Retallack (2014) claims that our observations of structures associated with Aspidella (Menon et al., 2013) interpreted as short horizontal trails and vertical equilibration traces are better explained by growth rugae and rhizomorphs of a sessile-growing organism in a shallow marine to intertidal environment. We find his interpretation and supporting observations unconvincing and speculative.
We focus here on Retallack’s critique of Menon et al. (2013) since his claims regarding trails described by Liu et al. (2010a) have already been countered (Liu et al., 2010b). Retallack states that the interpretation of upward migration is falsified by “the deformation of laminae both upward and downward” and “marked narrowing of the deformed zone downward” in the vertical sections shown in our figure 3. His meaning with reference to our figure is obscure, as neither systematic upward deflection nor narrowing is present. In any case, apparent narrowing can also be caused by non-axial sections of a broadly tubular object. To support his claim that the same features “are also seen in other thin sections of A. terranovica,” he refers to an image (his figure 1A) that is neither a thin section, nor a vertical section, but is the bedding plane expression of a relatively flat Aspidella. The reference to deformation of laminae both upward and downward in his material presumably relates to the low-relief concentric rings of the disc, most likely arising from the collapse and compression of sediment around an organism during decomposition. This has no relevance to the vertical structures we show, which occur in several consecutive laminae above the disc impression on the sole of a bed.
Retallack’s claim that our observations are “incompatible with the broad base of cnidarian polyps” is inaccurate. Broad bases are not found in all modern polyps. Burrowing polyps such as Peachia hastata have bases that are muscular, rounded, and highly flexible. The remark that our equilibration structures show no consistent direction of movement is also incorrect: all the traces show an oblique rise through the sediment.
We find Retallack’s explanation for the horizontal crescentic trails as growth rugae like those of a modern lichen (cf. Toninia sedifolia, his figure 1D) wholly unconvincing. The regular, short crescentic markings display raised margins and indications of typical Aspidella ornamentation; they bear no comparison to the amorphous, bulbous rugae illustrated. His lichen hypothesis has already been noted to lack evidential support from the fossil record (Antcliffe and Hancy, 2013).
We also rebut Retallack’s interpretation of the downward-deflected laminae as “downward-tapering rhizomorphs.” There is no evidence to associate the disc in his figure 1A with the one or two faint curved markings some distance away, nor to consider the latter as rhizomorphs. The supposed rhizomorphs in a “presumed uprooted specimen” (his figure 1B) we find equally unconvincing. The raised ridge lacks the detail to justify such a remarkable claim, and there is no direct association between the ridge and the disc figured. Raised, sometimes branched, filaments are often found on fossil-bearing surfaces in the Fermeuse Formation, and probably relate to the associated microbial mat (e.g., below the arrow in his figure 1B).
The irregular dark feature shown in vertical section in Retallack’s figure 1C is dissimilar to the vertical structures we have interpreted as equilibrium traces. Retallack’s vertical structure is not associated with a disc, nor is there any evidence for the downward growth he proposes. Such features are relatively common in the Fermeuse Formation and frequently contain pyrite. Their irregular morphology, and horizontal as well as vertical extension, would suggest the migration of sulfidic fluids, perhaps from the decay of microbial mat and dead organisms. The figure from Gehling et al. (2000, their figure 14) misquoted by Retallack as being “comparable tubular features attached to presumably uprooted Aspidella” was inferred by those authors to be a stem-like tissue lying above Aspidella discs, not below, as required by Retallack’s hypothesis.
We cannot directly comment on Retallack’s claims for flaser bedding as they are not presented. Our extensive sedimentological studies in the region, and those of others (e.g., Gehling et al., 2000), lead us to suggest that the author is probably mistaken in his interpretation of mud-draped ripples. Moreover, the isolated ripple trains that Retallack takes as confirming the intertidal setting his lichen model requires are well known to occur in many types of sediment-starved flows, especially dense hypopycnal flows that characterize pro-delta and continental slope-basin floor deposits.
These shortcomings in process-based sedimentology also characterize Retallack’s objections to the depositional environments of other classic Ediacaran fossil-bearing successions (see Callow et al., 2012). His views, often published as comments, are contrary to the peer-reviewed work of specialist sedimentologists on the same successions. His interpretations require special pleading and appear to be driven by the needs of his lichen model. Process-based sedimentology is an experimentally rigorous and well-established discipline; its interpretations should be heeded in preference to speculative models of Ediacaran habitats and macrofossils.
In conclusion, Retallack’s critique of our interpretations fails to convince, while his own interpretation is based on speculative claims wholly unsupported by the presented evidence, which bears little resemblance to the structures we describe.