Prqtqlepidqdendrales

Members of the Protolepidodendrales extend from the Devonian into the Mississippian (Lower Carboniferous) and fit the definition of a true lycopsid. They were either herbaceous or subwoody plants or small trees, and possessed small, helically arranged microphylls that branched at their tips and were vascularized by a single vein (Berry, 1996). Although one genus has been found to bear ligules (discussed below), they have not been found in any other taxa. The generic name Protolepidodendron has been used for a variety of Middle Devonian, dichotomously branched lycopsids (FIG. 9.10) (Grierson and Banks, 1963). Their stems are up to

figure 9.9 Transverse section of Smeadia clevelanden-sis showing pith, the ring of vascular tissue and cortex of stem (Devonian). Bar = 1 mm. (From Chitaley and Li, 2004.)
Protolepidodendron
figure 9.10 Suggested reconstruction of Protolepidodendron (Devonian). (Courtesy D. A. Eggert.)
figure 9.11 Suggested reconstruction of fertile leaf of Minarodendron cathaysiense (Devonian). (From Taylor and Taylor, 1993.)

2 cm in diameter and covered with helically arranged micro-phylls that were typically bifurcated at the tip. Schweitzer and Cai (1987) described P. cathaysiense from the Middle Devonian of southern China, which they believe is identical to Leclercqia (discussed below), based on the presence of highly bifurcated microphylls. C.-S. Li (1990) combined P. cathaysiense with P. scharyanum and erected a new genus, Minarodendron. Specimens of M. cathaysiense (Givetian) are ~3-4mm wide and generally exhibit a longitudinal series of elongate cushions on which the leaves are borne (FIG. 9.11) . The apex of each microphyll is trifurcate, with two tips directed up and a single, median one pointing down. The stems contain an exarch or mesarch strand of primary xylem that is toothed or triangular in cross section. Tracheids range from annular to bordered pitted, and the sporangia are globose-reniform and borne on the adaxial surfaces of unmodified sporophylls.

Some Early Devonian specimens that were initially described as species of Protolepidodendron are now called Estinnophyton (Fairon-Demaret, 1978, 1979) (FIG. 9.12). Like other protolepidodendrids, E. gracile was a small, herbaceous plant with axes up to 4mm wide. Leaves were helically arranged and up to 7 mm long. Each fertile leaf bore two pairs of sporangia, each attached by a small stalk, at a short distance from the tip (FIG. 9.13) , although in E. yunnanense (Lower Devonian of China) each fertile leaf contains two

Muriel Fairon Demaret
figure 9.12 Muriel Fairon-Demaret.
figure 9.13 Diagrammatic reconstruction of Estinnophyton yunnanense. (From Hao et al., 2004.)

single stalked sporangia (Hao et al., 2004). This species also contains poorly preserved trilete spores and tracheids with annular to helical wall thickenings. Estinnophyton is included here with Protolepidodendrales principally on the bifurcate nature of the leaves. However, the paired sporangia suggest affinities with the trimerophytes, whereas their recurved organization is similar to that of some Devonian sphenophytes (Hao et al., 2004).

Colpodexylon
figure 9.14 Suggested reconstruction of Colpodexylon deatsii (Devonian). (Courtesy D. A. Eggert.)

One of the better-preserved members of the Protolepidodendrales known from Middle and Upper Devonian rocks in New York is Colpodexylon deatsii (Banks, 1944). The dichotomously branched stems are up to 2.5 cm wide and reveal elliptical leaf bases arranged in a low helix or appearing as a pseudowhorl (FIG. 9.14). The characteristic feature of this fossil is the presence of trifurcate leaves, which reached 3 cm in length (FIG. 9.15). The primary xylem strand is lobed in cross section, with exarch-mesarch maturation and annular tracheids. Sporangia are borne on the upper surfaces of unmodified leaves that are scattered along the stem surface. In C. trifurcatum ( FIG. 9.16), a Middle Devonian taxon, the trifurcating leaves are 2 mm wide. The primary difference between the two species is the larger leaf base in C. trifurcatum, which may extend up to 5 mm long. In C. camptophyllum (FIG. 9.16) from the Devonian of Venezuela, the tips of the leaves are described as being shorter (Berry and Edwards, 1995).

Clwydia (formerly Archaeosigillaria) is a small, dichoto-mously branched, herbaceous lycopsid that extends from the Devonian into the Carboniferous (Lacey, 1962). The leaf bases range from fusiform on smaller axes (FIG. 9.17) to hexagonal on larger stems. Despite the helical arrangement of the small leaves, they appear decussate and organized into vertical ranks. Some specimens superficially resemble Lycopodites, differing only in the possession of decussate, needlelike leaves (FIG. 9.18). In other species, such as C. vanuxemii, the leaves are deltoid in outline and possess a toothed margin; extending from the apex of the leaf is an elongate hair (Fairon-Demaret and Banks, 1978). The leaves of Clwydia probably did not abscise. The vascular system consists of a lobed protostele with exarch primary xylem and scalariform metaxylem tracheids.

figure 9.16 Morphologies of various Colpodexylon leaves: A. C. trifurcatum; B. C. deatsii; C. C. camptophyllum; D. C. cachiriense; E. C. variabile; and F. C. coloradense (From Berry and Edwards, 1995.)

figure 9.15 Leaf of Colpodexylon trifurcatum (Devonian). (From Berry and Edwards, 1995.)

Phytokneme is a petrified lycopsid axis about 3 cm in diameter discovered in an Upper Devonian phosphatic nodule in the Chattanooga Shale of Kentucky (Andrews et al., 1971). The specimen is exquisitely preserved, with all cells and tissue systems intact (FIG. 9.19). In P. rhodona, the middle cortical zone, which is typically poorly preserved in fossil lycopsids, contains a network of radially aligned, ray-like strands; these appear similar to cells that characterize the axis of the Carboniferous cone Lepidostrobus kentuckiensis. Roy and Matten (1989) suggested that Phytokneme may have affinities within the Lepidodendrales.

A number of lycopsids have been described from the Devonian-Mississippian New Albany Shale of Kentucky and Indiana (Roy and Matten, 1989). One of these is Fodiodendron defractus, characterized by an exarch protostele with scalar-iform-reticulate metaxylem tracheids. The presence of two

Archaeosigillaria
figure 9.17 Leafy axis of Clwydia (=Archaeosigillaria) (Devonian). Bar = 1 cm.

strands of included phloem in the leaf traces distinguishes Fodiodendron from other permineralized lycopsids.

Probably the most completely known member of the Protolepidodendrales is Leclercqia (FIG. 9.20), a slender, herbaceous plant known from the Early-Middle Devonian of Australia (Fairon-Demaret, 1974; Meyer-Berthaud et al., 2003), North America (Banks et al., 1972; Gensel and Kasper, 2005; Gensel and Albright, 2006), South America (Berry, 1994), and Europe (Fairon-Demaret, 1981).

figure 9.18 Axis of Clwydia (=Archaeosigillaria) showing prominent leaves (Devonian). Bar = 2 cm.

Specimens of L. complexa are up to 46 cm in length and vary from 3.5 to 7mm in diameter. The dichotomously or pseu-domonopodially branched axes are covered by microphylls that attained lengths of 6.5 mm (FIG. 9.21). The leaves are

figure 9.19 Partial cross section of Phytokneme rhodona (Devonian). Bar = 3mm. (From Andrews et al., 1971.)

unusual in that they exhibit a pair of lateral divisions about half way up the lamina (Bonamo et al., 1988). Each of these divides into two acuminate tips, whereas the central portion of the leaf gradually tapers and recurves abaxially (FIG. 9.22). The closely spaced leaves, each with five slender tips, must have given the plant an unusual appearance. Stomata of the anomocytic type (Gensel and Albright, 2006) are present on the microphylls and stems, and a few have been observed on the wall of a sporangium. Ligules (FIG. 9.23) have also been reported on Leclercqia microphylls (Grierson and Bonamo, 1979). The function of these small flaps of tissue which occur only in lycopsids has been of historical interest dating back to Hofmeister (1851). Some of the many suggestions as to the function of ligules include secretion and accumulation of water, mucilage, enzymes, and/or nutrients, or superficial conduction of water (Pant et al., 2000). The presence or absence of ligules has been used to define some groups of lycopsids.

The Fossil Hunters Plants
figure 9.20 Leclercqia complexa axis (Devonian). Bar = 2 cm.

Some Leclercqia specimens are known in which the vascular cylinder has been preserved as a pyrite petrifaction. In cross section, the stele is circular with up to 18 external protoxylem points and exarch primary xylem maturation.

Protolepidodendron

figure 9.21 Suggested reconstruction of Leclercqia complexa (Devonian). (From Kenrick and Crane, 1997a.)

figure 9.22 A. Vegetative and B. fertile leaf of Leclercqia complexa (Devonian). (From Kenrick and Crane, 1997a.)

figure 9.21 Suggested reconstruction of Leclercqia complexa (Devonian). (From Kenrick and Crane, 1997a.)

figure 9.22 A. Vegetative and B. fertile leaf of Leclercqia complexa (Devonian). (From Kenrick and Crane, 1997a.)

Metaxylem tracheids are scalariform or have oval pits on their walls. Outside the stele is a narrow, parenchymatous cortex with scattered cells with thickened walls, which may be scler-enchyma. Each leaf is vascularized by a single trace that originates from one of the protoxylem ridges on the stele.

Sporangia are attached to the adaxial surfaces of sporo-phylls by a small pad of tissue just proximal to the lateral segments of the leaf. The distribution of sporophylls is similar to that in many species of Hupezia, in which fertile and sterile leaves are almost indistinguishable from one another and interspersed along the stems. Immature spores of Leclercqia are preserved in tetrads. At maturity, the spores are trilete, 60-85pm in diameter, and ornamented with numerous, closely spaced spines with expanded bases (FIG. 9.24); in situ spores of L. complexa from eastern New York State have been assigned to the dispersed spore genus Aneurospora (Streel, 1972), whereas others have been compared to Actinosporites lindlarensis (Richardson et al., 1993; Gensel and Albright,

Gesch Digtes Ohr Durch Rger
figure 9.23 Leaf of Leclercqia complexa with distal branched tips. Arrow indicates position of ligule (Devonian). Bar = 625 pm. (From Grierson and Bonamo, 1979.)
Recycling Art
figure 9.24 Tetrad of Leclercqia complexa spores with one spore removed from the tetrad (Devonian). Bar = 10 pm. (From Gensel and Albright, 2006.)

2006). Details of the spores, including those in tetrads, have been reported by Gensel and Albright (2006). The large number of fertile specimens with morphologically identical spores has been regarded as evidence that Leclercqia was homosporous. Cervicornus is a small herbaceous lyc-opsid with helically arranged leaves divided into eight segments (FIG. 9.25) that superficially appear similar to those of Leclercqia (C.-S. Li and Hueber, 2000).

Hubeiia dicrofollia is a herbaceous member of the Protolepidodendrales from the Upper Devonian Xiejingsi Formation of Hubei Province in China (Xue et al., 2005). Protoxylem occurs in ridges on the outer surface of the stele and consists of annular tracheids; metaxylem tracheids are scalariform with Williamson striations, delicate, vertical strands of secondary wall material that extend between the scalariform bars, also called fimbrils. These are characteristic of the tracheids in the Lepidodendrales (discussed below). Primary phloem forms a narrow band surrounding the stele. The cortex is relatively thick. Mesarch leaf traces depart directly from the protoxylem. Leaf bases are circular or slightly elliptical in shape and arranged in low helices or in pseudowhorls with the leaf bases alternating. Leaves are

Lycopsid Leaf
figure 9.25 Leaf of Cervicornus wenshanensis with eight segments (Devonian). Bar = 0.5mm. (From C.-S. Li and Hueber, 2000.)

subdivided into four segments as a result of two successive dichotomies.

Another interesting lycopsid from China is Wuxia bis-trobilata from the Upper Devonian (Berry at al., 2003). Compressed specimens of branches are up to 1.4 cm wide and possess sterile leaves inserted in whorls of six. Megasporangiate structures occur at the dichotomies of the axes and consist of elongate megasporophylls, each with a prominent midrib. Spines occur at irregular intervals along the megasporophyll. Megaspores are up to 4 mm in diameter. Morphologically W. bistrobilata shares most features with Minarodendron cathaysiense from the Middle Devonian (C.-S. Li, 1990). Another Chinese lycopsid of Devonian age with megasporophylls, each bearing 4-6 megasporangia, is Chamaedendron multispo-rangiatum (Schweitzer and Li, 1996). Microsporangia are stalked and contain spores of the Longhuashanispora type. Chamaedendron is reconstructed as a narrow tree-like plant lacking secondary xylem. Longostachys latisporophyllus from the Middle Devonian of China is reconstructed based on numerous specimens from the same site. It bears elongate megasporophylls with the distal lamina recurved upward and trichome-like appendages arising from the margin and is believed to represent another small tree-like lycopsid (Cai and Chen, 1996). The stele changes from a protostele, surrounded by secondary xylem near the base, to a medul-lated stele that lacks secondary tissues at higher levels. It is suggested that L. latisporophyllus is intermediate between the herbaceous Protolepidodendrales and the arborescent lepidodendrids. Monilistrobus yixingensis ( FIG. 9.26 ) is a species from the Late Devonian Wutung Formation (Famennian) of Jiangsu, China, that is similar in overall structure to Longostachys and Chamaedendron, but can be

Fossil Lycopsid
figure 9.26 Monilistrobus yixingensis showing partial reconstruction of the fertile axis (Devonian). (From Y. Wang and Berry, 2003.)

distinguished from these two taxa (and from all other fossil and most living lycopsids) based on the occurrence of sporangia on modified, proximally widened sporophylls which are compactly arranged into distinct, cone-like fertile zones separated by vegetative regions with more lax microphylls (Y. Wang and Berry, 2003).

Another plant believed to represent a transitional stage between the Protolepidodendrales and arborescent lycopsids of the Late Devonian and Carboniferous is Zhenglia radiata from the Lower Devonian of southeastern Yunnan, China (Hao et al., 2006). This plant is characterized by undivided microphylls, sporophylls arranged helically to form a compact area resembling a cone, and ovoid-elongate sporangia positioned adaxially on the widened proximal portion of the sporophyll. Leaf scar arrangement is similar to that seen in Lepidodendrales. Another arborescent lycopsid assigned to the Protolepidodendrales is Protolepidodendropsis pul-chra from the Middle and Upper Devonian of Spitsbergen (H0eg, 1942; Schweitzer, 1965, 1999). The largest compressions discovered to date indicate that the stems of this plant, which were originally described as Bergeria mime-rensis by H0eg (1942), reach a thickness of ~10cm and a height of 1.5-2m. Stem surfaces are covered with helically arranged, broad rhombic leaf cushions that vary considerably in size, depending on the position in the stem, and reach 1.5 X 1.2 cm in the proximal portions of the stem. In the middle of each of the leaf cushions is a prominent leaf scar; parichnos scars and ligule pit are lacking. Distally the stems dichotomize to form two branches that, in turn, dichotomize up to five times to form a loose crown. Branch fossils do not display leaf cushions, but rather show only leaf scars, which are spindle shaped, that is pointed at both ends, with a thin ridge from the lower scar tip to the upper tip of the scar below (Schweitzer, 1999). Leaves are small and simple (H0eg, 1942). Protolepidodendropsis frickei, a second species in that genus, has been described from the Upper Devonian of Bogendorf-Liebichau near Waldenburg (today Walbryzch) in Silesia (Poland) by Gothan and Zimmermann (1937). In contrast to P. pulchra, stems of P. frickei remain unknown to date, and Banks (1960) suggested that this plant was herbaceous.

As we learn more about Devonian and Mississippian lyc-opsids, it is becoming increasingly clear that not all of them fit into present classification schemes. Such forms as Linietta and Lycopogenia from the Famennian-Tournaisian of North America (Roy and Matten, 1989), and Trabicaulis and Landeyrodendron from the Montagne Noire (Tournaisian) of France (Meyer-Berthaud, 1984a) underscore the necessity of continuing to reevaluate early lycopsids (Meyer-Berthaud, 1984a).

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