Age and diversity of the La Ciénega assemblage

Without more recent paleontological and paleoecological evaluation, McMenamin's (Reference McMenamin1985) original descriptions of the skeletal fossils of the La Ciénega Formation in 1983 and 1985 remain the primary sources of information on this assemblage. Grant (Reference Grant1990) reconsidered McMenemin's (Reference McMenamin1985) published La Ciénega fossils but did not agree on either the taxonomic identities of the organisms or the age of the deposit. In the ~40 years since, little work has been done on this early skeletal assemblage, with the exceptions of index fossil examination by Sour-Tovar et al. (Reference Sour-Tovar, Hagadorn and Huitrón-Rubio2007) to constrain the Ediacaran-Cambrian boundary in the region and more recent integrative geochronological, chemostratigraphic, and lithostratigraphic work by Hodgin et al. (Reference Hodgin, Nelson, Wall, Barrón-Díaz, Webb, Schmitz, Fike, Hagadorn and Smith2021) that included a biostratigraphic summary of the formation in context. As an important note, McMenemin's assessments (McMenemin et al., Reference McMenamin, Awramik and Stewart1983; McMenemin, Reference McMenamin1985) did sample fossil materials from sections other than that reported herein from the Cerro Clemente section, including the Cerro Rajón, Cerros Calaveras, and Cerros Pitiquito; his reported fossils were comparable across the examined sites. Below, we summarize these contributions and conundrums with regard to the paleontological assessment of the La Ciénega fossils.

The original reports of the La Ciénega fossils—first by McMenemin et al. (Reference McMenamin, Awramik and Stewart1983) without taxonomic assignments and subsequently with details of affinity by McMenemin (Reference McMenamin1985)—described them as a polytaxic assemblage, occurring as a coquina within the host sandy dololimestone. Indeed, the first report (McMenemin et al., Reference McMenamin, Awramik and Stewart1983) used a morphological grouping system much like we employed herein, including the following categories: (1) smooth, single-walled tubes (tube diameters ranging from 1.2–1.8 mm; lengths, fragmented, to 10 mm); (2) smooth-surfaced, multiple-walled tubes (2–3 mm in diameter; lengths, fragmented, to 10 mm); (3) robust, irregularly annulated, single-walled tubes (averaging 2.4 mm in diameter; lengths, fragmented, to 10 mm); and (4) a single regularly annulated tube (1.7 mm in diameter; fragmented length 4.8 mm). The annulated tubes, in his 1985 contribution, were all united under a novel species, Sinotubulites cienegensis McMenamin, Reference McMenamin1985. His smooth tubes, on the other hand, were suggested to represent circothecid hyoliths and/or sinuously curving conoid specimens of Cambrotubulus cf. C. decurvatus Missarzhevsky in Rozanov et al., Reference Rozanov, Missarzhevsky, Volkova, Voronova and Krylov1969. Where our groups are comparable, specifically the smooth-walled tubes (our forms 4 and 5) and the sinotubulitids (our form 1), our morphometric data are relatively comparable, with a nearly identical mean maximum diameter for the sinotubulitids at ~2.4 mm and generally smaller on average, but still in range, for the smooth tube groupings to ~1.8 mm in diameter (Table 1). Our material appears slightly more fragmented, however, with notably shorter lengths, although this could be relative to discrepancies in the methods used for acid extraction and cleaning.

Based on his identifications of Cambrotubulus and circothecids, McMenamin (Reference McMenamin1985) suggested a lower Cambrian age, specifically Meishucunian/Tommotian—terminology which now corresponds to the Terreneuvian Series, likely Series 2. He noted, however, that the presence of Sinotubulites required a range extension of this taxon into the Cambrian from older rocks, based on (at the time) the primary identifying report of the genus from the Ediacaran-Cambrian transition in China (Chen et al., Reference Chen, Chen and Qian1981). The alternative inference—that Cambrotubulus could be extended downward into older units—was ignored based on the suggested co-occurrence of Sinotubulites and Coleoloides Walcott, Reference Walcott1889 in the similar Deep Spring assemblage of Nevada (Signor et al., Reference Signor, McMenamin, Gevirtzman and Mount1983). Largely because of the presence of Coleoloides, although notably the least common faunal element in the Deep Spring Formation, this assemblage was deemed to be lower Cambrian in age and comparable to other well-known Coleoloides-bearing Cambrian units (Landing and Brett, Reference Landing and Brett1982; Bengtson and Fletcher, Reference Bengtson and Fletcher1983; Signor et al., Reference Signor, McMenamin, Gevirtzman and Mount1983).

In 1990, Grant published an overview of Cloudina, its global distribution, and its time constraints, meant to argue for the use of this ubiquitous fossil as an index for the terminal Proterozoic. Within this contribution, Grant (Reference Grant1990) also detailed the shell structure, construction, and taphonomy of these organisms, which in several cases led to the re-evaluation or reconsideration of other published tubular shelly taxa from the same time interval—e.g., including Sinotubulites, Nevadatubulus Signor, Mount, and Onken, Reference Signor, Mount and Onken1987, and Wyattia Taylor, Reference Taylor1966, which were therein suggested to be “probably either con-generic with, or closely related to, Cloudina” (Grant, Reference Grant1990, p. 269). Later in this contribution, Grant (Reference Grant1990) commented on McMenemin's (Reference McMenamin1985) likely misinterpretations, noting that the wave-invaginated circothecids instead showed the characteristic multilayered shell wall construction known of Cloudina and that the longitudinal ribs used to indicate the presence of sinotublitids were likely folds formed during compaction. In summary, Grant (Reference Grant1990) surmised that the La Ciénega assemblage—as well as the contemporaneous Deep Spring shelly fauna (Signor et al., Reference Signor, Mount and Onken1987)—contained only Cloudina and refuted a lower Cambrian placement in favor of the terminal Proterozoic. Similarly, Sour-Tovar et al. (Reference Sour-Tovar, Hagadorn and Huitrón-Rubio2007) noted the presence of the cloudinid-bearing coquinas in Units 1 and 4 of the La Ciénega and noted their comparisons to other late Ediacaran skeletal packstones globally but suggested only the presence of Cloudina hartmannae on the basis of size and morphological comparisons. Nevertheless, it is challenging to identify the fossils in either the polished slab or bedding surfaces as shown (Sour-Tovar et al., Reference Sour-Tovar, Hagadorn and Huitrón-Rubio2007). For instance, the close-up that they illustrated (Sour-Tovar et al., Reference Sour-Tovar, Hagadorn and Huitrón-Rubio2007, fig. 2d), without extracting the fossil, could instead represent the wrinkled transverse corrugations of Sinotubulites as opposed to the funnel-in-funnel form of cloudinomorphs.

Recent work has provided better age constraints to these units using radiometric dating, bio-, and chemostratigraphic approaches (Smith et al., Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016; Hodgin et al., Reference Hodgin, Nelson, Wall, Barrón-Díaz, Webb, Schmitz, Fike, Hagadorn and Smith2021; Nelson et al., Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023). Smith et al. (Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016), for example, showed that the skeletal fauna of the Deep Spring Formation lies below the first appearance of Treptichnus pedum and the BACE. Although it is true that the global utility and correlation of the BACE has lately been questioned (e.g., Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields and Zhou2022; Warren et al., Reference Warren, Inglez, Xiao, Buatois and Mangáno2023), these data, along with new reports from Hodgin et al. (Reference Hodgin, Nelson, Wall, Barrón-Díaz, Webb, Schmitz, Fike, Hagadorn and Smith2021) and Nelson et al. (Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023) help to refine the relative age of these correlative southwestern USA and northwestern Mexico units to the latest Ediacaran–earliest Cambrian. From the regional litho-, chemo-, and biostratigraphy presented by Nelson et al. (Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023), tube-bearing fossiliferous horizons along the southern Nevada-California boundary (from west to east: Hines Ridge, Mount Dunfee, Boundary Canyon, Montgomery Mountains, and Spring Mountians) are constrained by a minimum ID-TIMS U-Pb age of 532.83 ± 0.98 Ma as assessed directly from detrital zircons at the Spring Mountains locality, Nevada. From their proposed framework (Nelson et al., Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023), this date precedes all regionally correlative occurrences of Treptichnus pedum and postdates all tubular fossil horizons and any reported occurrences of erniettomorphs. Comparably, Hodgin et al. (Reference Hodgin, Nelson, Wall, Barrón-Díaz, Webb, Schmitz, Fike, Hagadorn and Smith2021) added xircon U-Pb geochronology to their integrated litho-, chemo-, and biostratigraphy of the Sonoran La Ciénega and Cerro Rajón formations, as measured at the Cerro Rajón, Cerro Clemente, and Cerro San Agustín sections. The relative upward positioning of skeletal coquina in Unit 1, the onset and nadir of the BACE in Units 2 and 3, the zircon-bearing sandy dolostone in uppermost Unit 3 (providing a maximum depositional age of 539.40 ± 0.23 Ma), and T. pedum in the overlying Cerro Rajón Formation, collectively place the La Ciénega Formation in the late Ediacaran. When viewed in the broader context across the southwestern USA localities and those in northwestern Mexico, as presented by Nelson et al. (Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023), the tubular fossils generally fall prior to the inferred correlation of the 539.40 Ma date, largely hung proximal to the BACE—although some sections show closer stratigraphic proximity between the fossil-bearing horizons and the nadir of the BACE than others (e.g., tens of meters or less in Spring Mountains, Montgomery Mountains, and Hines Ridge, versus >100 m in Cerro Rajón). The Mount Dunfee section, on the other hand, shows multiple horizons of tubular fossils ranging across the Reed Dolomite and Deep Spring Formation (e.g., Smith et al., Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016; Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020; Nelson et al., Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023).

From our assessments presented herein, we favor a polytaxic view of the La Ciénega tubular assemblage as opposed to Grant's (Reference Grant1990) monotaxic view. Based on observed morphologies and size groupings, we suggest the presence of at least three distinguishable taxa and another morphological grouping that remains unresolved. Given the extent of taphonomic overprinting observed within the extracted samples, we stress our apprehension in assigning fossils observed within this unit below the genus-level, although we can make provisional recommendations at the species level. We present these recommendations below.

Form 1: Sinotubulites

From the original description of Sinotublites (Chen et al., Reference Chen, Chen and Qian1981) from the late Ediacaran Dengying Formation, China, fossils of this genus and its junior synonym Qinella Zhang, Li, and Dong in Ding et al., Reference Ding, Zhang, Li and Dong1992 are characterized as cylindrical, multilayered tubular fossils with a tube-in-tube construction, an irregularly but strongly corrugated outer wall, and a weakly ornamented to smooth inner wall (as emended by Cai et al., Reference Cai, Xiao, Hua and Yuan2015 and Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022). This genus is known to have a cosmopolitan distribution during the late Ediacaran, with a range spanning China, Namibia, Spain, Brazil, Mexico, and the southwestern USA (see occurrences presented by Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022), and it often co-occurs with Cloudina—leading to the suggestion of the Cloudina-Namacalathus-Sinotubulites Assemblage Zone by Zhu et al. (Reference Zhu, Zhuravlev, Wood, Zhao and Sukhov2017), preceding the Anabarites-Protohertzina Assemblage Zone of the lowermost Cambrian. McMenamin (Reference McMenamin1985) established a new species of this genus from the Sonoran material, Sinotubulites cienegensis, based on his observations of less strongly oblique and bifurcating annulae (= corrugations) as compared to the type species of the genus, Sinotubulites baimatuoensis. Taking into account both taphonomic considerations and interspecific variation, however, Cai et al. (Reference Cai, Xiao, Hua and Yuan2015) synonymized Sinotubulites cienegensis—both from Sonora (McMenemin, Reference McMenamin1985) and the White-Inyo region of the southwestern USA (Signor et al., Reference Signor, Mount and Onken1987)—along with several other taxa, under the type species.

In addition to synonymies, Cai et al. (Reference Cai, Xiao, Hua and Yuan2015) erected three novel species of Sinotubulites, specifically owing to the polygonality of the tube cross sections: Sinotubulites triangularis Cai et al., Reference Cai, Xiao, Hua and Yuan2015; Sinotubulites pentacarinalis Cai et al., Reference Cai, Xiao, Hua and Yuan2015; and Sinotubulites hexagonus Cai et al., Reference Cai, Xiao, Hua and Yuan2015. Although Sinotubulites cienegensis might not be valid on its own, it is possible that the La Ciénega assemblage contains representatives of these polygonal Sinotubulites species. Indeed, McMenamin (Reference McMenamin1985) noted specifically that the cross sections of his observed La Ciénega Sinotubulites representatives ranged in shape, including circular, subellipsoidal, oblate, and irregularly polygonal. Without enough figured cross sections by McMenemin (Reference McMenamin1985) to illustrate their polygonal nature, resolving these taxa is not possible—although the singular cross section, appearing slightly oblique to the transverse plane, shown in his figure (McMenemin, Reference McMenamin1985, fig. 3.2) might tenuously suggest a pentagonal profile akin to Sinotubulites pentacarinalis. It is important to comment, however, that Signor et al. (Reference Signor, Mount and Onken1987) did not indicate observation of any polygonal cross sections from the contemporaneous southwestern USA White-Inyo fossil materials, nor were we able to assuredly identify any transversely polygonal specimens in our examination. From our acid-extracted materials, μCT volume viewing, and cross sections observable in thin- and thick-section, all fossils that we were able to assign as sinotubulitids show either the generally rounded profile of Sinotubulites baimatuoensis or taphonomically altered variations, e.g., flattened but recognizable sinotubulitid tubes (Fig. 5.1). As such, we suggest that all of our observed sinotubulitids be classified as Sinotubulites baimatuoensis, although we can also cautiously submit that Sinotubulites triangularis, Sinotubulites pentacarinalis, and/or Sinotubulites hexagonus might be present in the La Ciénega assemblage based on McMenemin's (Reference McMenamin1985) description. Nevertheless, we suggest that they would be only rare components and, at present, cannot confirm their occurrence.

Based on the emended diagnoses by Cai et al. (Reference Cai, Xiao, Hua and Yuan2015) and Yang et al. (Reference Yang, Warren, Steiner, Smith and Liu2022), the morphometric measurements assessed, and morphological features observed, the La Ciénega material fits well within the generic dimensions and descriptions. Our La Ciénega Sinotubulites baimatuoensis specimens are only marginally smaller in diameter (1.10–5.23 mm; N = 33) than the formally described range of the species (1.5–6 mm; Cai et al., Reference Cai, Xiao, Hua and Yuan2015) and relatively comparable to McMenemin's (Reference McMenamin1985) measurements of fossils from the same unit (< 0.9–4.3 mm; N = 21). Although crest-to-crest spacing of the corrugations can be regular or irregular, as well as densely or sparsely arranged (Cai et al., Reference Cai, Xiao, Hua and Yuan2015), our specimens tend to show relatively regular spacings of < 1 mm. Similarly, the total wall thickness in the La Ciénega materials tends to be highly variable, reported at 0.2–1.1 mm BY McMenemin (Reference McMenamin1985) and observed here ranging from 0.10–0.61 mm. Like the argument posed by Cai et al. (Reference Cai, Xiao, Hua and Yuan2015), we suggest that these size distinctions are a function of interspecific variation and/or taphonomic differences between the Mexican and Chinese materials, rather than warranting the establishment of a new species. Taphonomic quality almost certainly plays a large role, considering the distinctions in fidelity between the finely phosphatized Chinese materials and the coarsely silicified Mexican materials.

Forms 2 and 3: Cloudinomorphs

As suggested by Grant (Reference Grant1990), many of the forms identified in the original McMenemin (Reference McMenamin1985) report could reflect cloudinomorphs, e.g., the eccentrically nested funnel-in-funnel forms figured as transverse sections by McMenemin (Reference McMenamin1985, fig. 5.3, 5.6). Our extracted materials more confidently identify the presence of two cloudinomorphic forms, described below.

If we consider the correlative tubicolous fauna of the White-Inyos in the southwestern USA for reference, our form 2, with nonrimmed funnels or collars, appears most similar to the contemporaneous cloudinomorph Nevadatubulus dunfeei Signor, Mount, and Onken, Reference Signor, Mount and Onken1987 from the Mt. Dunfee, Nevada location of the Deep Spring Formation. This fossil taxon is abundant in its noted localities, is coarsely calcified, ranges from 0.5–1.2 mm in diameter, and appears to be structurally very similar to our extracted materials. Nevadatubulus, however, has been suggested to be synonymized with the type species of Cloudina, Cloudina hartmannae Germs, Reference Germs1972, by independent authors on the basis of comparable tube structure (Yang et al., Reference Yang, Steiner, Zhu, Li, Liu and Liu2016, Reference Yang, Warren, Steiner, Smith and Liu2022; Cai et al., Reference Cai, Cortijo, Schiffbauer and Hua2017). Differences in coarser (Nevadatubulus) versus finer (Cloudina) tube-wall layering was suggested, like above, to be a result of differential taphonomy between localities. Although there is a very wide diameter range of what has been classed as Cloudina, in a right-skewed distribution from hundreds of μm to > 4.5 mm (see Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022), our materials tend to be at the larger end of the range, with maximum diameters extending to 2.61 mm. Of note, this would have also been large compared to the former Nevadatubulus range (0.5–1.2 mm, as reported by Signor et al., Reference Signor, Mount and Onken1987). We anticipate, however, that the measurements of our specimens are exaggerated owing to the pervasive silica overgrowth on the fossil tube walls, but by what magnitude much remains unknown. Based on our observations and notable morphological similarities to well-preserved specimens reported from Lijiagou, China (Cai et al., Reference Cai, Cortijo, Schiffbauer and Hua2017), we consider our form 2 fossils to be most logically identified as representatives of Cloudina sp. indet. Although Cai et al. (Reference Cai, Cortijo, Schiffbauer and Hua2017) reported other species based on the presence and extent of transverse funnel-surface annulation, our specimens are too coarsely preserved to capture such details, and hence our suggestion for open nomenclature. Nevertheless, Cai et al.'s (Reference Cai, Cortijo, Schiffbauer and Hua2017) novel species Cloudina ningqiangensis Cai et al., Reference Cai, Cortijo, Schiffbauer and Hua2017, with smooth-sided funnels and a maximum diameter ranging from 0.22–2.67 mm, might be the appropriate taxonomic home if better preserved samples can be found, although its tightly nested funnels (minimal funnel spacing) is notably smaller than what we observed.

Our form 3 shows thickened funnel rims, a marginally smaller mean diameter (1.43 mm, N = 22), and a more prominent funnel-rim sag or droop along the sagittal plane (Fig. 2.4). Reminiscent of Conotubus hemiannulatus Zhang and Lin in Lin et al., Reference Lin, Zhang, Zhang, Tao and Wang1986, this parabolic funnel rim appearance and imbrication was argued by Cai et al. (Reference Cai, Hua, Xiao, Schiffbauer and Li2010) to result from angled shear of a nonbiomineral (but robust) tube during burial and compaction. Like Cloudina, Conotubus Zhang and Lin in Lin et al., Reference Lin, Zhang, Zhang, Tao and Wang1986 exhibits a wide range of tube diameters from hundreds of μm at narrowest to >10 mm at widest (Cai et al., Reference Cai, Schiffbauer, Hua and Xiao2011).

With more recent descriptive work analyzing pyritized tubular fossils from the Mt. Dunfee and Wood Canyon localities in Nevada (e.g., Smith et al., Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016, Reference Smith, Nelson, Tweedt, Zeng and Workman2017; Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020) that are notably similar to Conotubus and other cloudinomorphs (e.g., Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022), these taxa should also be considered as possibilities. The closest in form is likely Saarina hagadorni Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020, which shows flared funnel rims and comparable tube dimensions, although distinct in preservational mineralogy—pyritization in Nevada versus silicification here. These two genera—Conotubus and Saarina Sokolov, Reference Sokolov1965—are obviously close in character, with Smith et al. (Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016, Reference Smith, Nelson, Tweedt, Zeng and Workman2017) initially reporting these and comparable forms under the Conotubus moniker. Indeed, the funnel rims of Saarina hagadorni can also show the same drooping, imbricated appearance as that noted in Conotubus hemiannulatus (see, e.g., Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020, fig. 2E–F)—although, at least when using high-resolution electron microscopy, Saarina tends to have a more pronounced outward flare of the funnel rims than Conotubus (Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020). Given this, and the paleogeographic proximity of the Sonoran and southwestern USA localities, we recommend that specimens of our form 3 be considered cloudinomorphs cf. Saarina hagadorni, with remaining taxonomic uncertainties owing to striking differences in taphonomic resolution between the pyritized versus silicified specimens.

It remains possible that representatives of our form 3 are congeneric with those of form 2, with differences imposed only by taphonomy. For example, in the description of Nevadatubulus (Signor et al., Reference Signor, Mount and Onken1987), there are two figured specimens (holotype UCMP 37537 and paratype 37541, their fig. 4.1–4.4) that also exhibit apparently flared funnel rims—more so than any of the other figured paratypes, which look more convincingly similar to our form 2 grouping. However, no mention of the dissimilarity in the funnel-rim pronunciation was made (Signor et al., Reference Signor, Mount and Onken1987). If these thickened- and flared-rim variants of Nevadatubulus (Signor et al., Reference Signor, Mount and Onken1987) can also be considered cf. Saarina hagadorni, an interesting continuation of this work would be to examine this and other possible community-level parallels between the Sonoran and White-Inyo/Mt. Dunfee assemblages.

Forms 4 and 5: Smooth-walled forms

As compared to the other forms reported herein, the smooth-walled specimens (forms 4 and 5) are perhaps even trickier to assign, although it is clear that the same forms had been observed previously (McMenemin, Reference McMenamin1985). With regard to the curved, narrowly conical, smooth-walled tubes (our form 5), McMenemin (Reference McMenamin1985) presented a case that they represented comparators of Cambrotubulus decurvatus. He additionally cited personal communication with Signor for a similar occurrence in the Mt. Dunfee assemblage, although no such mention was made by Signor et al. (Reference Signor, Mount and Onken1987). Even though the presence of this taxon was the exemplar used to argue a lower Cambrian age for the unit, the description provided was somewhat hesitant or indecisive, citing the possibilities of these fossils being steinkerns or silicified microcoprolites (McMenemin, Reference McMenamin1985). Similarly, McMenemin (Reference McMenamin1985) reported the straight, conical, smooth-walled forms (our form 4) as likely circothecid hyoliths, although indeterminate at the genus and species levels given the lack of any surface ornamentation that could be useful for taxonomic identification. Given the abundance of possible genera to which these forms could belong—for example including Cambrotubulus, Hyolithellus Billings, Reference Billings1871, Coleolella Missarzhevsky in Rozanov et al., Reference Rozanov, Missarzhevsky, Volkova, Voronova and Krylov1969, Conotheca Rozanov et al., Reference Rozanov, Missarzhevsky, Volkova, Voronova and Krylov1969, Cupitheca Duan in Xing et al., Reference Xing, Ding, Luo, He and Wang1984, and some of the less-descript anabaritids—compounded with a dearth of identifying features, we suggest keeping these as unresolved tubiform problematica at present until further information can be provided. Given the significant overlap in sizes from the sinuous and straight forms that we observed, we cannot even confidently suggest that these represent more than interspecific, ontogenetic, or taphonomic variation within one taxon.

Taphonomy of skeletons

Whereas terminal Ediacaran skeletal tubes show a wide range of taphomodes, from calcification and phosphatization to pyritization and silicification (Yang et al., Reference Yang, Steiner, Schiffbauer, Selly, Wu, Zhang and Liu2020), the extent to which late Ediacaran tubular organisms were originally biomineralized has been a point of question in past literature. Although most commonly preserved in brittle material through replacive mineralization, regardless of final taphonomic composition, the original shell might have been at least somewhat pliable. For instance, Grant (Reference Grant1990) observed evidence of plastic deformation in the materials assigned to Cloudina. As a primitive form of biomineralization preceding true matrix-mediated or biologically controlled pathways, Grant (Reference Grant1990) proposed that the tubes of cloudinids might have originally been composed of organic materials with periodically deposited calcium carbonate contributing added rigidity and structural integrity. This suggestion was echoed by Hua et al. (Reference Hua, Chen, Yuan, Zhang and Xiao2005), who proposed that calcium carbonates were secreted and suspended in an organic matrix, which then cured to create the skeleton. Shore and Wood (Reference Shore and Wood2021) also argued that Cloudina skeletons mineralized on an organic template, perhaps easily assimilated from high alkalinity seawater in the late Ediacaran (Wood et al., Reference Wood, Ivantsov and Zhuravlev2017). Chen et. al. (Reference Chen, Bengtson, Zhou, Hua and Yue2008) inferred the same for Chinese examples of Sinotubulites, suggesting that their shell might have been comprised of a combination of both organic and calcareous materials. All of these inferences and suppositions were supported or argued by suggestions of flexure in the skeletal tubes of these terminal Ediacaran organisms—which were provided empirical support by Yang et al. (Reference Yang, Steiner, Schiffbauer, Selly, Wu, Zhang and Liu2020), who showed finely interlayered calcium phosphate and carbonaceous laminae in an ultrastructural study of exceptionally well-preserved cloudinomorphs, including Cloudina specimens from Paraguay and Namibia as well as the newly described Zuunia Yang et al., Reference Yang, Steiner, Schiffbauer, Selly, Wu, Zhang and Liu2020 from Mongolia. Ultimately, however, the level of biological control of skeletonization within these earliest biomineralizing metazoans might have varied by paleoenvironment, seawater chemistry and alkalinity, and ecological pressures, among other plausible drivers (Wood, Reference Wood2018). Here, we provide additional data supporting an original flexibility in the shells of the La Ciénega sinotubulitids and cloudinomorphs.

As supported by our combination of analyses, the La Ciénega assemblage exhibits two prominent modes of preservation—silicification and calcification—that both represent taphonomically replacive processes and thus do not reflect an original mineralogy of the fossils. From acid-extraction methods, we captured only a selective glimpse of the total diversity of this assemblage—that is, the portion of the assemblage that was silicified, because the calcareous shells were dissolved along with the dolomite packstone host. The calcareous shells, on the other hand, are only observable in petrographic section, polished slab view, or in μCT tomographs (Fig. 7). With regard to the latter, although indications of calcareous shells were visible, being able to resolve their three-dimensional structure apart from the host packstone matrix was challenging at best with little X-ray attenuation contrast; thus, determining the affinities of these ‘ghost tubes’ was also hindered. Nevertheless, we are confident that calcareous shells are indeed present and abundant within the La Ciénega assemblage from petrographic and thick-section observations and SEM-EDS analyses. In cross section, these do not appear substantially different from those that are silicified; however, for reliable assessments of morphology, size, and taxonomy reported herein, we focused only on the silicified materials, because the extractable materials provided our best three-dimensional views.

Figure 7.

μCT views of fossils in hand sample. (1) Photogrammetric view of μCT-scanned sample, viewing the y-axis face. (2) Three-dimensional μCT tomographic reconstruction; red = segmented fossil data, brown = host rock matrix; y-axis to right, x-axis to left. (3) Two-dimensional slice example along x-axis showing fossil abundance by volume viewing; darker gray indicates fossils that were segmented in red in the volume view (2). Scale bars = 1 cm.

Our petrographic observations provided insight into the nature of deposition of these fossils but proved curious in explaining the taphonomic progression of this fossil assemblage, owing to the disarranged juxtaposition of silicified and calcareous tubes. Within the silicified portion, we suggest that it is possible to identify at least two phases of tube replacement—an initial phase of silicification followed by a second phase of infilling the internal cavity of the tube with carbonate cement. In conjunction with the observations of the calcareous tubes, we also infer that there was another phase of mineralization wherein formerly unsilicified tubes were remineralized, although this might have taken place simultaneously with the carbonate infilling of the silicified shells. The patchy silicification within the fossil packstone along with evidence of tube malleability within the silicified fraction could suggest that some of the tubular fossils were transported, likely during a storm event. In these cases, it is possible that postdepositional silicification occurred specifically within unoccupied tubes (those with no remaining organism or carcass), not only allowing for easier tube deformation, e.g, flattening or twisting, but also where percolation of burial fluids would have been less restricted than in tubes still filled with a recently deceased carcass. The latter, where carcasses were still intact in the buried tube, were likely instead remineralized during later phases of carbonate dissolution and cementation. A similar scenario was evoked by Warren et al. (Reference Warren, Fairchild, Gaucher, Boggiani, Poiré, Anelli and Inchausti2011) from analyses of the Itapucumi tubular assemblage in Paraguay, in which they inferred that tubes buried with an intact carcass would have inhibited both tube breakage during transport and either sediment or cement infill until later stages of decomposition. Alternatively, the taphonomic selectivity could also hint toward some taxonomic or histological selectivity of the observed mineralization processes, but as described above, we are hesitant to attempt identification of the ghost calcareous tubes with the currently available data.

The silicified fossils additionally revealed a propensity for elliptical and noncircular cross sections seems to point toward some level of plastic deformability. The presence of both circular and elliptical tube cross sections within the same taxon have usually been interpreted as differential tube compression during sediment compaction, in which those exhibiting circular cross sections were mineralized prior to compaction and those with elliptical cross section were not (Cai et al., Reference Cai, Schiffbauer, Hua and Xiao2011). However, Mehra and Maloof (Reference Mehra and Maloof2018) found that elliptical Cloudina cross sections were not oriented in the same direction (i.e., compaction stresses were not uniform) in proposed cloudinomorph reef structures, and therefore were most likely deformed in vivo. From this account, Mehra and Maloof (Reference Mehra and Maloof2018) inferred that Cloudina was likely only weakly biomineralized to account for the ease of deformation either before or during initial burial. Alternatively, this might not have been a deformational feature but instead a biological one. Like the reconstruction by Becker-Kerber et al. (Reference Becker-Kerber, Pacheco, Rudnitzki, Galante, Rodrigues and Leme2017), Cloudina might have had a recumbent life mode and thus a flatter bottom side of the skeletal tube (also hinted by Wood, Reference Wood2011), sometimes also inferred of Conotubus (Cai et al., Reference Cai, Schiffbauer, Hua and Xiao2011) and contemporaneous organic-walled tubes (e.g., Cai et al., Reference Cai, Hua and Zhang2013; Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020). Like these many examples before, the La Ciénega samples herein show a variety of both elliptical and circular cross sections (Figs. 2, 4), which could indicate early but differential taphonomic mineralization of initially weakly-to-nonmineralized tubes, and/or little stress imposed by sediment compaction. Others, however, show complete flattening, like the sinotubulitid in Figure 5.1, likely induced during initial burial and sediment compaction before silicification. In addition to funnel-unit imbrication (Fig. 2.4) or even marginal helical twisting (Fig. 5.2) one of our Cloudina specimens shows apparent torsion of a detached funnel unit, but another funnel with a clear generally circular cross section (Fig. 5.3), ruling out deformation affecting the whole organism. Our findings indicate that tubes of many taxa within this assemblage were prone to ductile deformation prior to sediment compaction. Any of these plausible inferences allow for the skeletons to deform biologically, during transport, or during burial, but also show brittle deformation after mineralization and burial stresses, nonetheless indicating a likely pliable in-vivo form.

Tentative inferences of predation

The last features of note, which might not expound further on the original mineralogy (or lack thereof) of the skeletal tubes, but coiuld provide evidence of their sturdiness, are what appear to be circular to subcircular punctures or borings (Fig. 5.4, 5.5) similar to those previously observed in Cloudina (Bengtson and Yue, Reference Bengtson and Yue1992; Hua et al., Reference Hua, Pratt and Zhang2003; Becker-Kerber et al., Reference Becker-Kerber, Pacheco, Rudnitzki, Galante, Rodrigues and Leme2017). Interpreted to be the result of drilling predators and an indication of both increasing trophic complexity and the protective function of tubicolous life modes, the generally < 100 μm diameter borings observed by Hua et al. (Reference Hua, Pratt and Zhang2003) were interestingly taxon-specific. In a skeletal assemblage that contained both Cloudina and Sinotubulities, only Cloudina shells were observed to have these plausible borings. In our samples, these features were quite rare (only a handful, < 5 convincing examples, were seen in our 106 total specimens) but were observed in both our Cloudina sp. indet. (Fig. 5.4) and smooth-tube morphogroup (Fig. 5.5). Our holes were ~300–400 μm in maximum diameter, which is comparable to the range reported by Bengtson and Yue (Reference Bengtson and Yue1992), only slightly larger than those figured in Becker-Kerber et al. (Reference Becker-Kerber, Pacheco, Rudnitzki, Galante, Rodrigues and Leme2017), and 4–5× the size range reported by Hua et al. (Reference Hua, Pratt and Zhang2003). Although we are not entirely confident that these punctures are traces of predation, their elliptical to circular regularity (Kowalewski, Reference Kowalewski1993) is a tantalizing indicator, considering the surface roughness of these samples imparted by the silicification process. However, it is perhaps equally as likely that these represent included and rounded grains that were captured during the silicification process, that either were later weathered or lost during our acid-extraction protocol. Nonetheless, finding more of these punctures during continued investigation could prove fruitful in uncovering their origin.