Exploring the drivers of early biomineralization.

The first biomineralized 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, which was initially acquired with minimal biological control in the highly saturated, high-alkalinity carbonate settings of the Ediacaran, where the 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 stabilizing oxygen levels. By the Early Cambrian, the diversity of biomineralization styles may have been an escalating defensive response to increasing predation pressure, with skeletal hard parts first appearing in abundance in clastic settings by the Fortunian. This marks full independence from ambient seawater chemistry and significant biological control of biomineralization.