Isqetales

Isoetaleans are ligulate, heterosporous, and mostly herbaceous in habit. One or two genera of living plants are included in the Isoetales: Isoetes, which has an extensive distribution of almost 200 species ranging from the tropics to the sub-Arctic, and Stylites, which includes two species restricted to the high Andes of Peru. Most authorities regard Stylites as simply another morphologic form of Isoetes. Isoetes is characterized by a short, squat stem (usually less than a few centimeters long) that produces helically arranged, monarch roots from the lower surface and elongated, ligulate leaves in a dense rosette from the upper portion. Both micro- and megaspor-angia are produced on the same plant. Microspores of Isoetes are bilateral and monolete; megaspores are radial and trilete, although many of the fossils placed in this order have tri-lete megaspores and microspores; sporangia have trabeculae (sing. trabecula), sterile plates of tissue that extend into the sporangium. The Isoetales is now recognized as having an extensive fossil history, probably dating back to the Devonian (Pigg, 1992, 2001); the earliest forms with a morphology similar to that seen in the extant Isoetes have been reported from the Triassic (reviewed in Skog and Hill, 1992; Srivastava et al., 2004). Fossils from the Jurassic (e.g., Isoetites rolan-dii; see Ash and Pigg, 1991) are morphologically similar to extant forms in the presence of a bilaterally symmetrical rhi-zomorph. The modern genus exhibits great morphological and genetic uniformity (Schuettpelz and Hoot, 2006).

Several Late Devonian and Early Mississippian forms have been interpreted as representing early members of the isoetalean lineage. These include Clevelandodendron ohioensis, a compressed, almost entire lycopsid plant, 1.25 m tall, from the Cleveland Shale member of the Upper Devonian Ohio Shale (Chitaley and Pigg, 1996). This plant consists of an unbranched, slender, monopodial axis, up to 2 cm wide, arising from a base bearing thick appendages. Terminally the axis bears a bisporangiate strobilus (FIG. 9.114), 9 cm long and 6 cm wide. The decorticated stem surface shows helically arranged elongate leaf traces and laterally compressed, slender leaves along the stem margin. Megaspores obtained from the cone are trilete and laevi-gate, and lack a gula; microspores are trilete, indistinctly punctate, and may be assignable to the dispersed spore genera Calamospora or Punctatisporites. Clevelandodendron demonstrates that lycopsids with a habit similar to the

figure 9.114 Strobilus of Clevelandodendron ohioensis showing sporangia and attachment of sporophylls to cone axis (Devonian). Bar = 2 mm. (From Chitaley and Pigg, 1996.)

Carboniferous genera Chaloneria and Sporangiostrobus (discussed below), and the Triassic Pleuromeia and related forms (see above), were present as early as the Late Devonian. Meyen (1987) suggested that several, of what he termed satellite genera, should be included in the Isoetales. One of these is Tomiodendron (FIGS. 9.27, 9.115) an unbranched, protostelic plant that may have reached 30 cm in diameter and had elongate leaf cushions on the stem surface.

A second lycopsid in this group is Wexfordia hookense from the uppermost Famennian (Upper Devonian) type locality at Sandeel Bay, County Wexford, in southeastern Ireland. This lycopsid has a forked axis (~3 cm in diameter) (Matten, 1989). The permineralized specimens consist of axes with medullated steles; tracheids with scalariform secondary wall thickenings contain fimbrils between the bars. Subsequent research on W. hookense indicates that secondary xylem with uni- to biseriate vascular rays is present in mature axes of Wexfordia (Klavins, 2004). Leaf bases are oval and the crowded leaves are each about 4 mm long. Although initial reports suggested that Wexfordia hookense shared features with anatomically preserved axes of other lycopsids, perhaps including the Protolepidodendrales and Carboniferous Lepidodendrales, the study by Klavins figure 9.114 Strobilus of Clevelandodendron ohioensis showing sporangia and attachment of sporophylls to cone axis (Devonian). Bar = 2 mm. (From Chitaley and Pigg, 1996.)

figure 9.115 Stem of Tomiodendron peruvianum showing leaf cushions (Mississippian). Bar = 1.5 cm. (Courtesy H. W. Pfefferkorn.)

hypothesizes that Wexfordia was a small tree with anatomical features closer to members of the isoetalean lineage.

A third plant with possible affinities in the order Isoetales is Otzinachsonia beerboweri from the Famennian (Upper Devonian) of north-central Pennsylvania (Cressler and Pfefferkorn, 2005). Stems are up to 10 cm wide and the stem base forms a characteristic, four-lobed cormose rhizomorph with rootlets and circular rootlet scars arranged in orthos-tichies. A transitional zone between the rhizomorph (basal portion of the stem) and the distal part of the stem lacks evidence of rootlet scars. Distal stem portions display spirally arranged leaf scars in parastichies, but no leaf cushions.

Carboniferous lycopsids with suggested affinities in the isoetalean lineage include Chaloneria cormosa , one of the most completely known fossil lycopsids within the group. This plant is known from both vegetative and fertile remains from the Upper Pennsylvanian of North America (Pigg and Rothwell, 1979, 1983a, b). The unbranched plant was ~2m tall, with small, ligulate leaves helically arranged about the stem. Leaf cushions were not produced. The base of the plant was cormlike. A limited amount of secondary xylem surrounds an exarch protostele, and in some specimens a thin band of periderm is produced (Pigg and Rothwell, 1985). These authors have been able to correlate various levels of decortication in C. cormosa with axis surface features of Knorria (FIG. 9.42), Asolanus, Bothrodendron, Pinakodendron, Jurinodendron (FIG. 9.43), and Stigmaria. A compressed, cormose lycopsid base of Middle Pennsylvanian age named Cormophyton also exhibits some of the same features as Chaloneria (Pigg and Taylor, 1985). Pigg and Rothwell (1983a,b) placed Chaloneria in its own family, Chaloneriaceae, in which they also included Sporangiostrobus and Polysporia.

Chaloneria cormosa is heterosporous, with the fertile region (~10cm long) consisting of alternating mega-and microsporophylls. Sporangia contain trabeculae. Microspores are monosaccate and of the Endosporites type, whereas the megaspores can be compared to Valvisisporites. Megagametophytes of Chaloneria, some with archegonia, have also been described (Pigg and Rothwell, 1983b). Their structure and embryology are similar to those known in Bothrodendrostrobus, suggesting perhaps that the latter should be allied with the Isoetales rather than the Lepidodendrales (Stubblefield and Rothwell, 1981).

A Pennsylvanian fructification that may be related to Chaloneria is Porostrobus. This bisporangiate cone (2.5 cm long) possesses megasporophylls with hairlike tips, and produced Setosisporites-type megaspores (750-1150 pm in diameter) and Densosporites-type microspores (Leary and

figure 9.116 Stem surface of Bodeodendron hispanicum showing leaf cushions (Pennsylvanian). Bar = 1 cm. (Courtesy R. H. Wagner.)

Mickle, 1989). The distal end of the megaspore is extended into a structure termed a gula that has been interpreted as a germ tube (Jha and Tewari, 2006). Polysporia includes compressed cones with the same types of spores as those in Chaloneria (Grauvogel-Stamm and Langiaux, 1995), but this genus has also been reported as a permineralized specimen (DiMichele et al., 1979).

Sporangiostrobus encompasses relatively large, mono- and bisporangiate lycopsid cones, known from both compression and anatomically preserved specimens (FIG. 9.117-; they are characterized by a massive axis occupying more than half of the total cone diameter (Bode, 1928; Chaloner, 1956, 1962; Remy and Remy, 1975a; Bek, 1996). The basic organization of this cone is similar to that of other arborescent lycopsid fructifications, but differs in having large, trira-diate megaspores characterized by a broad equatorial flange formed by fused and anastomosing hairs (Zonalesporites type) and triradiate microspores (FIG. 9.118) that exhibit an extended range of morphologic variability that includes such dispersed spore taxa as Densosporites , Radiizonates , Cingulizonates, and Vallatisporites, sometimes found in

figure 9.117 Cross section of microsporangium of Sporangiostrobus kansanensis showing parenchyma tissue (Pennsylvanian). Bar = 1.5 mm. (From Leisman, 1970.)
Nathorstiana Arborea
figure 9.119 Sporangiostrobus axis showing sporophyll bases (Pennsylvanian). Bar = 2cm.
figure 9.118 Several microspores macerated from the sporangium of Sporangiostrobus cone (Pennsylvanian). Bar = 50 pm.

the same sporangium (Bek and Strakova, 1996; Bek and Oplustil, 1998). One species from the Middle Pennsylvanian of Kansas, S. kansanensis, measures ~16cm long and is nearly 12 cm in width (Leisman, 1970). Wagner and Spinner (1976) have demonstrated that Sporangiostrobus was the cone of Bodeodendron (FIG. 9.116) based on their constant association and similar morphology. Exceptionally well-preserved specimens of Sporangiostrobus of late Stephanian age from Puertollano, Spain (FIG. 9.119) occur in large numbers in a tuff band at the base of a coal seam (Wagner, 1989) (FIG. 9.120). Some cones exceed 30 cm in length and are hypothesized to have been produced at the tips of vegetative branches. Microsporangia appear to have been intermingled with megasporangia in the cones. Sporangiostrobus has also been correlated with vegetative remains of Omphalophloios (Brousmiche-Delcambre et al., 1995).

Mesozoic representatives of the isoetalean lineage include Nathorstiana, a Cretaceous genus that possesses the same root arrangement and stelar morphology as living members

figure 9.120 Robert H. Wagner.

of Isoetes (Richter, 1909, 1910; Karrfalt, 1984). The genus is known from numerous specimens representing various developmental stages of the plant. M├Ągdefrau's (1932) reconstruction of Nathorstiana arborea depicts the plant as being <20 cm tall, with elongate, grasslike leaves attached to the stem apex on conspicuous bases (FIG. 9.121). The underground parts were bulbous and produced helically arranged roots at the lobed bottom of the plant. Although nothing is known about the internal structure of the plant or the method of reproduction, Karrfalt (1984) hypothesized the developmental pattern of the fossil based on his studies of living Isoetes (Karrfalt and Eggert, 1977a, b, 1978). A detailed morphological study of this type in which fossils are used in association with a living developmental model (Isoetes) represents an excellent way to infer homologies among the various lycopsid rooting structures and thus more accurately determine phylogenies within the group.

Nathorstianella is another Mesozoic fossil that has historically been included in this order based on cast specimens from the Lower Cretaceous of Australia (Glaessner and Rao, 1955). Although it differs in size, several features suggest a close correspondence with Isoetes. Karrfalt (1986) confirmed the isoetalean relationship, indicating that N. babbagensis was divided into five basal lobes, each bearing roots arranged

Nathorstiana
figure 9.121 Suggested reconstruction of Nathorstiana arborea (Cretaceous). (From Delevoryas, 1962.)

like those in extant Isoetes. He believes that the basal corm supported an arborescent aerial axis.

Several fossils that morphologically resemble modern Isoetes have been described from rocks as early as the Triassic, and the generic name Isoetites has been used for many of these (Pigg, 2001), whereas others have been assigned to the extant genus Isoetes (see Srivastava et al., 2004). Isoetites serratifolius is the name used for compressed sporophylls from the Triassic of India (Bose and Roy, 1964); they extend up to 6.6 cm in length and are characterized by a serrate margin. A single vascular bundle extends the length of the sporophyll. The position of the sporangium is indicated by an elongated impression near the base of the sporophyll. In another species of the same age, I. indicus, sporophylls with megasporangia were preserved. The sporophylls are slightly smaller than those of I. serratifolius. Megasporangia contain up to 1500 trilete spores ranging 285-430 pm in diameter. Isoetites rolandii occurs in the Middle Jurassic of Oregon and Idaho (Ash and Pigg, 1991). Because the specimens are preserved as both mold-casts and impression-compressions they offer an excellent example of how different preservation types can be used to reconstruct an entire plant. The plants are ~10cm tall and consisted of linear leaves, up to 8 cm long. Casts of sporophyll bases contain structures interpreted as megaspores. What is especially noteworthy is that this discovery represents the oldest occurrence of Isoetes-like plants in western North America. The authors also noted that the occurrence of Isoetites provided the first evidence for an aquatic to semiaquatic environment within the Coon Hollow Formation.

Isoetites serratus is a well-known form from Upper Cretaceous rocks (Frontier Formation) in Wyoming (Brown, 1939). The plant consists of rosettes of narrow, spathulate leaves with serrate margins; the leaves arise from the edge of a round corm that is 1.3 cm in diameter. The upper surface of the sporophyll has two rows of rectangular cavities interpreted as either wrinkles or the remains of collapsed internal air sacs. At the base of each sporophyll are compressed elliptical sporangia that contain impressions of either megaspores or microspores. Dichotomously branched roots are attached to the corm base beneath the outer rosette of sporophylls.

Isoetes ermayinensis is a Triassic isoetalean from China that includes both vegetative and reproductive material (Z.-Q. Wang, 1991). The plant is small, with leaves up to 7 cm long, each with an elongate, lacunate band on either side of the vascular bundle; the corm is unknown. Megaspores are of the Dijkstraisporites type (50-200 megaspores per sporangium), whereas the microspores are assignable to Aratrisporites. Mature megaspores in stages of germination morphologically resemble Laevigatisporites .

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