Role of low intensity environmental disturbance in structuring the earliest (Ediacaran) macrobenthic tiered communities

摘要

Rangeomorphs were important components of Ediacaran macrobenthic ecosystems, yet their biology and ecology remain poorly constrained. They formed high-density, tiered communities that were subjected to intermittent burial events, the largest of which killed entire communities. Abundant thin event beds in the Ediacaran succession of Charnwood Forest indicate the additional, frequent impact of minor obrution events. The type surface of Charnia masoni is immediately underlain by one such lamina (a tuff) and preserves a distinctly bimodal population. It is dominated by Charnia fronds that are of smaller or comparable length to the holotype (19.4 cm), but also includes notably larger specimens (>45 cm) that would traditionally have been assigned to Charnia grandis. Multiple morphological- and morphometric parameters (length, width, spacing of primary branches) demonstrate that these are indistinguishable from the holotype of C masoni, affirming the synonymy of the two taxa. Nevertheless, these outsized individuals are distinguished by their proportionally fewer primary branches per unit length. Taphonomic evidence indicates that they were survivors of an incumbent population, the rest of which was culled by a minor ashfall. We suggest that this temporary reduction in competition from neighbours allowed the survivors to grow larger and thereby gain access to a greater proportion of the water column. As the community recovered, their large size would have continued to provide them with an advantage, divorcing them from the density-dependent competition seen in the new understory. The interlude between cohorts implies that new recruits were substrate-sensitive, presumably awaiting re-establishment of the biomat. Sub-lethal disturbance events thus played a significant role in structuring Ediacaran communities, and help explain the observed bed-by-bed variability. Taken as a whole, the growth trajectory of C masoni resembles that of extant organisms with indeterminate growth programmes and no genetically-controlled upper size limit. (C) 2015 The Authors. Published by Elsevier B.V.