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figure 8.4 Stratigraphic ranges of microfossils, megafossils and various anatomical features. Numbers across the top refer to (1) Dyads and tetrads of possible bryophytic origin. (2) Single spores with trilete marks. (3) Cooksonia megafossils. (4) Bifurcating axes of putative vascular plants. (5) Nematophyte cuticle. (6) Higher plan cuticle. (7) Stomata on axial fossils and lycophytes. (8) Banded tubes. (9) C-type conducting elements. (10) G-type conducting elements. (11) Zosterophylls. (12) Baragwanthia, Drepanophycus, lycopsids. (13) S-type conducting elements. (14) P-type conducting elements. (15) Trimerophytes. (From Edwards, 2003.)

figure 8.4 Stratigraphic ranges of microfossils, megafossils and various anatomical features. Numbers across the top refer to (1) Dyads and tetrads of possible bryophytic origin. (2) Single spores with trilete marks. (3) Cooksonia megafossils. (4) Bifurcating axes of putative vascular plants. (5) Nematophyte cuticle. (6) Higher plan cuticle. (7) Stomata on axial fossils and lycophytes. (8) Banded tubes. (9) C-type conducting elements. (10) G-type conducting elements. (11) Zosterophylls. (12) Baragwanthia, Drepanophycus, lycopsids. (13) S-type conducting elements. (14) P-type conducting elements. (15) Trimerophytes. (From Edwards, 2003.)

Since the discovery and publications by Kidston and Lang (1917-1921), additional simple land plants have been described from Devonian and pre-Devonian rocks. For many years most of these early plants were placed in a single order, Psilophytales. These fossils, along with two living plants, Psilotum and Tmesipteris, made up a separate subdivision of vascular plants, Psilopsida (or Psilophyta). Today this classification is rarely used. Some include Psilotum and Tmesipteris in their own division (Gifford and Foster, 1989), whereas others regard the two extant genera as having their figure 8.5 Longitudinal section of cell wall thickenings in fossil S-, G-, and P-type conducting elements and tracheids of extant seed plants showing primary cell wall components. (Modified from Cook and Friedman, 1998.)

Resistant layer

Spongy layer

Resistant layer

Spongy layer

Primary cell wall

S-type conducting cells (Rhyniopsida)

Resistant layer r

Degradation-prone layer

Primary cell wall

G-type conducting cells (early Lycophytina)

Resistant layer

Degradation-prone layer

Primary cell wall

P-type conducting cells (early Euphyllophytina)

Resistant layer

Primary cell wall

Seed plant tracheids (recent Euphyllophytina)

figure 8.6 John W. Dawson. (Courtesy H. N. Andrews.)

closest affinities with certain ferns (Bierhorst, 1968; 1971), a placement that is supported by molecular studies (Manhart, 1995).

As additional Silurian-Devonian plants were discovered and carefully evaluated (Andrews et al., 1977), it became apparent that there were suites of characters that might be used to define larger taxonomic groups among the fossils (Hoeg, 1954). Harlan Banks (1975) (FIG. 8.11) was the first to propose abandonment of the Psilophytales, which had become a repository for all types of unrelated early plants. In its place he established three subdivisions—the Rhyniophytina, Zosterophyllophytina, and Trimerophytina. In the earliest cladistic analysis of early land plants (Kenrick and Crane, 1997a, b) the rhyniophytes and trimerophytes were not considered monophyletic, whereas the zosterophyllophytes were similar to the Zosterophyllophytina of Banks, with the inclusion of several other taxa. These fossils are now included in the polysporangiates, a clade of all land plants that bear multiple sporangia in the sporophyte phase, which includes both vascular plants and nonvascular plants (e.g., Aglaophyton). The Eutracheophytes contain all extant vascular plants and most vascular plant fossils, and are further subdivided into the Euphyllophytina and Lycophytina. The characters that are interpreted as plesiomorphic or derived (apomorphic) will be continually debated in the case of certain fossil plants. It is especially difficult to characterize basal groups, such as the earliest land plants, as by definition they will contain multiple plesiomorphies that are shared by all later-evolving land plants. With those limitations in mind, this chapter will discuss the Late Silurian-Early Devonian record of "vascular" land plants by reference to Banks' original three groups.

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