The best-known Ediacaran fossil, Dickinsonia, promises to provide a tremendous amount of information concerning the paleobiology of these organisms. If only we knew what it was. Dickinsonia has the distinction of being the only fossil to be described as a jellyfish, a coral, a sea anemone, an annelid worm,58 a polychaete worm, an arthropod, a bacterium, a protozoan, a member of a new phylum, a member of a new kingdom, and even an alien creature from outer space. In 1992 Rudolf Raff asked seven colleagues to identify what type of organism Dickinsonia was. He received seven different answers.59
My first encounter with specimens of Dickinsonia came as a graduate student in the early 1980s at the University of California at Santa Barbara in what was then the Biogeology Clean Lab.60 The central corridor of the building is lined with gray steel specimen cabinets, holding Cloud's collection of Cambrian and Precambrian fossils. Cloud had a few specimens of Dickinsonia from Australia. They weren't particularly good or complete specimens, but they did give a good sense of what such fossils were supposed to look like, permanently impressed into the yellowish-tawny colored Pound Quartzite.61
My next encounter came in 1988, when I convened, for the American Association for the Advancement of Science, a symposium at the annual meeting in Boston titled "The Dawn of Animal Life or Aliens Here on Earth? Paleobiology of the Ediacaran Fauna." Richard Jenkins, whom I had invited to speak at the meeting, handed me a plaster cast of a small slab of quartzite bearing some of the best-known specimens of Dickinsonia costata. This cast is a marvelous thing to behold (figure 2.18); my students and I spend hours looking at it, trying to unleash its secrets. The slab has two large dickinsoniids, a number of juvenile specimens, and several specimens of the small Ediacaran Parvancorina.
Here is how a formal paleontological description of Dickinsonia might be written:
Dickinsonia is oval in shape, broad and flat. It is bilaterally symmetric, with a plane of symmetry bisecting the oval along its long axis. In some specimens there is a raised ridge running along the
symmetry line. Both halves of the creature are divided into tubular partitions that run approximately perpendicular to the plane of symmetry at the point where they meet the midline. Moving away from the midline, the tubular partitions become wider and curve gently toward the nearest end of the flat oval body. Adjacent tubular partitions are fused along their lateral edges for almost all of their length. At one end of the oval the tubes are long and constitute approximately one-third of the length of the midline of the organism. At the end opposite to this one, the tubular structures are much shorter. There may be an inward indentation in the perimeter of the oval at this end. The end of the oval with shorter tubules is presumably the end at which new tubular partitions are added during growth. The margins of the oval may show concentric wrinkling.
Four species of Dickinsonia are known: D. costata, D. lissa, D. tenuis, and now (with apologies to Tyrannosaurus) D. rex. Thus, dickinsoniids have more well-delineated species than any other member of the Australian Ediacaran assemblage.
D. rex is 43 cm long, appropriately described by Richard Jenkins as looking like a beaver's paddle. The type specimen of D. rex has a pronounced medial ridge, interpreted by Jenkins as the animal's lower intesti nal tract, although in light of Seilacher's perspective this interpretation is controversial. Jenkins claims to be able to identify a fossil mouth in one dickinsoniid specimen, but the photograph has been retouched with pen and ink and it is difficult to say whether this single specimen is reliable.62
Much discussion has focused on the flexibility of Dickinsonia and its ability to exchange gas and nutrients through its cuticle. A major unsolved problem is how the cuticle could be thin and soft enough to be flexible and easily contracted, yet firm enough to stand up to the grains of a sandstone under the crushing pressures of rock lithification and, assuming it absorbed food, admissive of nutrients dissolved in water.
It is also difficult to determine which end of a Dickinsonia is its head and which is its tail, a major reason that the worm interpretation of its affinities has not been accepted by all paleontologists. Jenkins assumes that the end with the longest (and hence oldest) segments is the head or anterior end. However, one of the participants in my Chautauqua course ("The Ediacaran Biota" held May 18-20, 1995, Northern Illinois University, DeKalb), made the interesting suggestion that the enlarged tubes at the supposed anterior end of a dickinsoniid represent swollen gonads. Another suggestion is that these first two segments were modified to become sense organs. I discuss in chapter 11 why Dickinsonia's reproduction probably occurred at the growing ("posterior") end.
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