Cloudina is one of the main index fossils of the Ediacaran Period commonly occurring in carbonate platform settings as in situ clusters or loosely packed parautochthonous bioclastic deposits in close association with microbial facies. However, despite these taphonomic and sedimentologic attributes found in almost all Cloudina accumulations worldwide, many doubts persist regarding the (a) phylogenetic affinity, (b) palaeoecology and (c) taphonomy of this biomineralized metazoan. One of the greatest issues concerning the interpretation of Cloudina palaeobiology is related to the preservational quality of the fossils, which varies according to distinct rock matrixes, in some cases precluding the precise interpretation of its tridimensional morphology. This also hampers the use of detailed morphometric parameters (i.e., oblique/transverse annulations at the exterior surface of funnels) that can be easily or better observed in those individuals preserved in phosphatized material (Cai et al., 2017). When the preservation occurs in carbonate rocks, the lack of compositional contrast between recrystallized shell walls and carbonate rock matrix that embeds them, hindering mechanical or chemical separation of individuals, or even the assessment through micro-CT scanning. On the other hand, size distribution of tubes in autochthonous Cloudina assemblages has demonstrated to be a useful, and sometimes the only approach possible to define taxonomic characters and assess information on the structure of palaeocommunities, modes of reproduction and ontogeny. Indeed, at the type locality of the Nama Group, taxonomic classification within two species of Cloudina (C. hartmanae and C. riemkeae) was mainly based on differences in tube diameter (Germs, 1972). Additionally, morphometry was also used to infer size variations in response to environmental changes, suggesting generalist and adaptable growth strategies for this taxon (Wood et al., 2017).

The fossil-bearing Tagatiya Guazu Formation (Itapucumi Group), on northeastern Paraguay is characterized by an intercalation of grainstone, mudstone and microbial facies interpreted as deposited in a protected lagoonal environment. This unit is particularly rich in bioclastic concentrations of Corumbella, Namacalathus and Cloudina skeletal remains (Warren et al., 2017). When associated to microbialite facies, previous taphonomic analysis (Warren et al., 2013) has demonstrated that Cloudina occur as either vertically oriented tubes or as prostrate carapaces with little or no taphonomic signatures (fragmentation, abrasion) indicating extensive lateral transport and prolonged residence time at the sediment-water interface (Fig. 1). These features strongly suggest that they possibly represent autochthonous individuals.

Here we analyzed both types of occurrences in (i) naturally weathered surfaces, (ii) polished slabs and (iii) thin sections in order to identify faciological, taphonomical and morphological parameters. We have taken systematic measurement of the carapace diameters from distinct types of assemblages and use Shapiro-Wilk test to determine the normality of their distribution. The Kolmogorov-Smirnov tests was employed to statistically compare the distinct assemblages. Based on detailed observation and quantitative analysis we identified that autochthonous Cloudina assemblages can occur with two different patterns of carapace concentrations (subdivisions of the Type 2 shell concentration proposed by Warren et al., 2013).

The first accumulations (Pattern A) (Fig. 1A-D), is characterized by relatively sparse to dense aggregations of mainly large rectilinear to slightly curved Cloudina carapaces (between 0.25 and 2.77 mm in diameter). Sometimes diagnostic morphological features such as well-pronounced flaring apertural rims (Fig. 1B, D) and transverse annulations in funnels outer surface are preserved. Tubes generally possess constant diameters along its length, but few may present a slight enlargement in diameter towards the apertural end (modes of growth A and B after Wood et al., 2017). They either occur vertical, horizontal or obliquely oriented in massive to clotted (thrombolitic) carbonatic matrix. This association often preserves fragmentary and sparse remains of Corumbella and Namacalathus skeletal elements. The second type of accumulation (Pattern B) (Fig. 1E-J), is characterized by sparse to dense concentrations of visibly smaller specimens of Cloudina than those observed in the Pattern A (between 0.12 and 0.90 mm in diameter). The specimens are highly recrystallized and mostly filled with sparry calcite cement, which precludes the observation of detailed external morphological features in the carapaces. Tubes possess constant diameters (modes of growth B or C, after Wood et al., 2017) and have a high degree of sinuosity, with curves sometimes approaching 90º (Fig. 1H-J). They occur associated with thin-laminated grainstone and planar microbialites mostly horizontally to bedding, but also in vertically oriented.

Our preliminary data suggests a strong correlation between sedimentary facies, and distinct morphotypes and sizes of Cloudina carapaces from in situ assemblages. In general, autochthonous accumulations dominated by small and highly sinuous specimens are preserved in laminated grainstone in contrast to assemblages of large and rectilinear carapaces that are usually found in thrombolitic facies. Further investigation is necessary in order to test whether these characteristics might represent paleoecological responses to different environmental contexts, or could be controlled by taphonomic processes.