The Dickinsonia (1947)
Phylum : Proaticulata
Class : Dipleurozoa
Family : Dickinsoniidae
Genus : Dickinsonia
Species : D. costata, D. lissa, D. tenuis, D. menneri, D. rex
- Precambrian (560 - 555 Ma)
- 1 cm - 1,4 m long
Dickinsonia was first described by Reg Sprigg, the original discoverer of the Ediacaran biota in Australia, who named it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg.
Dickinsonia is known from unskeletonised impressions in late Ediacaran quartz sandstones in Ediacara and elsewhere in the Flinders Ranges of South Australia, as well as, Podolia of Ukraine, and the White Sea area and Central Urals of Russia, and has an estimated time range of 560-555 Myr.
Dickinsonia is a “resistant” fossil – that is to say, it is preserved as a (usually concave) cast on the underside of overlying bedding planes—unlike most Ediacaran fronds. Where part and counterparts of the same impression are known, they are separated by as much as 3 mm, with the ribbing most prominent on the top surface; this suggests that the ornament was displayed on the top surface only, and that underlying sand supported the impression.
Arcing trackways of Dickinsonia fossils, termed Epibaion, have been found, but their interpretation too is insecure. They may be impressions the organism made while it rested on the sediment surface – perhaps by secreting slime in order to form a platform on the underlying microbial mat,or by sitting and dissolving the underlying microbes in order to devour them. They have also been interpreted as “tumble tracks” created by an organism rolling along the sea floor, perhaps as it was buffeted by currents, and as the bases of lichens or “mushrooms arranged in fairy rings”. However, in some cases these trackway imprints overlap. Ridges apparently produced by the channelling of sediment in digestive tubes seem to indicate that the trackways do indeed represent feeding traces; the sedimentary disturbance expected of tumbling-induced impressions is not observed.
Halo-like “reaction rims” surround specimens. Adjacent specimens deform as if to avoid entering their neighbour’s halo, suggesting they competed with one another. No body fossils have been found to overlap.
Some spectacular fossils which can be attributed to Dickinsonia appear to preserve internal anatomy, believed to represent a tract that both digested food and distributed it throughout the organism.
The organisms are preserved in positive or negative relief, usually in coarse sandstone, and are usually preserved by virtue of imprinting on microbial mats, though their preservation may also reflect the abundance of aerobic environments or microbial pyritisation in the Ediacaran era—or, if they are protists, possibly agglutination (although this hypothesis is not mainstream).
Where Dickinsonia is found to be folded or bent, it is not deformed in a brittle manner, as a “death mask” would be; indeed, it is not very flexible at all.
The height of the specimens preserved bears little relation to their length or width, suggesting that the mode of decay resembled that of a lichen, leaf or mushroom. Assuming their pneus were originally cylindrical, they were more rigid than worms, jellyfish or logs.
Organisms of all sizes are found on bedding plane assemblages; this shows that they were commonly preserved in life position, as currents would preferentially remove smaller specimens.Further, their preservation on the top of certain sedimentary structures shows that they must have been firmly attached to the substrate at their time of burial.
Dickinsonia is found in sedimentary beds 8 mm thick; allowing for compaction, this allows these specimens a maximum height of 1 cm.