Genselia compacta (JONGMANS et al.) KNAUS et GILLESPIE comb. nov.:New Insights Into Possible Developmental Pathways of Early Photosynthetic Units
Knaus, Jane Margaret;Gillespie, William H.
published: Feb 6, 2001
A new Lower Mississippian fossil plant locality in southeastern West Virginia, USA yields specimens representing new taxa including numerous specimens assignable to Genselia. These in particular provide new information on the systematics and possible developmental pathways of early plants with frond-like leaves. Specimens are sufficiently complete to allow measurement of within frond variation and sufficiently abundant to provide statistically significant sample sizes, thus providing opportunities to assess w ithin-plant variation and to infer a possible evolutionary progression in the originations of developmental constraints. The several hundred Genselia specimens exhibit ranges of variation that do not correspond to any revised species; the absence of character discontinuities among this suite of specimens implies that they represent one highly variable species. Five species previously described by Jongm ans et al. from specimens collected from Pennsylvania and assigned to Triphyllopteris and Sphenopteridium fall within this range of variation. All specimens are thereby assigned to Genselia compacta (Jongm ans et al.) Knaus et Gillespie comb. nov. Bipinnate fronds of Genselia exhibit a smoothly abscised separation layer at the proximal end of the petiole indicating that they were shed as a unit. Characters that show high levels of variability between fronds are 1) frond breadth, 2) spacing of pinnae along the axis, 3) depth of pinnule dissection; and to a lesser extent, 4) spacing between pinnules, and 5) length of pinnule attachment (stalk). However, within fronds, spacing of pinnae along the rachis, their angles of attachment, and the number of pinnules per pinna remains notably constant resulting in linear frond shapes. We interpret this pattern of constraint vs. indeterminacy, that characterizes the frond architectures of many Lower Mississippian plants, to be intermediate between a shoot system controlled by indeterminate apical meristematic growth and a fully laminate system controlled by determinate marginal meristematic growth. Through comparisons with later Carboniferous and extant leaves, a stepwise progression of evolutionary innovations can be inferred. Based on the premise that relative degrees of developmental constraint are added as terminal additions in a time ordered manner, relative stratigraphic ages can be inferred for the various species of Genselia based on the relative degree of organization of their frond architecture. This may provide a potentially valuable tool for improved resolution of megafossil age determinations within the Appalachian depositional province. If comparable developmental transitions are found to have occurred more or less synchronously across unrelated taxa, analogous to that seen in the case of reticulate venation, then an improved understanding of these grade transitions may provide the groundwork for improved circumequatorial floral zonation for the Mississippian.