Elsevier

Precambrian Research

Volume 332, 15 September 2019, 105385
Precambrian Research

Putting the cart before the horse: An example of how the lack of taphonomical approaches can mislead paleobiological inferences for the late Ediacaran

https://doi.org/10.1016/j.precamres.2019.105385Get rights and content

Highlights

  • Cloudina from the Tamengo Formation are susceptible to taphonomic alterations.

  • Size distributions are an unreliable taxonomic characteristic for Cloudina.

  • Biostratinomic and diagenetic processes obstructed diagnostic features in Cloudina.

Abstract

The arrival of animals with hard parts at the end of the Ediacaran Period was an important evolutionary innovation. Biomineralized structures serve a number of biological functions and pose environmental challenges. Those same hard parts that once played a role in living organisms also affect their postmortem histories. Taphonomic scenarios may create biases that can impact perceptions on the systematic, morphological, biostratigraphic, and paleogeographic patterns in the fossil record. This is well exemplified by the taxonomic controversies regarding Cloudina, the most geographically widespread and abundant shelly fossil of the uppermost Ediacaran. In this study, we discuss new taphonomic data on Cloudina-bearing strata deposits from the Tamengo Formation (Corumbá Group, Brazil) and how influential this taphonomy is on a robust taxonomy of this fossil. Our observations suggest that allochthonous Cloudina deposits from the Tamengo Formation present evidence of taphonomic influences on the transporting/reworking of fragmentation and disarticulation of Cloudina tubes. Differences in size distributions between some of the localities have demonstrated that this trait is not reliable for defining or synonymizing species of Cloudina, and these differences probably reflect a myriad of taphonomic and paleobiological phenomena. Moreover, in some outcrops of the Tamengo Formation, shell walls are usually poorly preserved due to plastic deformations and diagenetic dissolution/recrystallization processes, which conceal morphological diagnostic features used in Cloudina taxonomy. Similar taphonomic biases may be of equal importance to the taxonomy of Cloudina preserved in other upper Ediacaran carbonates. Hence, earlier claims in favor of the synonymization of Cloudina species from the Tamengo Formation cannot currently be justified.

Introduction

Ediacaran strata offer some of the first evidence for many evolutionary innovations, including the appearance of macroscopic complex biotas (Shen et al., 2008), motility (Ivantsov and Malakhovskaya, 2002, Ivantsov, 2013, Liu and McIlroy, 2015) and skeletonized animals (Penny, 2018). Given the complexity of these scenarios, disregarding the influence of taphonomy when creating taxonomic databases could induce erroneous assumptions regarding species (Lucas, 2001). For example, discoidal organisms from the Ediacaran were first described and differentiated into several different species and genera, likely based on the interpretation that they were impressions of medusa-like animals. As other evidence was considered, it was shown that a number of such discoid forms were holdfast impressions of frondose organisms (with preserved pieces of fronds) (Droser et al., 2006) and that their morphology was, in many cases, related to taphonomic biases (Tarhan et al., 2010, Tarhan et al., 2015). Thus, taphonomy will always be imperative to understanding the taxonomic composition of such important biotas (e.g., Gehling et al., 2000, Grey and Willman, 2009, Liu et al., 2011) and improving our comprehension of paleobiological (Laflamme et al., 2007, Flude and Narbonne, 2008, Elliott et al., 2011, Meyer et al., 2012, Narbonne et al., 2014), geobiological (Mapstone and McIlroy, 2006), and sedimentological processes (Warren et al., 2013).

This is the case for the shelly fossil Cloudina, whose taxonomy is still controversial despite being one of the most geographically widespread and abundant fossils in the latest Ediacaran carbonate rocks (Germs, 1972, Grant, 1990, Conway-Morris et al., 1990, Hofmann and Mountjoy, 2001, Hua et al., 2003, Hua et al., 2005, Cortijo et al., 2010, Kerber et al., 2013, Becker-Kerber and Pacheco, 2018, Adorno et al., 2017). With the exception of recent works (Cortijo et al., 2010, Yang et al., 2016, Cai et al., 2017), most of the species attributed to the genus Cloudina have been distinguished by their external diameters: Cloudina hartmanae Germs, 1972 (2.5–6.5 mm); Cloudina riemkeae Germs, 1972 (0.3–1.3 mm); and Cloudina lucianoi (Beurlen and Sommer, 1957) (0.2–3.8 mm in Zaine and Fairchild, 1987). Other than size, these three species were described by the same diagnostic characteristic: a conico-tubular shell constructed by excentrically nested funnel segments, with concentrically placed circular to ellipsoid layers in cross section (Germs, 1972, Zaine and Fairchild, 1987, Grant, 1990, Hua et al., 2005).

Recently, Cai et al. (2017) proposed a new and emended diagnosis for C. hartmanae and described two new species, C. ningqiangensis and C. xuanjiangpingensis, based on other morphological characteristics (e.g., external ornamentations) considered to be more reliable diagnostic features than size classes (Cai et al., 2017). Particularly for C. hartmanae, the differentiating feature was the presence of oblique and/or transverse annulations.

In this study, we discuss new taphonomic data regarding Cloudina deposits from the Tamengo Formation (Corumbá Group, Brazil) and how taphonomy impacts taxonomic interpretations for this fossil. As already stated, the species C. lucianoi from the Tamengo Formation was previously classified by considering its differentiation from other species by size distribution only (Zaine and Fairchild, 1987). This led some authors to propose Cloudina lucianoi as the senior synonym of C. hartmanae (Adorno et al., 2017; but see discussions in Becker-Kerber and Pacheco, 2018, Adorno et al., 2018), considering the priority of the publication date in Beurlen and Sommer (1957).

However, Tamengo Formation carbonates yielding Cloudina tubes were mostly accumulated in relatively shallow marine environments influenced by storms (e.g., indicated by hummocky bedding and packstones and grainstones consisting of Cloudina bioclasts) and intercalated with occasional, relatively deep-water sediments (e.g., fine pelites with Corumbella remains) (Spangenberg et al., 2013, Pacheco et al., 2015). Thus, it is expected that taphonomic factors (e.g. transporting/reworking) could have impacted their original morphology. To investigate this, we hypothesized that Cloudina deposits containing more tubes with few and incomplete segments in cross section, as well as a higher proportion of mud-filled tubes, represent taphocoenoses that suffered a high degree of fragmentation and transportation. We further hypothesized that diagenetic processes, such as dissolution/recrystallization, can also alter the diagnostic features used in Cloudina taxonomy.

Section snippets

Geologic context

The Corumbá Group is a Neoproterozoic succession from the Mato Grosso do Sul State (Southwest of Brazil) that occurs in the southern portion of the Paraguay Belt (Fig. 1). It is a basin that likely evolved under a rift-to-drift scenario (Alvarenga et al., 2000). The Corumbá Group overlies Marinoan glaciogenic diamictites of the Puga Formation (Alvarenga and Trompette, 1992, Boggiani et al., 2010).

The Corumbá Group has a thickness of approximately 600 m (Gaucher et al., 2003). From the base to

Materials and methods

We examined thin sections (n = 87) already housed (see Supplementary Table 1) at the Paleontological Scientific Collection of the Institute of Geosciences at the University of São Paulo (IGc-USP). Most of this material was collected during earlier studies from outcrops in the cities of Corumbá and Ladário (Zaine and Fairchild, 1985, Zaine and Fairchild, 1987, Zaine, 1991, Meira, 2011): specifically, Porto Sobramil (Saladeiro), the Laginha and Corcal quarries, and the Marina Gattass park

Results

We found that the Corcal and Porto Sobramil assemblages showed significantly higher occurrences of tubes with few segments (1, 2) in cross section compared to the Laginha and MG assemblages (Table 1), while the Laginha and MG samples presented significantly higher occurrences of tubes with more segments (4, 5) than the Corcal and Porto Sobramil samples (Table 1). This pattern appeared to be maintained when we analyzed the ratio of complete/incomplete segments (Fig. 5). For example, the Laginha

Discussion

Our taphonomic parameters suggest that alterations in the original morphology (mainly fragmentation) are higher in the Corcal and Porto Sobramil material than in other localities. This is supported by the predominance of individuals with few segments in cross-section and the additional rarity of specimens with four or more segments in cross-section (Table 1). Furthermore, the higher proportion of tubes with incomplete segments in cross-section (“crescentic fragments” in Grant, 1990) strongly

Conclusions

After identifying several taphonomic and diagenetic processes that have influenced the original morphological information of Cloudina from the Tamengo Formation, we reiterate the need for caution when using specific traits for defining or synonymizing species within the Cloudina genus. Variation in size could mean a number of things: ontogeny, intraspecific differences, depositional bias, etc. Hence, other important characteristics need to be considered when proposing taxonomic definitions for

Acknowledgments

This work was conducted during a master scholarship supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Funding Agency at the University of São Paulo. This work also had assistance from the FAPESP Funding Agency, under the Proc. 2013/17835-8. The authors would like to thank the Brazilian Nanotechnology National Laboratory (LNNano) for providing the facilities used for the SEM investigations (project SEM – 16826 and SEM - 23684), and the helpful assistance of

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      Citation Excerpt :

      According to the authors, this glacial event would be younger than the Gaskiers glaciation, and would correlate in part with the Shuram-Wonoka isotopic anomaly. There exists a prolific debate regarding taxonomic affinities of the Cloudina genus in the four decades of its study (Germs, 1972; Zaine and Fairchild, 1987; Grant, 1990; Conway Morris et al., 1990; Conway-Morris, 1993; Hua et al., 2003, 2005; Adorno et al., 2017), including morphology (Grant, 1990; Seilacher, 1999; Adorno et al., 2017; Mehra and Maloof, 2018), mineralogy (Grant, 1990; Brain, 2001; Hua et al., 2005; Becker-Kerber et al., 2017), paleoecology (Germs, 1972; Grant, 1990; Seilacher, 1999; Hua et al., 2005; Cortijo et al., 2010; Becker-Kerber et al., 2017; Mehra and Maloof, 2018) taphonomy (Cortijo et al., 2010; Meira, 2011; Warren et al., 2012; Becker-Kerber et al., 2017, 2019; Mehra and Maloof, 2018) and extinction (Brain, 2001; Hua et al., 2003; Becker-Kerber et al., 2017). In contrast, information about the sedimentary environment in which these fossils inhabited are restricted to the Nama Group in Namibia (Germs, 1972; Seilacher, 1999; Grotzinger et al., 2000) and the central region of the Iberian Peninsula (Vidal et al., 1994).

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