First macrobiota biomineralization was environmentally triggered

Abstract

Why large and diverse skeletons first appeared ca 550 Ma is not well understood. Many Ediacaran skeletal biota show evidence of flexibility, and bear notably thin skeletal walls with simple, non-hierarchical microstructures of either aragonite or high-Mg calcite. We present evidence that the earliest skeletal macrobiota, found only in carbonate rocks, had close soft-bodied counterparts hosted in contemporary clastic rocks. This includes the calcareous discoidal fossil Suvorovella, similar to holdfasts of Ediacaran biota taxa previously known only as casts and moulds, as well as tubular and vase-shaped fossils. In sum, these probably represent taxa of diverse affinity including unicellular eukaryotes, total group cnidarians and problematica. Our findings support the assertion that the calcification was an independent and derived feature that appeared in diverse groups where an organic scaffold was the primitive character, which provided the framework for interactions between the extracellular matrix and mineral ions. We conclude that such skeletons may have been acquired with relative ease in the highly saturated, high alkalinity carbonate settings of the Ediacaran, where carbonate polymorph was further controlled by seawater chemistry. The trigger for Ediacaran biomineralization may have been either changing seawater Mg/Ca and/or increasing oxygen levels. By the Early Cambrian, however, biomineralization styles and the range of biominerals had significantly diversified, perhaps as an escalating defensive response to increasing predation pressure. Indeed skeletal hardparts had appeared in clastic settings by Cambrian Stage 1, suggesting independence from ambient seawater chemistry where genetic and molecular mechanisms controlled biomineralization and mineralogy had become evolutionarily constrained.

Keywords: Ediacaran; biomineralization; carbonate supersaturation; macrobiota; metazoans.

Figure 1.

Suvorovella aldanica Vologdin and Maslov, 1960, Aim Formation, Ediacaran; Siberian Platform, Republic of Sakha (Yakutia), Russia. (a) Detail of shell inner lower surface showing outer flat zone with diamond-shaped pattern and concentrically arranged chambers. (b) Detail of shell showing folded apex and outer flat zone with diamond-shaped pattern, latex mould of inner lower surface, PIN no. 5119/1063. (c) Reconstruction, diameter 100 mm. (d) Field photograph of shell bed of whole and broken Suvorovella shells. (e) Photomicrograph of thin section showing micritic envelopes (arrowed) around Suvorovella, encrusted by an isopachous crust of early diagenetic radial fibrous dolomite cement. (f) SEM image of detail of outer upper surface of shell at the apex showing concentric ribs and uneven surface. (g) SEM image of broken shell edge showing micrite envelope (M) encrusted by early marine radial fibrous dolomite cement (D). Specimens figured in a, b, f, and g are housed in the Palaeontological Institute named after A.A. Borisyak, Russian Academy of Sciences, Moscow (PIN).

Figure 2.

(a–d) Ediacaran skeletal and (e–h) non-skeletal counterparts. (a) Skeletal Suvorovella. (b) Skeletal Cloudina (Photo: Shuhai Xiao). (c) Skeletal Sinotubulites (Photo: Shuhai Xiao/Yaoping Cai). (d) Skeletal Protolagena (Photo: Shuhai Xiao/Hong Hua). (e) Organic Eoporpita medusa. (f) Organic Conotubus (photo: James Schiffbauer/Yaoping Cai). (g) Organic Corumbella (photo: Lucas Warren). (h) Organic Sicylagena (Photo: Shuhai Xiao/Hong Hua).