The reproductive units of the lepidodendrids consist of stro-bili or cones borne on distal branches in the crown of the tree. In Synchysidendron cones occur on late-formed crown branches, whereas in Diaphorodendron they are borne on deciduous lateral branches (DiMichele and Bateman, 1992). Lepidodendrid cones could reach considerable size (FIG. 9.58), for example cones assignable to Lepidostrobus goldenbergii could be more than 50 cm long. The basic organization of lepidodendrid cones consists of a central axis with helically arranged sporophylls (FIG. 9.59). Sporangia are located on the adaxial (upper) surface of sporophylls which are upturned at their distal ends so that they overlap the sporophylls above. A portion of the lower surface of the sporophyll often extends downward to form a heel or distal extension. A ligule is present in a small pit just distal to the sporangium. Lepidostrobus is the most common designation for lycopsid cones of this type (FIGS. 9.60, 9.61). The name has been used, however, for cones demonstrating all forms of preservation, and for both monosporangiate (having only
one type of spore) and bisporangiate (having two types of spores) forms, so that taxonomic problems within the genus are considerable.
Microsporangiate and Bisporangiate Cones In an attempt to better define the taxonomic limits of arborescent lycopsid cones, Brack-Hanes and Thomas (1983)
suggested that the name Lepidostrobus be used for cones containing only small spores (monosporangiate) and Flemingites for bisporangiate strobili containing both microspores and megaspores (FIG. 9.62). Other authors have used cone morphology and in situ spores to more accurately circumscribe individual species within Lepidostrobus (Bek and Oplustil, 2004, 2006).
One of the oldest lepidodendrid cones is Lepidostrobus oldhamius, known from Lower and Middle Pennsylvanian deposits in both North America and Great Britain. Specimens range from 2-6 cm in diameter and exceed 30 cm in length. Sporangia are massive and, depending on the stage of development, may have an irregularly shaped pad of sterile tissue extending from the sporophyll into the lumen of the sporangium. All of the sporangia contain small (20-30 pm) spores that have a trilete suture on the proximal surface and delicate spines on the distal face. Dispersed spores of Lepidostrobus are often preserved, and one of the common generic names applied to these grains is Lycospora (Willard, 1989). Lepidostrobus shanxiense is a slightly smaller cone from the Xishan Coal Field in China (Wang et al., 1995). An interesting Lepidostrobus cone, L. xinjiangensis, also from China, occurs in Upper Devonian rocks (Wang, Q. et al., 2003a). The discovery of this species suggests that segregation into micro- and megasporangiate cones had already taken place in the lepidodendrids by the Late Devonian.
Flemingites schopfii is a permineralized bisporangiate cone that resembles a massive modern Selaginella strobilus in general organization (Brack, 1970). Specimens are up to 8 cm long and 1.3 cm in diameter. The arrangement of parts and cone anatomy are identical with those of L. oldhamius , except for the presence of two types of spores in F. schop-fii. Distal sporangia (FIG. 9.63) contain a large number of Lycospora-like microspores (FIG. 9.64), some still in tet-rahedral tetrads; more basal sporangia contain 12-29 tri-lete megaspores (Brack-Hanes, 1978). The megaspores range from 700-1250 pm in diameter and are marked by an elongation of the proximal surface into an apical prominence or gula. Dispersed megaspores of this type are called Valvisisporites .
Both compressed and structurally preserved lepidostroboid cones are known from the Fayetteville Shale of Arkansas (Upper Mississippian). These cones, which all appear to have been monosporangiate, may have attained lengths of 22.5 cm. Spores extracted from sporangia of both cone types provide a means of comparing different preservational types. In Lepidostrobus fayettevillense, the small, trilete miospores are characterized by a perforated flange (cingulum), that encircles the spore at the equator (FIG. 9.64) (Taylor and Eggert, 1968). It is probable that apparently monosporangiate cones such as L. oldhamius and L. fayettevillense represent the microsporangiate cones of heterosporous plants. In other instances, however, monosporangiate cones may have been produced by homosporous plants reproductively similar to Lycopodium , in which the spores germinate into free-living gametophytes. In this regard, the neutral term miospore
might be more appropriate than microspore, as the exact function of these spores in the life history of the plant is currently unknown.
Mixostrobus givetensis, from the Middle Devonian of Kazakhstan, is a bisporangiate cone with an irregular arrangement of micro- and megasporangia. In some strobili, one type of sporangium is the dominant type, but this does not depend on the size of the cone (Senkevitsch et al., 1993). Cones assigned to the genus Kladnostrobus from the Pennsylvanian of the Kladno-Rakovmk Basin, Czech Republic, differ from other lycopsid reproductive structures in that the in situ spores are reticulate and resemble the dispersed spore genera Convolutispora, Camptotriletes, Reticulatisporites, and Dictyotriletes (Libertm et al., 2005).
Some lepidodendrid cones are monosporangiate, but produce only megaspores (FIGS. 9.65, 9.66). Possibly the
most common of these Lepidocarpon (FIGS 9.67, 9.68), which was borne on Lepidophloios stems. This cone type is regarded as the most highly evolved reproductive structure in the lycopsids, because the arrangement of the sporophylls closely approximates the function of integuments in seed plants. In Lepidocarpon, sporangia are borne on the adaxial surface of the sporophyll (FIG. 9.69), which consists of two lateral laminae (FIG. 9.70 ) and a distal extension or heel (Balbach, 1962, 1965) (FIG. 9.71) (as in Lepidostrobus and Flemingites ) (Thomas, 1978). The lateral laminae partially envelop the sporangium with only a slit-like opening on the top. Within the sporangium is one large, functional, trilete
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