|The Proterozoic Eon|
|Proterozoic||Proterozoic - 3|
We come, at last, to the most difficult and contentious of the Ediacaran animal issues. The February 6, 1998 issue of Science carried a brief report by Li et al. describing some microfossils from phosphorites of the Ediacaran Doushantuo Formation, at a site in Wengan (or Weng'an ) County in central Guizhou Province. The main point of the paper was that these Ediacaran remains included monaxonal sponge spicules (suggesting demosponges) and small structures interpreted as early sponge embryos and eggs. Then, Chen et al. (2000) (a large group including two of the three authors of Li et al., 1998) reinterpreted some of the spherical objects as specifically bilaterian embryos. This announcement generated a remarkable amount of press coverage; and, more to the present point, it opened a scientific free-for-all which continues to this day. Researchers have since referred the putative embryos to at least four different Linnaean kingdoms and any number of animal phyla. Some have argued that the embryos are pseudofossils and have no place on the Tree at all.
These embryos -- if that is what they are -- pose baffling problems of interpretation. Many of the authors who have addressed the issues are names to conjure with. However, many, perhaps most, have changed their views, or have at least heavily qualified them during the course of the last few years. For example, Prof. Shuhai Xiao has moved from being an unbeliever (Xiao et al., 2000) to championing a very specific cnidarian affinity for at least some of the specimens (Xiao et al., 2007). Prof. Whitey Hagadorn was one of the authors of Chen et al. (2000). However, he has become an apostate, relegating most specimens to "stem metazoan" status (Hagadorn et al., 2006) and co-authoring Xiao's (2007) paper assigning others to the Cnidaria. These examples might suggest convergence of scientific opinion on a cnidarian interpretation. In part, that seems to be the case. However, two groups have recently published thought-provoking arguments that the embryos are actually bacterial (Bailey et al., 2007) or protistan (Matz et al., 2008) structures.
We are reluctant to get involved in these issues. In addition to overcoming our usual handicaps of indolence and ignorance, a reasonable explanation of the problems would require a small book, possibly even a large one. Yet that book could come to no firm conclusion, and it isn't obvious to us that the project is important enough to merit the investment. The internal anatomy of these structures is very poorly preserved (Donoghue et al., 2006), dominated by phosphorite artifacts and inconsistent from specimen to specimen. Any real evolutionary signal may well be hopelessly swamped by taphonomic noise. Phosphorites can create an alarming variety of morphological artifacts in the process of phosphatization (Chen et al., 2002; Bengston & Budd, 2004) -- a problem made worse by the presence of two or three distinct modes of phosphatization at Doushantuo (Dornbos et al., 2005; Dornbos et al., 2006). In addition, occasional specimens preserved in chert (i.e., silicified, rather than phosphatized) are found in the same beds. Yin et al. (2004).
One reason for this variety is that the the embryo fossils are found in intraclasts. Intraclasts are typically grains of partially fossilized material ripped from the sea floor, or from some nearshore mudflat, during a storm or other high-energy event and thoroughly mixed with everything else which is being churned around at the same time. In the case of Doushantuo, the grains were deposited by storms above the fair-weather wave base (the depth at which normal waves don't stir things up). Zhou et al. (2002). As a result the intraclasts were knocked around, (presumably) abraded, and further mixed by wave action before finally settling again.
Plainly, no one has a really good handle on the environment(s) in which the embryos developed, or what their various histories may have been. Like these fossils, scientific opinion in recent years has been thoroughly mixed by high-energy currents, subjected to all kinds of alterations, and -- unlike the fossils -- has not yet had an adequate opportunity to settle and consolidate. Consequently, we will defer further discussion of these odd varmints (the fossils, not the scientists) to another day. ATW081219.