Lepidqdendrales

The Lepidodendrales includes the arborescent lycopsids that were the most conspicuous elements of the Carboniferous landscape. Members of this group are responsible, to a large extent, for the extensive quantities of plant material that resulted in the formation of Carboniferous coal seams around the world. It is estimated that up to 70% of the biomass in the extensive Westphalian coal-swamp forests of Euramerica was produced by members of the Lepidodendrales (DiMichele et al., 1985). Toward the end of the Westphalian, however, their numbers were in decline and, in the Stephanian epicontinental swamp forest ecosystems, these plants are responsible for merely ~5% of biomass production (see Kerp, 2000 and references therein). At the end of the Carboniferous, most arborescent lycopsids become extinct in Europe and North America, and are replaced by tree ferns that, for the first time in geologic history, formed a relatively closed forest canopy; in China, however, the arborescent lycopsids persist into the Middle Permian. It is suggested that the disappearance of the Euramerican Lepidodendrales toward the end of the Carboniferous was due to climate change (DiMichele and Phillips, 1996a). Others hypothesize that the Variscan tectonic activity considerably reduced the size of the coal-swamp ecosystems, which led to more dynamic environments and unstable conditions that were no longer suitable for the lepidodendrids (Kerp, 1996, 2000). Some authors have combined these individual assumptions and suggested that climatic changes were caused by dramatic changes in the floral composition that, in turn, were triggered by tectonic activity (Cleal and Thomas, 2005).

Because of the large number of specimens collected, and the variety of ways in which they were preserved (FIG. 9.27), members of the Lepidodendrales are easily the best understood of all fossil lycopsids. The occurrence of structurally preserved members of the order, as well as the extensive stratigraphic distribution of the group, have provided pale-obotanists with material to address a variety of geologic and biological aspects of these coal-swamp giants. The great diversity of fossil material has provided exceptionally detailed knowledge not only of the reproductive biology and developmental stages of the vegetative parts of these plants, but also of the role of these plants within the paleoecosystem and the structure of the coal-swamp ecosystem through time. Members of the Lepidodendrales are ligulate and characterized by the presence of secondary xylem, extensive periderm development, a three-zoned cortex, spirally arranged rootlike appendages (stigmarian rootlets) with a monarch vascular strand, and a single functional megaspore per megasporan-gium, which germinates within the sporangium (DiMichele and Bateman, 1996).

Historically specimens preserved in many different modes, for example impressions, compressions, and structurally preserved fossils, were included in Lepidodendron, a genus defined principally on features of leaf cushion external morphology. To alleviate ambiguity and provide better resolution of the sys-tematics and diversity of the arborescent lycopsids, DiMichele (1985) established Diaphorodendron for some structurally preserved specimens previously placed in Lepidodendron , including D. vasculare, D. scleroticum, and D. phillipsii. Later, Diaphorodendron was divided into two genera, Diaphorodendron and Synchysidendron, and placed in its own family, the Diaphorodendraceae (DiMichele and Bateman, 1992). Synchysidendron includes two species, S. dicentricum (formerly Lepidodendron) and S. resinosum (DiMichele and Bateman, 1992). According to DiMichele and Bateman (1996), synapomorphies of the family include a medullated

figure 9.27 Base of a stem of Tomiodendron peruvianum in growth position (Mississippian). (Courtesy H. W. Pfefferkorn.)

protostele (siphonostele) and a megasporangium that is dor-siventrally flattened with proximal dehiscence; the megaspore has a massa. Synapomorphies of the Lepidodendraceae (Lepidodendron, Lepidophloios, and Hizemodendron) include a bilaterally flattened megasporangium with distal dehis-cence and infrafoliar parichnos that extends below the leaf scar (discussed later), among other characters (DiMichele and Bateman, 1996). Today, the generic names Diaphorodendron and Lepidodendron are not only used for stem segments exhibiting cellular preservation but also to encompass the concept of entire plants (FIG. 9.28) , including leaves, underground parts, reproductive organs, and a variety of other parts (e.g., decortication stages, spores, isolated sporophylls). Each of these plant organs is assigned both generic and specific names, as relatively few of them have been found organically attached to each other. The generic name Lepidodendron is retained for impression-compression specimens that possess a particular type of leaf cushion morphology in addition to

Fossil Lycopsid
figure 9.28 Suggested reconstruction of an arborescent lycopsid (e.g., Lepidodendron). (Courtesy D. A. Eggert.)

certain anatomical features. Specimens of Lepidodendron have been discovered that indicate some trees attained heights in excess of 40 m and were at least 2 m in diameter at the base (Thomas and Watson, 1976). In the midconti-nent of North America, the genus appears to have reached its zenith at the end of the Middle Pennsylvanian (Phillips et al., 1974). The massive, erect trunks of some Lepidodendron species branched profusely to produce large crowns of leafy twigs (FIGS. 9.29, 9.30). Some leaves were at least 1 m long (most were much shorter) and, when they dropped from the branches, conspicuous leaf bases were left on the stem surface.

figure 9.29 Leafy lycopsid twig (Pennsylvanian). Bar = 2 cm.
Lycopsid Leaf
figure 9.30 Compression of Lepidodendron lycopodites (Pennsylvanian). Bar = 2.5 cm.

Strobili were borne at the tips of distal branches or in a zone at the top of the main trunk. The underground portions of the Lepidodendrales (sometimes called a stigmarian rhizomorph) consisted of dichotomizing axes that bore helically arranged, lateral appendages that presumably functioned as roots.

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