Abstract
Cloudinids have long been considered the earliest biomineralizing metazoans, but their affinities have remained contentious and undetermined. Based on well-preserved ultrastructures of two taxa, we here propose new interpretations regarding both their extent of original biomineralization and their phylogenetic affinity. One of these taxa is a new cloudinid from Mongolia, Zuunia chimidtsereni gen. et sp. nov., which exhibits key characteristics of submicrometric kerogenous lamellae, plastic tube-wall deformation, and tube-wall delamination. Multiple carbonaceous lamellae are also discovered in Cloudina from Namibia and Paraguay, which we interpret to have originated from chitinous or collagenous fabrics. We deduce that these cloudinids were predominantly originally organic (chitinous or collagenous), and postmortem decay and taphonomic mineralization resulted in the formation of aragonite and/or calcite. Further, based on our ultrastructural characterization and other morphological similarities, we suggest that the cloudinids should most parsimoniously be assigned to annelids with originally organic tubes.
Highlights
Cloudinids have long been considered the earliest biomineralizing metazoans, but their affinities have remained contentious and undetermined
Sediment sequences from the critical E-C transition are widespread on various blocks of Western Mongolia
The fossil assemblages described here were recovered from the basal Zuun-Arts Formation in Bayan Gol and the upper Salanygol Formation (Cambrian Stage 3) in Salany Gol, Zavkhan Block, Mongolia (Supplementary Fig. 1)
Summary
Cloudinids have long been considered the earliest biomineralizing metazoans, but their affinities have remained contentious and undetermined. Most studies have focused on comparisons of gross morphology with similar tubular constructions in either annelids (serpulids in particular3) or tube-forming cnidarians[3,6,7] 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, this interpretation has been challenged[10]. Despite uncertainties surrounding biomineralization and biological affinity, there is a broad consensus that Ediacaran tubular fossils, and those of the cloudinids, are among the earliest skeletal metazoans[1,3,14,15,16,17]. This work yields new information on the state of biomineralization and biological affinity of the cloudinids
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