Ediacaran characters

Antcliffe and Hancy (2013) raise questions concerning recent work on Ediacaran fossils (Retallack 2013a), here quoted in full. “What are the characters present in Ediacaran organisms that lead logically to the conclusion that these organisms are the remains of ancient lichens? How was it determined that such characters are unique to lichens to the exclusion of other similar morphological shapes or structures formed by abiological processes? The answers to these questions are not forthcoming. ” Answers are not forthcoming because they have already been published. Space here permits only a referenced list for Dickinsonia, a particularly iconic Ediacaran fossil: (1) unifacial structure, with finished and thick upper surface layer, but less distinct lower surface (Wade 1968, Figs. 17 and 20C; Gehling 1999, Fig. 7); (2) fractal tubular constructional and histological elements (Retallack 2007, Fig. 6); (3) indeterminate isometric growth in width and length to maintain proportions (Retallack 2007, Figs. 3 and 4); (4) indeterminate allometric growth in thickness to maintain ground‐hugging form (Retallack 2007, Fig. 3); (5) juvenile thallus unusually large and coarsely plicate compared with adult (Runnegar 1982, Fig. 7); (6) mature growth by radial addition of segments as well as diffuse marginal expansion (Runnegar 1982, Fig. 7); (7) hypothallial rims (“halo margin” of Retallack 2007, Fig. 1A); (8) allelopathic avoidance of other individuals (Retallack 2007, Fig. 1B); (9) fairy ring arrangements of individuals (Retallack 2007, Fig. 1E); (10) greater resistance to burial compaction than comparably scaled fossil tree trunks without any mineral or relict evidence of pyritization (Retallack 2007, Fig. 5); (11) series of decayed individuals showing loss of relief but not of outline (Retallack 2007, Fig. 1E); (12) basal fine rhizine‐like extensions down in tomatrix (Retallack 2013a, Fig. 2g); (13) attached stout connecting rhizomorphs (Seilacher’s “Aulozoon” for Retallack 2007, Fig. 7A); (14) limited marginal overturn and pull apart of thallus over expansion cracks indicating firm cementation or rooting to substrate (Glaessner 1969, Fig. 1E; Retallack 2007, Fig. 1D); (15) growth which effaces primary sedimentary structures such as ripple marks (Xiao 2013, Fig. 1); (16) association with complex microbial surfaces (“old elephant skin” or Rivularites repertus) characteristic of desert crusts (Retallack 2012a, Fig. 6A); (17) life position within oxidized, well drained gypsic and calcic paleosols (Retallack 2013a, Fig. 2b); (18) growth through time coordinated with proxies for paleosol development such as proportion of gypsum sand crystals (Retallack 2013a, Fig. 3). Anticipating Antcliffe and Hancy’s (2013) second question, most of these characters are tabulated for comparison with alternative biological affinities by Retallack (2007, Table 5), with the result that lichens and non‐lichenized fungus fully (100%) explain 16 characters of Dickinsonia, but only 81% are explained by xenophyophores and cnidarian polyps, 75% by cnidarian jellyfish, 69% by polychaete or annelid worms, and 63% by turbellarian worms. The recently proposed idea of Dickinsonia as a placozoan (Sperling and Vinther 2010), explains only 50% of the 16 characters of Retallack (2007), and only 28% of the 18 characters listed above. Antcliffe and Hancy (2013) summon support from Brasier and Antcliffe (2008) for the idea that Dickinsonia shrank, and so did not have a rigid or attached carapace. However, the specimen considered unshrunken on the slab with the one considered shrunken by Runnegar (1982) is now known to have been a different species on a different growth trajectory (Retallack 2007). Because both the large and small specimen have a rim of about the same size, it cannot be due to shrinkage of one like the large one to the small one, and is better regarded as a fungal hypothallus, or rim of incomplete hyphal expansion. Antcliffe and Hancy (2013) leave unspecified the “conclusive evidence that they were marine” from Callow et al. (2013), without citing refutation of that account by Retallack (2013b), who found their marine evidence far from conclusive (ripple marks) or in different beds from the living surfaces of Dickinsonia (hummocky cross stratification). Evidence from unusually light carbon and oxygen isotopic composition in outcrop and drill core persuaded Knauth (2013) that the carbonate nodules were pedogenic. Geochemical mass balance showing both volume and common cation loss from the paleosols is most persuasive to me, as it cannot form during sedimentation in which titanium‐bearing heavy minerals drop to the bottom of a bed (Retallack 2012b). Also compelling is evidence from distinctive complex associated mats formed by a highly deviatoric system of shrinking, swelling, overgrowth, and healing (Retallack 2012a). Other features of the paleosols with Dickinsonia include downward gradational destruction of bedding, drab‐haloed filament traces (Prasinema gracile), soil crust pedestals, loess‐like grain size and fabric, replacive (not displacive) sand crystals and nodules of calcite, and gypsum at characteristic depth below bed tops, desiccation cracks, ice heave and melt structures, needle ice impressions, red redeposited soil clasts in gray fluvial sandstones, and red colour EVOLUTION & DEVELOPMENT 15:6, 387–388 (2013)