A Solution to Darwin's Dilemma: Differential Taphonomy of Ediacaran and Palaeozoic Non-Mineralised Discoidal Fossils
MacGabhann, Breandán Anraoi
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The eldonides, a group of non-mineralised asymmetrical discoidal fossils characterised by a coiled alimentary canal with circumoral tentacles and radially arranged internal lobes, are perhaps the least-well known major group of Palaeozoic organisms. Their fossil record is primarily known from sites of exceptional preservation, ranging from the Cambrian to the Devonian. Eleven species are known in eight genera and three families (the Eldoniidae, Maoyanidiscidae, and Paropsonemidae), all placed in the Order Eldonida, Class Eldoniata, and the early deuterostome stem-group Cambroernids. The eldonides are of little biostratigraphical or palaeoecological utility. However, they are significant in that while eldoniids are generally preserved as compressions in shales, most paropsonemid specimens are found as moulds and casts in sandstones, a poorly-understood taphonomic mode previously thought to be unique to the oldest known animal fossils, of Ediacaran age. Paropsonemids from the Tafilalt Lagerstätte (Ordovician, Morocco), were apparently exceptionally preserved in this style by the adsorption of iron ions, produced during decay by iron reduction, to tissues composed of complex biopolymers. These ions provided sites for the nucleation of aluminosilicate clay minerals, and the formation of iron sulphides (by reaction with hydrogen sulphide produced during subsequent decay by sulphate reduction). These formed moulds of the paropsonemid dorsal surfaces, which were cast by adjacent sediment following the completion of decay. Importantly, comparison to Ediacaran-aged fossils indicates that these were preserved by the same processes, while comparison to fossil eldoniids confirms that this taphonomic style can only preserve tissues composed of complex biopolymers. This suggests that the oldest known animal fossils cannot have been the oldest animals, but merely the first animals to evolve such tissues. It thus appears that Charles Darwin was correct in his proposal that the oldest known animal fossils were preceded by a long period of animal evolution not represented in the fossil record.