Phase 1—the hypothesized earlier transition to land of prokaryotic and simple eukaryotic oxygen producers and fungi—is taken as a necessary precursor to the embryophytic life history and subsequent vascularization (40,66,97). Some of the earliest land plant spores have features consistent with a sphaerocarpal affinity. In none of the rhyniopsids tested do the stipules contribute significantly to trunk flexural stiffness.
In the primitive "trimerophyte" Psilophyton dawsonii, the hypodermal sterome of collenchymatous-sclerenchymatous elements contributes significantly to the bending stiffness of the whole stem (values of 96-99% result from calculations that enter either collenchyma or sclerenchyma as the outer tissue of the biomechanical model) (123). Stems of the derived (sagdonial) zosterophyll Gosslingia breconensis (basal diameter 4 mm) yield critical buckling lengths of 51–140 cm, depending on the tissue type used in the model to represent the newly acquired hypodermal sterome (more than 96% when modeled as collenchyma and more than 99% when modeled as sclerenchyma) (123).
PALEOECOLOGY
However, the contribution to the bending stiffness of the xylem was still only 42%, compared to 56% of the collenchymatous hypodermal steroma. Larger evaporation surfaces would require even more efficient water conduction than was provided by the primary steles in small-bodied early land plants. Secondary xylem was confined within the primary body of the stem in many aneurophytic "progymnosperms" and basal seed plants, providing little mechanical strength but likely improving water conduction.
Significant mechanical contributions of secondary xylem, such as that observed in archeopteridal "progymnosperms," were only possible after additional developmental innovations; most notable was the formation of periderm, which allowed the woody cylinder to extend beyond the limits of the primary stem body (119). By the end of the Devonian, landscapes were diverse and local plant communities were structurally complex, with a much greater diversity of body plans, life histories, and survival strategies (112). Given that typical contemporaneous plants were characterized by rhizoids or rudimentary true roots supported by turgor pressure and exhibited homosporous life histories, most vascular plants were probably restricted to wetter parts of the landscape.
The Rhynie Chert flora, now one of the best understood floras of the entire Paleozoic, offers a remarkable window into the Early Devonian ecosystem. Seed-bearing "pteridosperms" appear to have originated in wetter parts of the landscape and then spread as opportunists into areas of disturbance and physical stress, including relatively arid habitats (108). Studies on the developmental control of Devonian plants (127) similarly suggest to some of us that the initial increase in developmental complexity created many opportunities for morphological diversification with relatively few constraints.
The Late Devonian encapsulated all the major body plans of vascular plants, each of which characterizes one of the major "modern" clades: seed plants, ferns, sphenopsids, and various groups of lycopsids (the lack of high-level innovation after the Early Carboniferous, with the arguable exception of the rise of the angiosperms, presumably reflecting increasing morphological and ecological constraints).
CHARACTERS AND PHYLOGENY RECONSTRUCTION
The insights gained by applying phylogenetic techniques to the Siluro-Devonian radiation are remarkable given the many serious handicaps. First, extinct Paleozoic plants do not yield DNA, and large swaths of pioneer land plants have left no close living relatives—half of the Paleozoic plant groups traditionally considered taxonomic classes (albeit the most paraphyletic) are "extinct". Thus, molecular attempts to understand the evolutionary origins of bryophytes will be greatly aided by sequencing extinct rhyniophytes (9). Third, the Siluro-Devonian radiation occurred either rapidly (5100 million years; see 66) or very rapidly (35–50 million years; see 11) relative to the much longer period separating the radiation from present-day testable floras by sequencing.
Such deep and fast radiations are difficult to capture using clock-like molecules; those that change fast enough to capture the class and order relationships that emerge during radiation are now oversaturated with mutations, and the slower nonoversaturated molecules changed too slowly to capture the major events (10). Thus, molecular data have been less useful than might have been assumed in detecting the Siluro-Devonian radiation, although they have usefully revealed the error of viewing certain pteridophytes as "living fossils" unchanged since the radiation. Sequencing has also revealed that among truly primitive lineages, such as the homosporous lycopsid Huperzia, most extant species may nevertheless be of recent and distinct origin (139); the longevity of clades does not necessarily equate to the longevity of their component species.
However, the plants and resulting data are fragmentary for less easily preserved groups such as the "Bryophytes" and conservationally challenging periods such as the all-important Sensilur-earliest Devonian. Consequently, discussions of the Siluro-Devonian often focus on the total collection of phenotypic characters available to land plants during specific time periods. Although this approach can encompass a much larger portion of the fossil record, only suites of characters packed into a single plant (and thus representative of a single genome) yield meaningful evolution.
For example, the now widely accepted separation of lycophytes from the remaining eutracheophytes (5,6) leaves a group—the euphyllophytes of Kenrick & Crane (66)—diagnosed primarily by homologues of megaphyllous leaves, but these are questionably present in the basal leaf members of the clade (the paraphyletic "trimerophytes." Similarly, the synapomorphies of the ferns appear disturbingly "retrospective when sought in basal fern-like taxa such as the Iridopteridales, Cladoxylopsida, Stenocoleales, Stauropteridales, and Zygopteridaceae, where the pattern sensu latoae, which branches sensu. stellate anatomy is ill-defined and highly homoplastic ( 107).
INTERPRETING THE SILURO-DEVONIAN RADIATION(S)
However, in early Devonian ecosystems, salt mutants would have been produced even more frequently, given their weaker developmental channeling and, in the case of "bryophytes" and "protracheophytes" with long-lived haploid stages, their absence of buffering by second alleles of specific genes. Also, simple economic creation (possibly but not inherently associated with reproductive success; 24) would have been accomplished relatively easily in unsaturated habitats. When compared to the above model, the observed patterns of diversity suggest that the ecology of the Early and Middle Devonian provided weak constraints on evolution compared to the Late Devonian, given that much of the Earth's surface was either uncolonized or minimally occupied by vascular plants.
This conclusion also applies to the stomata found in most stomatophytes (tracheophytes plus "bryophytes" excluding liverworts), and the pronounced axial gametophyte, terminal gametangia and well-developed sporangiophore of the mosses plus tracheophytes clade. The basal members of the Polysporangiomorpha clade are "protraphyte"-grade genera such as Horneophyton and Aglaophyton. The unique combinations of characters in these extinct taxa provide us with key information about the sequence of character acquisition within the land plant clade, but again did not clearly cause a profound increase in the diversity of either phenotypic characters or species.
Even the subsequent dichotomy into Lycophytina and Euphyllophytina, and within Lycophytina into Lycopsida and more derived zosterophylls of the Sawdoniales, did not immediately add much to the overall diversity of characters. Later in the Devonian, tiiese ciiaacters were supplemented by the development of at least the early stages of secondary growth in perhaps five genera (66) and by at least the early stages of heterospory in perhaps ten genera (9,24). Secondary growth also provided the ability to adjust conductance and mechanical properties of the axial system according to local environmental conditions.
For this reason, many of the early land plants probably faced severe limitations in exploiting new sites and nodes compared to the later stages of the primary radiation of land plants.
CONCLUSIONS
Moreover, Bateman's ( 7 ) attribution of the ecologically driven “behavioral phase” of plant evolution to the post-Devonian is gradually being undermined as it becomes increasingly clear that paleobiologists have underestimated the role of interkingdom coevolution in early terrestrial ecosystems. When also considering the various carnivorous and phytophagous arthropods (74) and representatives of the sister group of embryophytes, the aquatic charophytes (134), de Chert graphically illustrates that understanding the origin and early diversification of terrestrial flora requires consideration of relationships required. between kingdoms, as well as relationships between classes and orders within Plantae. Nevertheless, even the available data are considerably better than those underlying the much-vaunted studies of the marine Cambrian 'explosion' of animal life, and at the level of exceptionally preserved Lagerstatten the terrestrial Rhynie Chert undoubtedly ranks alongside the marine Burgess Shale.
A molecular phylogeny of the bryophytes and their relationships to the tracheophytes based on chloroplast 16S and 23S ribosomal-encoding genes. Aspects of sedimentology, palynology and palaeobotany of the Upper Devonian of southern Kerry Head, Co. Morphology and taphonomy of root and stump deposits of the earliest trees (Middle to Late Devonian), Pennsylvania and New York, USA.
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The origin of land plants: phylogenetic relationships among charophytes, bryophytes and vascular plants inferred from complete small subunit ribosomal RNA gene sequences. Insect fluid feeding on upper Pennsylvanian tree ferns (Palaeodictyoptera, Marattiales) and the early history of the piercing-sucking functional feeding group. The amplification of two chloroplast tRNA introns marks the green algal ancestors of land plants.
