THE ENIGMATIC DEVONIAN FOSSIL PROTOTAXITES IS NOT A ROLLED-UP LIVERWORT MAT: COMMENT ON THE PAPER BY GRAHAM ET AL. (AJB 97: 268-275)

摘要

It should not be surprising that there are numerous examples of fossil organisms for which there are no known direct modern analogues. The seed ferns of the Paleozoic and Mesozoic, arborescent sphenophytes, and a number of Silurian and Devonian organisms sometimes placed in the artificial group Nematophytales (Lang, 1937) or Nematophyta (Strother, 1993) represent examples of such organisms. These organisms either combine morphological and anatomical features not found in the modern biota or are constructed differently from any other living or fossil life form (Edwards, 1982). The nematophytes are undoubtedly among the latter, and the biology and systematic affinities of these organisms have remained controversial since their first discovery more than 150 years ago (Hueber, 2001; Lindsay, 2005). In nematophytes that are structurally preserved, it can be seen that they are composed entirely of tubes of various size, shape, and orientation (Taylor and Wellman, 2009); some of the tubes are characterized by relatively complex cross walls with a roofed central pore resembling a dolipore or parenthesome (Schmid, 1976; Hueber, 2001; Taylor et al., 2009, fig. 6.9). One of the most unusual of these organisms is Prototaxites, a fossil that was initially described as partially degraded gymnosperm wood based on silicified specimens from the Gaspé of Canada (Dawson, 1857, 1859; Carruthers, 1872). Fossils interpreted as Prototaxites have been reported as compressed, coalified remains (Boyce et al., 2007), but the most interesting are silicified specimens that occur as "logs," which may be more than 1 m in diameter and 8 m long (Hueber, 2001). Hypotheses as to the affinities of these fossils have included seed plants (Dawson, 1859), several algal types (e.g., Kr?usel, 1936; Schweitzer, 2003; Niklas and Smocovitis, 1983), an early, terrestrial evolutionary dead end (Lang, 1937; Abbott et al., 1998), a lichen-like association of a fungus and an autotrophic carbon source (Selosse, 2002), and some type of terrestrial saprotrophic organism, i.e., a fungus, with affinities perhaps closest to the Basidiomycota (Church, 1919; Hotton et al., 2001; Hueber, 2001). Likewise, biomarkers and carbon-isotope signatures, while not identifying the organism, do suggest some type of heterotrophic nutritional mode (Boyce et al., 2007). The most recent suggestion is that Prototaxites represents extensive mats of a liverwort similar to modern Marchantia that were rolled up by wind, gravity, or water to form the so-called logs that are found silicified in Silurian–Devonian rocks (Graham et al., 2010). Based on a number of factors, including the anatomy of Prototaxites trunks, their mode of preservation as fossils, and the environment in which they lived, we take exception with the opinion of Graham et al. (2010) and with some of the methodology used to develop their interpretations as to the affinities of Prototaxites.