|Reconstruction of Dickinsonia costata|
In 1946, while investigating abandoned mines in the Ediacara Hills, South Australia, geologist Reginald Claude Sprigg stumbled upon some peculiar and barely visible trace fossils of jellyfish-like creatures. He realized that they must be far older than any fossil known at the time. He published his preliminary results in the Transactions of the Royal Society of South Australia (1947, 1948). He then tried to publish his findings in the prestigious journal Nature and made a presentation at the 1948 International Geological Congress, but his results were largely dismissed by the larger scientific community. Sprigg will have to wait until 1959 for his contribution to be recognized when paleontologist Martin Glaessner evaluated the age of the Ediacaran rocks to be Precambrian (~600 MYA) and fully appreciated the importance of the fossils found within. The Ediacara biota consists of a number of truly enigmatic trace fossils of soft bodied creatures that give us a glimpse of what life forms existed before the evolutionary “invention” of readily fossilizable parts such as hard shells.
|Spriggina, another enigmatic creature from the Ediacara biota|
One of the most famous fossils of the Ediacara biota is Dickinsonia, first described by Sprigg in 1947. Dickinsonia fossils were preserved as imprints of ovoid or ribbon- like creatures with bilateral symmetry. Their sizes range from a few millimeters to practically a meter in length. The animal also appeared to be segmented. The affinities of Dickinsonia were and is, still today, highly debated. Is it related to jellyfish (Sprigg, 1947) , corals (Valentine, 1992), comb jellies (Zhang & Reitner, 2006)? Is it a lichen (Retallak, 1994, Retallak, 2007), a sponge, a polychaete worm (Wade, 1972), a giant single cell organism (Seilacher et al., 2003) or something else altogether? In the absence of any discernable proof of affiliation to any modern phylum, a brand new name was erected to encompass these strange precambrian fossils, the Vendobionta (Seilacher,1992; Buss & Seilacher, 1994), that would have evolved before the reign of the eumetazoans (all bilateral animals plus the cnidaria [jellyfish, coral, sea anemones, …] and ctenophores [comb jellies]). The phylum Vendobionta regroups many of the familiar Ediacaran fossils such as Dickinsonia, Spriggina, and Charnia, and they are viewed as immobile creatures, possibly ancestral to the Cnidaria. Some authors have argued that Dickinsonia like other Ediacaran fauna (such as the above mentioned Spriggina) lacked true bilateral symmetry and placed them in a phylum called Proarticulata (Fedonkin, 1987; Fedonkin, 2003) that went totally extinct at the end of The Precambrian and left no descendant. Specimens of Dickinsonia indeed show that the segments do not join symmetrically at the median ridge but are made of portions called isomers that alternate along the longitudinal axis, as if one side of the body is shifted by half a segment period with respect to the other half.
|Charnia looked like modern sea pens but they were|
However extensive examination of hundreds of specimens show that the lack of bilateral symmetry in those individuals is only apparent and is, like the presence of the middle line rim, a preservation artifact (Gehling et al., 2005). Dickinsonia was thus a true bilateral segmented creature. Specimens showing folds and tears also indicate that it was a soft-bodied flexible organism, contradicting earlier claims that only rigid "wood-like" bodies could have been preserved in sandstone (Retallak, 1994). The preservation of series of oval traces “following” the body mold of some Dickinsonia specimens, that are about the same size and shape as the animal itself, also indicates that it was clearly mobile, so the hypothesis of its affinity to a phylum such as sponges or lichens with a sessile life can be rejected. These traces have been interpreted as resting or feeding traces, imprints of shallow depressions left by Dickinsonia as it fed and digest the layer of organic matter underneath. The total absence of anything that look like a digestive system such as a mouth (Gehling et al., 2005) indicates that the animal was most probably feeding through ventral external digestion. And there is actually only one phylum of animals alive today that eats in this way, the Placozoans. So, is Dickinsonia a placozoan (Sperling & Vinther, 2010)? The difficulty of this hypothesis is that modern placozoans,such as Trichoplax adhaerens, appear to be much simpler creatures. Although multicellular, they do look like amoebas, with no apparent bilateral symmetry, no segments and reproduced asexually by budding. However, the genome sequencing of Trichoplax (Srivastava et al., 2008) showed that they belong to the eumetazoans. Cnidarians (jellyfish, sea anemones, corals and the like) do possess a radial symmetry rather than a bilateral symmetry. However molecular and deep morphological clues reveal that cnidarians were originally bilateral (Matus et al., 2006). Thus bilaterality was probably an ancestral feature to all eumetazoans including placozoans. So is Dickinsonia a placozoan? Well, we still can’t say for sure and the debate is far from closed, but that's one definite intriguing possibility.
Acknowledgment: I am indebted to Prof. James G. Gehling who has kindly provided me with first-hand information about Dickinsonia.
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