical points of view, such a consideration involves a metabolic web in which boron, lignification and peroxidases intermesh with the metabolism of hormones, especially auxin, indol-3yl-acetic acid (IAA) (Fig.…”

“…The first macrofossils of vascular plants appear in rocks of late Ludlovian beds of the Silurian (Edwards and Davies, 1976; Addendum to Gray and Boucot, 1977). They pre-date Cooksonia (Table 1).…”

“…With this information and their own interpretation of Silurian microfossils. Gray and Boucot (1977) argue that 'the inference is inescapable that considerable pre-Ludlovian vascular plant evolution must already have occurred' and that land (not necessarily vascular) plants existed some 30 milhon years before macro-fossils were preserved. Their interpretation of the microfossils, hotly disputed by Schopf (1978), is that they represent cuticularized epidermal fragments, durable-walled spores with triradiate scars and tracheid-like tuhes, features they regard as the sine qua non of non-algal terrestrial plants.…”

“…Following the acquisition of the biosynthesis of lignin, it is reasonable to suppose selection would favour those plants in which it was deposited in an existing water-conducting tissue, e.g. the hydrome-like tissue of bryophyte-like organisms [see Hebant (1977) and Gray and Boucot (1977) for a discussion of the hydrome of bryophytes]. Among extant bryophytes, tracheid-like cells are found principally in the gametophytes of mosses and not in those of leafy liverworts.…”

New records of fertile Rhyniophytina from the late Silurian of Wales

“…Much of the recent discussion on the early colonization of the land by plants has been based on dispersed microfossils (Banks, 1975 b;Gray & Boucot, 1977). We consider it highly unlikely that tracheids with annular thickenings such as have been described from late Silurian and early Devonian plants would remain intact on the disintegration of the remaining tissues of the plant, and hence would not be found as isolated elements.…”