Figure 2.3: The medusoid phase of a living aequorian hydroid. Diameter approximately 1 cm.
compaction, would have been greater. In chapter 5 we see how recognition of the convex-side-down aspect of Ediacaran medusoids was the precursor to Garden of Ediacara theory, the concept that these medu-soids were actually bowl-shaped solar collectors.
Many Ediacaran medusoids are three-dimensional fossils filled with fine sediment. The Mexican specimen (figure 2.1) is one of these, as is the Irish find. How did sand get inside these medusoids? Do they represent a sand casting of the impression in the sediment where a soft body once was, or was the sand part of the creature in life?
Uncertainties about this question have led to wide divergences in the interpretation of the most spectacular medusoid fossil, Mawsonites (figure 2.4). This ornate form, which has graced the covers of magazines in full color,18 has been interpreted as a medusoid and as a trace fossil.19 As noted in Stuart A. Baldwin's catalog of fossil reproductions, in the description of the reproduction of the holotype of Mawsonites spriggi,
A superb specimen in very strong positive relief showing the many arcs of large irregular bosses which increase in size outwards towards the lobate periphery. . . . When originally described it was thought
to be an unusual form of Medusa but Prof. A. Seilacher (personal communication, 1987) on seeing one of our replicas for the first time diagnosed it as a TRACE FOSSIL with a central burrow surrounded by backfill structures (the bosses).20
Seilacher published this suggestion in a scientific paper as well21 but later abandoned the idea. Runnegar suggests that Mawsonites is a holdfast.22
I think that the problem with the interpretations discussed here is that, with the medusoids, we are dealing with sand creatures. I am not talking here about Seilacher's Psammocorallia concept.23 Seilacher has inferred that certain discoid fossils formed of sand were the internal, organically cemented sand skeletons of a sea-anemone-like creature. He felt that this weighting of sand in the bottom of the organism would help the creature remain upright and act as an anchor that would cause the base of the anemone to be automatically implanted into sandy surfaces when it was rocked by currents (figure 2.5). I suggested to Seilacher the phrase "rock in a sock" to describe this arrangement, and he uses the phrase in his drawings of the psammocorals.24
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