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

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 tubewall 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.

Ediacaran metazoan fossils with siliceous skeletons from the Digermulen Peninsula of Arctic Norway

“…Cloudina also has transverse and/or oblique annulations that are distributed differently along the funnels in the various species (Cai et al, 2017). This fossil wall was thought to have a calcareous mineral composition (Germs, 1972;Grant, 1990;Cortijo et al, 2010Cortijo et al, , 2015Zhuravlev et al, 2012;Penny et al, 2014;Wood et al, 2017), but it has been revealed to be originally organic and only taphonomically mineralized (Yang et al, 2020).…”

Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period

“…Moreover, at least some of these alleged diagnostic features (or lack thereof) may be taphonomic noise rather than primary biological signal. For instance, although a homogenous microgranular tube ultrastructure is commonly reported for Cloudina [14][15][16] , lamellar construction has also been observed 16,23 , raising the question as to the influence of diagenetic recrystallization on retention of primary ultrastructure 24 , or, for that matter, original composition 23 . It thus follows that the degree of tube wall biomineralization in addition to the original biomineral chemistry has been met with differing interpretations 19,22,23 , likely compounded by varying preservational and diagenetic histories between localities 23,24 .…”

“…For instance, although a homogenous microgranular tube ultrastructure is commonly reported for Cloudina [14][15][16] , lamellar construction has also been observed 16,23 , raising the question as to the influence of diagenetic recrystallization on retention of primary ultrastructure 24 , or, for that matter, original composition 23 . It thus follows that the degree of tube wall biomineralization in addition to the original biomineral chemistry has been met with differing interpretations 19,22,23 , likely compounded by varying preservational and diagenetic histories between localities 23,24 . The absence of substrate attachment structures 15 may be a consequence of displacement and transport during storm events 24,25 , a common mode of deposition of cloudinomorphs that yields fragmental tubes in detrital hash resembling biohermal or reefal buildups 9 .…”

Ediacaran diversity and paleoecology from central Iran

“…The Nama assemblage is typically composed of low-diversity populations of Ediacaran clades such as Rangeomorpha ( Rangea ; Vickers-Rich et al, 2013) and Erniettomorpha ( Swartpuntia , Ernietta , Pteridinium ; Narbonne et al, 1997; Ivantsov et al, 2016), and diverse organic-walled (e.g., Gaojiashania , Wutubus ; Chen et al, 2014) or weakly biomineralized (e.g., Corumbella ; Walde et al, 2019; cloudinomorphs; Selly et al, 2019; Yang et al, 2020) tubular metazoans of controversial affinities (Schiffbauer et al, 2016). The difficulty with evaluating the proposed terminal-Ediacaran extinction and subsequent Cambrian explosion is the scarcity of biologically diverse fossil localities spanning this interval (Wood, 2011; Darroch et al, 2018a; Muscente et al, 2018).…”