This group includes the oldest known lycophytes (Banks, 1960). In some treatments these Devonian plants are regarded as prelycopsids and classified based on their stratigraphic position and lack of some typical lycopsid characters (Gensel and Andrews, 1984), whereas in others they are considered as transitional between the zosterophylls and the true lycopsids. They represent a diverse collection of presumably herbaceous plants, although anatomical details and complete specimens are not known for all taxa. The upright axes have exarch primary xylem maturation and bear helically arranged appendages that sometimes appear to be in a near-whorled or pseudowhorled pattern. These appendages are either non-vascularized or partially vascularized, that is, the vein runs into the proximal portion of the appendage, but does not extend through the entire structure, so in this way they are similar to microphylls, rather than being true leaves. The appendages do not dichotomize at their tips as do microphylls in the Protolepidodendrales, but this may not be a sufficiently well-defined character, as it can vary with preservation. Fertile specimens suggest that sporangia are not aggregated into cones. In contrast to the true lycopsids, the sporangia of most Drepanophycales are not borne adaxi-ally on sporophylls, but rather arise directly from the axis shortly above an appendage, that is, in an axillary position. The lack of definitive information on this feature, however, has been used to suggest that many of the fossils are better placed in the Lycopodiales (Bateman, 1996b). All members are homosporous.

One of the oldest lycopsids is Baragwanathia (FIG. 8.1), known from the Siluro-Devonian rocks of Australia and the lower Middle Devonian of Canada (Lang and Cookson, 1935; Hueber, 1983; Garratt et al., 1984; Rickards, 2000). Baragwanathia longifolia is similar to a modern Lycopodium or Huperzia in that it had dichotomous branches that bore small, closely spaced, helically arranged appendages. The plants are much more robust than those of Lycopodium, however, with stems reaching 6.5 cm in diameter and appendages up to 4 cm long. Associated with some of the appendages are reniform sporangia that produced trilete spores 50 pm in diameter. The compressed nature of the fossil material makes it difficult to determine whether the sporangia are attached to the upper surfaces of the appendages or borne on short stalks in the axils of appendages. The stele is stellate in cross-sectional outline, and the exarch xylem has annular tracheids.

Baragwanathia abitibiensis is based on fossils from the Emsian (Lower Devonian) of Canada and consists of axes up to 3.2cm in diameter with helically arranged, microphyll-like appendages that bend downward (Hueber, 1983). Anomocytic stomata are randomly scattered on the appendages, but are far more common on the stems. The exact configuration of the xylem cylinder could not be determined, but the stele is constructed of tracheids with both annular and helical wall thickenings that closely conform to the G-type wall thickening. Sporangia were not found on any of the specimens. One interesting feature of B. abitibiensis is the truncated end of the mature appendages. It is believed that this unusual tip morphology is the result of postmortem changes. A similar cause may be used to explain the truncated or cup-tipped spines that have been described in some zosterophylls and trimerophytes.

Drepanophycus spinaeformis (FIG. 9.3) was once regarded as a good index fossil of Lower Devonian strata; however, specimens have now been discovered in Middle and Upper Devonian rocks as well. Although traditionally placed with the lycopsids, Schweitzer (1980b) included the genus in the

figure 9.3 Suggested reconstruction of Drepanophycus spinaeformis (Devonian). (Courtesy D. A. Eggert.)

zosterophyll complex. Aerial axes were probably produced from horizontal rhizomes (Banks and Grierson, 1968) (FIG. 9.4). Branching in D. spinaeformis is in an H or K configuration (as in zosterophylls). This pattern forms when a branch departs from a rhizome at a right angle and then dichotomizes at right angles to produce two stems that parallel the primary axis (Li, 1995). Some specimens from the Emsian of Scotland possess axes interpreted as endogenously formed roots (Rayner, 1984). Drepanophycus spinaeformis axis fragments are up to 27 cm long and 4.2 cm wide. On some specimens from the Canadian Arctic buds occur in the position of branches (Gensel et al., 2001). The surface of the stem in D. spinaeformis is covered with raised mounds (FIG. 9.5) (leaf cushions), representing bases of microphyll-like appendages that have broken off. Other specimens have been preserved so that the inner surface of the outer part of the stem is exposed. In these, the appendages are surrounded by the matrix under the specimen and, instead of mounds, one sees

figure 9.4 James D. Grierson. (Courtesy M. A. Millay.)

depressed horizontal or circular areas that denote the position of the appendages still in place. Microphyll-like appendages up to 2 cm long were borne in a shallow helix. Stomata have been described as occurring randomly among elongate, polygonal epidermal cells. The stomatal apparatus of D. spinaeformis is the paracytic type, consisting of two guard cells and two reniform subsidiary cells surrounded by a ring of epidermal cells that vary in number. Permineralized axes of D. spinaeformis reveal a lobed, exarch protostele ~2 mm in diameter. Individual tracheids are —70 pm in diameter and > 1 mm long. Secondary wall thickenings are annular-helical and possess a perforated reticulum in the position of the middle lamella (Hartman, 1981), now interpreted as G-type thickenings. Within the genus, sporangia are borne in either an axillary position or adaxially on appendages. Little is known about the spores, although the plants are regarded as being homosporous.

Another species, D. gaspianus, had more robust axes characterized by rhombic leaf bases that bore microphyll-like appendages with broad bases and recurved tips. Appendages were produced in a flat helix that contained 18-22 rows. Specimens from the Lower Devonian of New York

figure 9.5 Drepanophycus spinaeformis (Devonian). Bar = 2 cm.

include a lobed, stellate protostele with annular trac-heids and a perforate reticulum similar to that in D. spinae-formis (Grierson and Hueber, 1968). Drepanophycus gujingensis is a species from the Emsian of Yunnan Province in China (C.-S. Li and Edwards, 1995). It has sporangia attached to the stems by short stalks and adventitious roots attached to both the fertile and sterile axes. The stalked, adaxial or axillary sporangia are considered to represent an intermediate feature between plants like Asteroxylon (discussed below) and later lycopsids with sporangia borne directly on sporophylls. Another early lycopsid that is morphologically similar to Drepanophycus is Halleophyton (Early Devonian of China). It has rhomboidal to hexagonal swollen leaf bases and sporangia that split into two equal valves (C.-S. Li and Edwards, 1997).

Haskinsia is a herbaceous lycopsid that was once regarded as a species of Drepanophycus (Grierson and Banks, 1983; Xu and Berry, 2008). The helically arranged appendages are falcate and ~3mm long (FIG. 9.6). Metaxylem tracheids are characterized by various patterns of bordered pits. In H. hastata appendages are arranged in a pseudowhorl and are ~5mm long (Berry and Edwards, 1996a, b); sporangia are globose (Yang et al., 2008). These authors placed the genus with the Protolepidodendrales based on the presence of deltoid-shaped sporophylls. Another presumably herbaceous Devonian lycopsid is Haplostigma. In H. baldisii the

figure 9.6 A. Haskinsia sagittata and B. H. hastata (Devonian). (From Berry and Edwards, 1996a.)

microphyll-like appendages are simple and possess subhexagonal bases (Gutiérrez and Archangelsky, 1997).

Asteroxylon mackiei (FIG. 8.41) (Chapter 8) is sometimes included in the Drepanophycales, or within a clade that includes Baragwanathia and Drepanophycus as a sister group to all other lycopsids (Kenrick and Crane, 1997a). As noted in Chapter 8, Kidston and Lang originally described terminal sporangia that were thought to belong to this taxon, but subsequent studies showed that the sporangia were borne laterally on the stems near the axils of microphyll-like appendages and not in a terminal position (Lyon, 1964). Whereas A. mackiei is known exclusively from the Lower Devonian Rhynie chert, a second species, A. elberfeldense (FIG. 9.7), is based on impressions with partial anatomical preservation from the Middle Devonian of Germany, Scotland, and

Asteroxylon Mackiei
figure 9.7 Asteroxylon elberfeldense, proximal and distal and narrow, naked axes fragments of Stolbergia spiralis (Devonian). Bar = 6 cm. (Courtesy BSPG.)

Norway. The best-preserved specimens come from Kirberg near Elberfeld, western Germany (Kräusel and Weyland, 1926; Weyland et al., 1969). Axes of A. elberfeldense (originally named Thursophyton milleri) are dichotomous to sym-podial, up to 1m long and >0.5 cm in diameter. They bear

figure 9.8 Distal end of Smeadia clevelandensis showing terminal cone (Devonian). Bar = 5mm. (From Chitaley and Li, 2004.)

helically arranged and densely spaced, short microphyll-like appendages. Often found in association with the spiny A. elberfeldense axes are narrower, dichotomously branched, naked axes of the Hostinella hostimensis type (sometimes misspelled as Hostimella), which were originally believed to represent the terminal portion of A. elberfeldense (Krausel and Weyland, 1926). Fairon (1967), however, demonstrated that these axes do not belong to A. elberfeldense, but rather represent a different plant, for which she introduced the name Stolbergia spiralis. Although superficially similar to A. mackiei, the systematic affinities of A. elberfeldense remain unclear. Anatomically preserved specimens from the Middle Devonian of the Aachen region in Germany suggest that A. elberfeldense, as well as S. spiralis, belong to the lycophytes (Fairon, 1967).

Hestia eremosa is a putatively herbaceous and phyloge-netically primitive lycopsid with uncertain affinities that has been described based on isolated stems from a Mississippian sequence of tuffs and lacustrine deposits at Oxroad Bay, East Lothian, Scotland (Bateman et al., 2007). Stems are characterized by the combination of a stellate stele, scalariform xylem pits, and perforate sheets of wall material partially infilling the pits, a complement of features largely consistent with that seen in Huperzia, the most plesiomorphic extant genus of Lycopodiaceae. The limited number of characters preserved in the fossils, however, cannot preclude placement of H. eremosa within the Drepanophycales. An excellent example of a lycopsid that possesses characters of several groups is Smeadia (FIG. 9.8) from the Upper Devonian Cleveland Shale of Ohio (Chitaley and Li, 2004). This plant was herbaceous with a siphonostelic stem (FIG. 9.9) and helically arranged leaves. At the distal end was an erect stro-bilus (FIG. 9.8 ) that contained trilete spores 40-80 pm in diameter.

Was this article helpful?

0 0

Post a comment