Although a great deal of information is known about Protosalvinia (initially named Foerstia), the biological affinities of this Devonian organism remain problematic (Niklas and Phillips, 1976; Niklas et al., 1976; Schopf, 1978b ; Gray and Boucot, 1979). Like Spongiophyton, it has been interpreted as a fern, alga, bryophyte, or some form of semiaquatic organism. It is typically found compressed (FIG. 6.15 ), but may assume a variety of morphologic shapes, ranging from nearly circular to clavate. Some of the largest specimens approach 1 cm in diameter. At least three taxa, P. arnoldii , P. ravenna, and P. furcata ( FIG. 6.16 ), have been suggested as representing different growth forms of a single biological species. According to this developmental chronology, the thallus becomes more clavate and develops depressions
containing spores as development progresses (Phillips et al., 1972). On the surface of the thallus are cell patterns that suggest that thalli dichotomized, and specimens with an apical notch are known.
At the tips of the thallus are slight depressions (FIG. 6.17), termed hypodermal conceptacles, which contain tetrads (FIGS. 6.18, 6.19) of large (200 pm), thick-walled spores (Niklas and Phillips, 1976). Each conceptacle is slightly <0.5 mm in diameter and constructed of two distinct cell layers. Spores are large and thick-walled (FIG. 6.20). In the Curiri Formation (Famennian) of the Amazon Basin, Melo
and Loboziak (2003) correlated spores of the sporae disper-sae genus Retusotriletes with Protosalvinia, where they characterized a distinct biostratigraphic unit throughout the basin. It has been debated whether the spores represent meiotic
or mitotic products (FIG. 6.20), and thus how they may have functioned in the life history of the organism. The spore wall ultrastructure suggests that they are the result of meiosis, even though they do not appear to have been constructed of sporopollenin (W. Taylor and T. Taylor, 987). The presence of tetrads of spores does not unequivocally establish Protosalvinia as a land plant, since some red and brown algae also produce spores and eggs that are morphologically similar to the spores recovered from Protosalvinia , but the similarity does not extend to the ultrastructural level. The discovery of lignin-like compounds in the fossils suggests land plant affinities for Protosalvinia (Romankiw et al., 1988), but it has also been interpreted as related to the brown algae based on biochemical evidence (Niklas, 1976b). Others consider Protosalvinia inhabited to be an alga that shows some land-adapted features, which may result from convergent evolution at a time when land plants were first becoming established. Gutschick and Sandberg (1991) suggested that Protosalvinia inhabited brackish coastal swamps. Despite all of the tools available to paleobotanists to examine the nature of the organic matter in Protosalvinia, the biomo-lecular signature is neither distinctly marine nor terrestrial (Mastalerz et al., 1998). Some Protosalvinia specimens have been suggested to represent parts of the cephalopod Sidetes (Hannibal, 1994).
Although the biological affinities of Protosalvinia may remain unclear, it has served as a significant index fossil, that is the Protosalvinia Zone, within Upper Devonian black shale sequences of the eastern United States (Schopf and Schwietering, 1970; Murphy, 1973; Schwietering and Neal, 1978). It has also been useful for stratigraphic correlation
among Devonian rocks of the Michigan, Illinois, and Appalachian Basins (Matthews, 1983). More recent studies, however, have shown that the age of the Protosalvinia Zone locally varies from middle to late Famennian, which reduces its value as an index fossil and correlation aid. As noted earlier, a Protosalvinia Zone has also been documented in the Amazon Basin of northern Brazil (Grahn, 1992; Loboziak et al., 1997; Melo and Loboziak, 2003).
Another Late Silurian-Early Devonian thalloid organism of uncertain affinity is Parka decipiens (Fleming, 1831; Reid et al., 1897; Don and Hickling, 1917; Neuber, 1979). This fossil attracted the attention of biologists because of its morphological similarity to some members in the Coleochaetales, an extant order of green algae that many believe is significant in deciphering the ancestry and origin of land plants (discussed below). Specimens of P. decipiens are ~7 cm in diameter with an oval outline and slightly undulating margin (FIG. 6.21). Impressions of cells on the surface of some compressed specimens suggest that the underlying tissue was pseudoparenchymatous, and growth simulation models suggest that the thallus grew by means of both apical and anticlinal intercalary growth (Niklas, 1976c). On the surface of the thallus are disk-shaped structures which have been interpreted as some type of sporangium (FIG. 6.22); many contain small (25-45 pm) compressed bodies thought to represent spores. None possess haptotypic features such as a trilete mark. At the ultrastructural level, the wall consists of lamellae of various thicknesses (Hemsley, 1989b) similar to those in the spores of certain extant liverworts (Hemsley, 1990). Parka has also been suggested as a potential ancestor to Coleochaete in the transition to a terrestrial habitat (Niklas, 1976c). Although this hypothesis is intriguing, major obstacles remain, including the time gap of more than
400 million years between the two taxa, together with uncertainties regarding the life history of the fossil. Moreover, antheridia have not been found in P. decipiens, and if these two taxa are similar, the presumed spores in Parka would actually represent zygotes.
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