Most of our knowledge of Neoproterozoic metazoans comes from Lagerstätten preserving fossils as moulds and casts in siliciclastic sediments. Until recently, such Lagerstätten were believed to be unique to the Neoproterozoic, and although significant advances have been made in recent years, our incomplete understanding of this taphonomic mode remains the limiting factor in our comprehension of these fossils. A key factor in this limited understanding has been that all specimens known in this style of preservation are to some extent enigmatic, including representatives of early stem groups, extinct lineages, and many specimens which are simply incertae sedis. These complexities have drastically hindered our understanding of what biological components and tissue types are, and more importantly, are not preserved, in this taphonomic mode.A recently discovered Lagerstätte in Ordovician siliciclastic sediments of the Tafilalt region, south-eastern Morocco, comparable to Neoproterozoic Lagerstätten in both lateral (70km) and temporal (10Myr) extent, may help to advance our knowledge of this preservational mode. Thousands of fossil moulds and casts have been collected, the vast majority of which are eldonids: a Palaeozoic group of primitive deuterostomes, the biology of which is well-known from hundreds of specimens of Eldonia from the starkly contrasted taphonomic modes of the Cambrian Burgess Shale of Canada and Chengjiang in China.The large volume of material at Tafilalt has permitted destructive analytical techniques which are generally not an option for Neoproterozoic specimens, including SEM-EDS, and Laser Raman microscopy. These have allowed the establishment of a complete taphonomic and diagenic sequence of the preservation of the Tafilalt eldonids. The available evidence for Neoproterozoic specimens, although more limited, is consistent with the Tafilalt model. Together with the comparison between the Tafilalt and Burgess Shale eldonids, this differential taphonomic analysis allows the first evaluation of what particular biological components are, and are not, preserved in this taphonomic mode, with significant implications for the interpretation of the oldest known fossil metazoans.