Exploring the drivers of early biomineralization

Abstract

points:  The first skeletons are known from protists, 810 Million years ago (Ma)  Large, putative metazoan, calcareous skeletons first appear in the terminal Ediacaran, ~550 Ma. Calcification was an independent and derived feature that appeared in diverse groups. The presence of a pre-existing organic scaffold, which provided the framework for interactions between extracellular matrix and mineral ions, can be inferred in many skeletal taxa. Calcareous biomineralization may have been favoured in the highly saturated, high alkalinity carbonate settings of the Ediacaran, where carbonate polymorph was further controlled by seawater chemistry. By the Early Cambrian, there is a marked increase in the diversity of skeletons as an escalating defensive response to increasing predation pressure, and biomineralization may have come under stronger biological control. The initial triggers for widespread biomineralization in the Ediacaran remain unclear. AbstractThe first biomineralised hard-parts are known from ~810 Million years ago (Ma), consisting of phosphatic plates of probable protists formed under active biological control. Large skeletons in diverse taxa, probably including total-group poriferans and total-group cnidarians, first appear in the terminal Ediacaran, ~550 Ma. This is followed by a substantial increase in abundance, diversity, and mineralogy during the early Cambrian. The biological relationship of Ediacaran to early Cambrian skeletal biota is unclear, but tubular skeletal fossils such as Cloudina and Anabarites straddle the transition. Many Ediacaran skeletal biota are found exclusively in carbonate settings, and present skeletons whose form infers an organic scaffold which provided the framework for interactions between extracellular matrix and mineral ions. Several taxa have close soft-bodied counterparts hosted in contemporary clastic rocks. This supports the assertion that the calcification was an independent and derived feature that appeared in diverse groups, that was initially acquired with minimal biological control in the highly saturated, high alkalinity carbonate settings of the Ediacaran, where carbonate polymorph was further controlled by seawater chemistry. The trigger for Ediacaran-Cambrian biomineralization is far from clear, but may have been either changing seawater Mg/Ca ratios that facilitated widespread aragonite and high-Mg calcite precipitation, and/or increasing or stabilising oxygen levels. By the Early Cambrian, the diversity of biomineralization styles may have been an escalating defensive response to increasing predation pressure, with skeletal hardparts first appearing in abundance in clastic settings by the Fortunian. This marks full independence from ambient seawater chemistry and significant biological control of biomineralization.

Ultrastructure of Ediacaran cloudinids suggests diverse taphonomic histories and affinities with non-biomineralized annelids

“…2E2). Our observation more clearly points to a diagenetic origin of the calcite spars in the tube wall 1,4 , and does not support notions that the walls consist of micritic primary layers fusing to secondary laminae 7,28 .…”

“…More specifically, Cloudina itself has been documented with calcitic 1,2,28 , phosphatic 3 , and siliceous tube compositions 14,32 . Of these varying taphomodes, phosphatization and silicification are broadly considered to be secondary replacement of pre-existing carbonate 1,2,7,28 , while the possibility of phosphatization of an organic exoskeleton has been unexplored. Indeed, it has been widely accepted that phosphatization in the cloudinids, as well as in most Cambrian small shelly fossils, is of early diagenetic origin.…”

“…Interpretations on the affinity of cloudinids, and especially on Cloudina, have been controversial since its first descriptions. Most studies have focused on comparisons of gross morphology with similar tubular constructions in either annelids (serpulids in particular 3 ) or tube-forming cnidarians 3,6,7 . Partly based on both the premise of its biomineralization and inferred affinity with anthozoan-like cnidarians 8,9 , recent studies have implicated Cloudina as an early reef-builder, although this interpretation has been challenged 10 .…”

Forced Biomineralization: A Review

“…Consequently, environmental metal contamination can change the diversity of microbial communities via the domination of metal-resistant species (for example, see [ 266 , 267 ]). As an intriguing example, metal resistance co-occurring with antibiotic resistance has been reported in bacteria isolated from metal-contaminated soils, waters, and sewage [ 1 , 75 , 133 , 266 , 268 , 269 , 270 , 271 , 272 , 273 , 274 , 275 , 276 , 277 , 278 , 279 , 280 , 281 , 282 , 283 , 284 , 285 , 286 , 287 , 288 ]. The ecological impacts of these essentially manmade organisms are yet to be understood, although the harm possibly caused by forced biomineralizing organisms might plausibly be overshadowed by the metal-concentrated environments they inhabit.…”

Phosphatic scales in vase‐shaped microfossil assemblages from Death Valley, Grand Canyon, Tasmania, and Svalbard

“…Mineralized skeletons evolved dozens of times within the eukaryotes and are distributed widely across the tree, occurring in every major clade (Knoll, 2003). It is therefore puzzling that, despite the fact that eukaryotic fossils are found in rocks as old as 1.6 Ga (Javaux, 2007;Lamb et al 2009;Peng et al, 2009), fossils of skeletal-mineralizing eukaryotes are virtually absent from rocks >550 Ma (Wood, 2018).…”