Isoetes Sporangium

figure 9.68 Compressed Lepidocarpon cone (Pennsylvanian). Bar = 1 cm.

megaspore and three aborted spores. The wall of the megaspore (called Cystosporites when found dispersed) is unique in that it is constructed of loosely arranged strands of sporopollenin. In some specimens, cellular megag-ametophytes are preserved, a few containing archegonia. Embryos have been described from Lepidocarpon cones and they are ellipsoidal, unvascularized, and characterized by an isoclinally folded epidermis (Phillips et al., 1975). As the embryo develops, there is a dichotomy of the vascularized axis; one branch of the dichotomy develops into the stigmarian rooting system and the other into the aerial stem. In more mature embryos, what has been termed secondary

figure 9.68 Compressed Lepidocarpon cone (Pennsylvanian). Bar = 1 cm.

figure 9.69 Longitudinal section of Lepidocarpon sporangium (below) and transverse section (above). Arrows indicate lateral laminae (Pennsylvanian). Bar = 5 mm.
Lepidocarpon

figure 9.70 Cross section of Lepidocarpon lomaxi megaspo-rangium showing lateral laminae, megasporangium, and multicellular megagametophyte (From Hirmer, 1927.)

figure 9.71 Margaret K. Balbach.

figure 9.70 Cross section of Lepidocarpon lomaxi megaspo-rangium showing lateral laminae, megasporangium, and multicellular megagametophyte (From Hirmer, 1927.)

tissues are present. When only the lepidostroboid sporo-phyll is preserved in the compressed state, the generic name Lepidostrobophyllum is sometimes used (FIGS 9.72, 9.73).

Embryos have also been found in the cone Bothrodendrostrobus (FIG. 9.74) (Stubblefield and Rothwell, 1981). In the earliest stage of development, the embryo consists of an unvascularized globular structure embedded within megagametophyte tissue. In more mature specimens, two vascularized appendages extend through the trilete suture, one representing the first shoot, the other the first root. It was initially suggested that embryology in Lepidocarpon and Bothrodendrostrobus was sufficiently different to demonstrate two evolutionary paths within the lycopsids (Stubblefield and Rothwell, 1981). What was originally interpreted as bipolar development in Bothrodendrostrobus is now considered to be a derived condition of the unipolar developmental pattern demonstrated in Lepidocarpon (Rothwell and Erwin, 1985).

Another megasporangiate cone that had an unusually ornamented megaspore is Caudatocorpus . This cone type was apparently monosporangiate, with helically arranged megasporophylls lacking lateral laminae (Brack-Hanes, 1981). The sporangium is large and the wall is constructed of columnar cells. Each sporangium contains a tetrad of megaspores with one large (FIG. 9.75 ), presumably

Isoetales Fossils
figure 9.72 Detail of Lepidostrobophyllum showing sporangium attachment scar (arrow) (Pennsylvanian). Bar = 1 cm.

figure 9.73 Compressed Lepidostrobophyllum showing the point of sporangium attachment (arrow) (Pennsylvanian). Bar = 1 cm.

functional spore (<4mm long) and three smaller (200500 pm) aborted spores. If dispersed, these spores would be included in the genus Lagenicula (Scott and Hemsley, 1993). The sporoderm of the functional megaspore is ~10pm thick and has two layers. The outer surface is covered with numerous spines about 50 pm long; on the proximal surface is a conspicuous apical prominence. The four spores are enclosed in a granulose spongy structure that represents a

figure 9.74 Bothrodendrostrobus embryo showing two appendages extending from spore wall. Arrow indicates vascular element (Pennsylvanian). Bar = 150pm. (From Stubblefield and Rothwell, 1981.)

figure 9.75 Section of Caudatocorpus arnoldii megaspor-angium showing spiny megaspores of the Lagenicula type (Pennsylvanian). Bar = 400 pm. (From Brack-Hanes, 1981.)

figure 9.73 Compressed Lepidostrobophyllum showing the point of sporangium attachment (arrow) (Pennsylvanian). Bar = 1 cm.

figure 9.75 Section of Caudatocorpus arnoldii megaspor-angium showing spiny megaspores of the Lagenicula type (Pennsylvanian). Bar = 400 pm. (From Brack-Hanes, 1981.)

distal, winglike attachment to the large functional spore. This extension may have been involved somehow in dispersal, or it may have functioned to orient the apical prominence of the trilete suture so as to enhance the possibility of fertilization.

Achlamydocarpon is a monosporangiate cone with reduced lateral laminae and a single large, functional megaspore in each sporangium. Species of both Lepidodendron and Diaphorodendron are known to have borne this cone type (DiMichele, 1985). In Achlamydocarpon, the orientation of the trilete suture is toward the cone axis rather than away, as in Lepidocarpon. The suture of the functional spore in Achlamydocarpon is covered by a massa (FIG. 9.76) or cap of sporopollenin that may have functioned to protect the developing gametophyte and perhaps to help retain moisture in the region of the suture (Taylor and Brack-Hanes, 1976). In Lepidocarpon cones, this protection could have been provided by the conspicuous lateral laminae of the sporophyll, whereas in Achlamydocarpon, the developing megagameto-phyte may have been protected by both the reverse orientation of the proximal suture and the sporopollenin cap on the megaspore. Microsporangiate cones are assigned to A. var-ius on the basis of similarities in epidermal structure, pedicel alations, and other histologic details (Leisman and Phillips, 1979). The trilete microspores average 64 pm in diameter and exhibit scattered papillae over their distal surfaces, which may represent tapetal residues in the form of orbicules. If found dispersed, such grains would be included in the genus Cappasporites (Ravn et al., 1986). Achlamydocarpon pingquanensis is a megasporangium-sporophyll unit from the Lower Permian of China (Y. L. Zhou et al., 2006). The structure is ~ 1.6 cm long and contains a large, presumably functional megaspore of the Cystosporites type, and three

figure 9.76 Aborted megaspore of Achlamydocarpon varius with proximal massa (arrow) (Pennsylvanian). Bar = 250 pm.

smaller abortive spores, each with a massa near the proximal suture. Based on the organization of A. pingquanensis , it is suggested that this disseminule adds additional support to the idea that there is a group of arborescent lepidoden-drids from China that is distinct from those in Euramerican Carboniferous deposits.

A single large megaspore within each sporangium is also a feature of the cone Suavitas imbricata collected from Upper Pennsylvanian marine deposits of Texas, USA (Rice et al., 1996). The cone is permineralized and the sporangium is located at the distal end of the sporophyll. Although the affinities remain conjectural, the analysis of characters suggests some relationship with the rhizomorphic lycopsids.

Achlamydocarpon is believed to represent the cone of several different species of Diaphorodendron (DiMichele, 1981, 1985), based on an analysis of several hundred permineralized specimens of Diaphorodendron from Lower and Middle Pennsylvanian rocks. DiMichele recognizes different morphological groups among the Euramerican forms of lep-idodendrids, each distinct relative to reproduction, habitat, and evolutionary history. One group, consisting of D. vasculare, D. scleroticum, and D. phillipsii, included trees 8-20 m tall (FIG. 9.77) that had deciduous lateral branches bearing cones of the A. varius type. Synchysidendron trees were smaller (from 10 to 15 m tall) and produced A. varius cones, but these occurred in large numbers near branch tips toward the end of the growing season. DiMichele (1981) suggested that the coal-swamp environments may have acted as evolutionary refugia for some of the arborescent lycopsids, such as Diaphorodendron and Lepidophloios, a habitat preference no doubt dictated by their reproductive biology, which was well adapted for aquatic dispersal. DiMichele (1980) also speculated that speciation in this group may have taken place outside the swamp habitat.

Arborescent lycopsid megaspores of several types have been examined at the fine-structural level in an attempt to determine the affinities of the spores and also to investigate the development of the spore wall (T. Taylor, 1974). Wilson Taylor (1990) was able to correlate megaspore ultrastructure with the dispersal strategy in these lycopsids. His study indicates that some of the Carboniferous megaspores share both developmental and dispersal features with some modern species of Selaginella. Others appear to possess a uniquely organized sporoderm pattern (FIG. 9.78) that reflects the degree to which megaspores enlarge within the sporangium. W. Taylor, (1989) distinguished three basic types of construction (laminar, laterally fused spherules, and ordered units) in the walls of Selaginella. All megaspores possess an

Diaphorodendron
figure 9.77 Suggested reconstructions of several arborescent lycopsids (left to right): Diaphorodendron vasculare, D. scleroticum, D. phillipsii, and Synchysidendron dicentricum. (From DiMichele, 1981.)

inner separable layer, which may be involved in regulating water balance. It is also important to characterize megaspore wall development at the ultrastructural level for its systematic value (Hemsley and Scott, 1989; Hemsley and Galtier, 1991). This type of study is useful in identifying the parent plants of dispersed spores, and the floral composition of particular assemblages where megafossils are absent or poorly preserved. Ultrastructural studies also provide important information about the development and evolution of lycopsid spore walls (Glasspool et al., 2000).

figure 9.78 Detail of sporopollenin units forming the wall of a functional Lepidocarpon megaspore (Pennsylvanian). Bar = 3 pm.
figure 9.79 Polar view of Flemingites schopfii arche-gonium (arrow) showing four neck cells (N) (Pennsylvanian). Bar = 100 pm.

Gametophytes

Knowledge about the gametophyte generation of the lepi-dodendrid arborescent lycopsids is generally meager, and is based on only a few specimens (Renault, 1893; Gordon, 1908, 1910; MacLean, 1912). One interesting feature of Flemingites schopfii cones (discussed earlier) is the exquisite preservation of both the micro- and megagametophyte phases (Brack-Hanes, 1978). Within some of the megaspores near the trilete suture is a parenchymatous, cellular meg-agametophyte (FIG. 9.79 ) . Some of the surface cells of the megagametophyte bear elongated tufts of rhizoids that extend from the trilete suture and actually penetrate the sporangium

figure 9.80 Polar view of several archegonia inside arborescent lycopsid megaspore (Mississippian). Bar = 80 pm. (Courtesy J. Galtier.)

wall. Other megagametophytes possessed archegonia at the time of fossilization, and several archegonial necks have been described interspersed among the rhizoids. Archegonia have from one to three tiers of neck cells (FIG. 9.82) and, in a few specimens, an enlarged cell, suggestive of an archegonial venter, occurs beneath the neck cells. Some microspores in the distal sporangia in F. schopfii reveal stages in the development of the microgametophyte, including partitions suggestive of the antheridial initial and prothallial cells. Some contain material that morphologically resembles chromosomes (Brack-Hanes and Vaughn, 1978) (FIG. 1.25) . When compared with the gametophytes of existing lycopsids, F. schopfii has microgametophytes that are more similar to those of extant Selaginella, whereas the structure of the megagametophytes more closely corresponds to that of Isoetes.

Other well-preserved lepidodendrid megagametophytes (FIGS. 9.80-9.82) have been discovered in spores assignable to Lepidodendron esnostense or L. rhodumnense that occur in late Visean (uppermost Mississippian) cherts from central France (Galtier, 1964b ) 1970a, b); these occur in dispersed spores as well as in specimens still within the megasporan-gium (FIG. 9.81) . The megagametophytes are multicellular structures that develop inside the megaspore wall (endosporic gametophyte development). As they mature, however, they protrude from the spore in the region of the trilete suture, where they form a mass of tissue, within which several arche-gonia are produced (FIG. 9.82); rhizoids appear to be lacking. Archegonia are embedded in gametophytic tissue, with only the uppermost ring of neck canal cells protruding from the surface. The microgametophyte of this taxon remains unknown to date.

Fossil Lycopsid
figure 9.81 Gametophyte tissue inside arborescent lycopsid megaspore (arrows) still inside megasporangium (white arrow) (Mississippian). Bar = 330 pm. (Courtesy J. Galtier.)
figure 9.82 Megagametophyte tissue containing several archegonia (arrows) rupturing lycopsid megaspore wall (S). Bar = 100 pm. (Courtesy J. Galtier.)
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