The first fossils of organisms with mineralized skeletons appear in strata from the Terminal Ediacaran, between about 550 and 538 million years ago. This appearance coincides with the decline of the organisms of the 'classical' Ediacaran White Sea Fauna. One of the most abundant of these early mineralized fossils is Cloudina, a diverse group of fossils with a cone-in-cone or funnel-in-funnel structure, although the extent to which Cloudina was mineralized appears to have been variable, with some forms heavily mineralized, some only lightly so, and others apparently having skeletons made from tough organic materials. Cloudina was first described from the Terminal Ediacaran Nama Formation of Namibia, but has since been found in Brazil, Spain, China, Oman, and the United States, and is considered to be a useful index-fossil for the Terminal Ediacaran.
While Clodina is often the dominant fossil in Terminal Ediacaran assemblages, it is often found alongside a variety of other tubular fossils, such as Namacalathus, Namapoikia, and Sinotubulites. Sinotubulites fossils have a tube-in-tube structure and often have annular or longitudinal ridges, as well as both circular and polygonal cross-sections. Originally described from South China, Sinotubulites has also been described from Mexico, the United States, Brazil, Spain, and Namibia. Interestingly, which specimens of Cloudina have been discovered in many places with what-appear to be drill-holes, possibly the oldest known example of predation by boring, this has not been observed in specimens of Sinotubulites, even when the two are found together, potentially representing the oldest example of prey-selection.
Ediacaran fossils in the Terminal Ediacaran La Ciénega Formation of Sonora State, Mexico, were first described in the mid 1980s. The initial descriptions suggested a variety of tubular fossils were present, including Sinotubulites but not Cloudina. However, subsequent studies of the material led to the conclusion that these 'Sinotubulites' fossils were in fact specimens of Cloudina which had undergone taphonomic alteration, causing them to develop compactional folds which were mis-interpreted as the longitudinal striae of Sinotubulites.
In a paper published in the Journal of Paleontology on 10 October 2024, James Schiffbauer of the Department of Geological Sciences and X-ray Microanalysis Laboratory at University of Missouri, Clara Wong also of the Department of Geological Sciences at University of Missouri, and of the Department of Geosciences at Smith College, Cassidy Davis, also of the Department of Geological Sciences at University of Missouri, Tara Selly, again of the Department of Geological Sciences and X-ray Microanalysis Laboratory at University of Missouri, Lyle Nelson of the Department of Earth Sciences at Carleton University, and Sara Pruss, also of the Department of Geosciences at Smith College, re-examine the La Ciénega Formation fossil assemblage, using modern methodologies to provide new insights into the community structure preserved there.
The Caborca Block in Sonora comprises a series of Late Neoproterozoic and Early Palaeozoic strata laid down in shallow-marine environments on the edge of first a rift zone and then the southern margin of the continent of Laurentia. Here the early Ediacaran Period is represented by the Clemente Formation, which is followed by the Late Ediacaran La Ciénega Formation, and the Cambrian Cerro Rajón Formation. Uranium/lead analysis of zircons from the uppermost layer of the La Ciénega Formation places the age of this at 539.4 million years, the latest part of the Ediacaran. Zircon is a volcanic mineral that forms as liquid magma slowly cools to form solid rock. As zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead. This is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium. Since the decay of uranium to lead occurs at a steady rate, it is possible to determine the age of zircons by measuring the ratio of uranium to lead within them.
Locality map and stratigraphy of the Cerro Clemente section: (1) Map indicating position of Caborca localities (yellow star) in northern Mexico, and plausibly correlative fossiliferous units (grey stars) in the southwestern USA. (2) Satellite image from Google Earth denoting the topography of the Cerro Clemente section with longitude and latitude markers. (3) Geologic map corresponding to the same map view in (2). (4) Photograph of collected coquina block. (5) Stratigraphic section with carbon isotope chemostratigraphy showing position of uranium-lead radiometric date (green star) and sampled fossil horizon (yellow star). Schiffbauer et al. (2024). Schiffbauer et al. obtained a block from a silicified coquina bed within the La Ciénega Formation at the Cerro Clemente section measuring 15 cm x 10 cm x 10 cm. This was partitioned into a number of fragments, which were then treated to different analysis regimes, including dissolution with acetic acid to allow the collection of fossil residues, scanning electron microscope analysis, elemental analysis using an energy dispersive X-ray spectrometer, and thin sectioning for visual microscopy.
These methods enabled them to identify five distinct forms of tubes. Firstly, there were tube-in-tube structures with annular ridges and possible laminae, interpreted as Sinotubulites. Secondly, there were tubes made up of a series of tightly-fitting funnel-in-funnel structures lacking rims, interpreted as Cloudina. Thirdly, there were funnel-in-funnel structures with thickened and/or pronounced rims, interpreted as the Cloudinomorph Saarina. Fourthly, simple straight tubes, and fifthly, simple curved tubes.
Morphological groupings of fossils (SEM). (1, 2) Form 1, Sinotubulitids in lateral (1) and cross sectional (2) views: (1) lateral view showing diagnostic transverse corrugations; (2) cross section illustrating multiple tube-in-tube construction, with substantial silica overgrowth. (3), (4) Forms 2 and 3, Cloudinomorphs, Cloudina sp. indet. (3) and cf. Saarina sp. indet. (4): (3) Cloudina with two nested funnel units and no thickened apertural rims; (4) the other Cloudinomorph form showing thickened apertural rims with observable drooping imbrication. Also note slight change in growth direction or plastic deformation at tube midpoint, along with slight tubular compression. (5), (6) Forms 4 and 5, smooth tubes that are either straight (5) or sinuous (6). Scale bars are 1 mm (1), (3), (4), and (6), 500 μm (2), and (5). Schiffbauer et al. (2024).
Of the identifiable individual specimens, 33 were tube-in-tube structures, 20 were non-rimmed funnel-in-funnel structures, 23 were rimmed funnel-in-funnel structures, five were straight smooth tubes, and ten were curved or sinuous tubes. Thus, Cloudinomorphs were the most abundant group, making up about 40% of the sample, with Sinotubulitids comprising about 31% of the sample, and unidentified tubes making up 28%.
The Cloudinomorphs ranged from 0.88 to 2.61 mm in diameter, with an average of 1.51. The rimless forms were on average slightly larger, ranging from 1.13 to 2.61 mm in diameter, with an average of 1.65 mm., while the rimmed forms ranged from 0.88 to 2.26 mm in diameter, with an average of 1.43 mm. The Sinotubulitids were generally larger than the Cloudinomorphs, ranging from 1.11 to 5.23 mm in diameter, with an average diameter of 2.41 mm. The indeterminate tubular fossils ranged from 0.36 to 1.84 mm in diameter, with an average of 1.21, and little difference between the two forms (the straight forms average 1.22 mm in diameter, the sinuous forms 1.17 mm). The length of the fossils was much harder to estimate, as fossils of this size are prone to fragmentation; the longest Cloudinomorph found was 4.49 mm long, the longest Sinotubulitid 5.82 mm, and the longest smooth tube 5.59 mm.
Seen in thin section, much of the block was made up of densely packed tubular fossils. The majority of these were calcareous in nature, with only a minority of examples being silicified. However, the acid-extraction method produced only silicified specimens, with the calcarious fossils apparently lost from the portion of rock treated this way. This is likely to have given a distorted view of the nature of the total assemblage. Furthermore, the silicified fossils within the block cut into thin sections often had very fine skeletal walls, sometimes as little as 2–3 μm thick, while all of those extracted by acid etching were much courser, with the thinnest being 0.15–0.32 mm thick, suggesting that finer silicious fossils had also been lost.
Petrographic thin section photomicrographs. (1) Silicified tube examples (brighter white material) in transverse section (left) and longitudinal section (right). (2) Silicified funnel-in-funnel tube in longitudinal section, non-orthogonal to the length of the tube. Note blocky calcareous infilling and potential fine layering in the tube wall. (3) Transverse plane of non-silicified tube, with apparent fine layering and blocky calcareous infill. (4) Longitudinal plane of non-silicified tubular fossil with fine layering and micritic infill. Scale bars are 1 mm. Schiffbauer et al. (2024).
The calcitic tubes visible in the thin sections appeared to be made from a fine micrite, with courser carbonate and even dolomite crystals separating them. This could also be seen in scanning electron microscopy images, where it could also be seen that while the fine structures of the fossils were preserved, they were altered by the development of courser crystals around them, with many Cloudinomorph fossils having an elliptical cross-section (interpreted as distortion of an original circular shape), whole Sinotubulitids showed flattening, irregular cross-sections, and corrugation. One smooth-walled tube appears to have been helically twisted along its length. This makes it unlikely that the original surface structure of any of the fossils was preserved.
Surface and deformative features of silicified fossils (scanning electron microscope images). (1) Sagittally flattened Sinotubulitid specimen. (2) Imbricated funnel rims (dashed white lines to guide orientation) of cf. Saarina specimen with little-to-no flattening. (3) Torted funnel (left dashed curve), broken funnel wall (arrow), and intact funnel aperture (right dashed curve) of Cloudina sp. indet. specimen. (4) Ovoid puncture (arrow) in smooth (curved) tube. (5) Subcircular puncture (arrow) in Cloudina sp. indet. funnel (infilled). Scale bars are 1 mm. Schiffbauer et al. (2024).
Backscattered electron imaging and energy-dispersive X-ray spectroscope mapping of the thin sections suggested that there were many 'ghost tubes' present, which could not be visually observed, which were very nearly identical in composition to the host rock. There were detectable primarily by a lack of iron, an element present in the dolomite matrix.
Scanning electron microscope imaging and energy-dispersive X-ray spectroscope elemental maps of fossils in polished slab. (1) Overview giga-macro photomosaic of a portion of polished thick section; labelled rectangles correspond to scanning electron microscope imaging and energy-dispersive X-ray spectroscope elemental maps image regions as indicated. (2), (3) Silicified fossil in transverse section: (2) backscattered electron (z-contrast) image, with (3) corresponding overlain elemental maps for calcium, silicon, and iron. (4), (5) Calcareous fossil in transverse section: (4) Backscattered electron (z-contrast) image, with (5) corresponding overlain elemental mabs for calcium, silicon, and iron. Scale bars are 5 mm (1), and 500 μm (2), (5). Schiffbauer et al. (2024).
The La Ciénega fauna was first described in 1985 by palaeontologist
Mark McMenamin of
Mount Hollyoak College, who assigned the fossils to a group of morphotypes rather than trying to assign them to taxa, and believed the sediments to be Early Cambrian in age, at least in part because PreCambrian fossils were thought to be extremely rare (if accepted at all) at this time. Subsequently, some of the fossils were identified as
Sinotubulites, an exclusively PreCambrian taxa (the Ediacaran Period had not been named as such at this time), although this led to the postulation that this taxon extended into the Cambrian, as this was the data assigned to the La Ciénega Formation. It was not until the recognition of
Cloudina, also exclusively PreCambrian, and later an index fossil for the Terminal Ediacaran, that the La Ciénega Formation was recognised as PreCambrian in origin, with geochemical dating methods later confirming an End Ediacaran age.
Schiffbauer et al. interpret the La Ciénega fauna as a multi-taxa, Terminal Ediacaran assemblage, including Sinotubulites, Cloudina, and other Cloudinomorphs, probably Saarina or Conotubus, as well as other, unidentified tubular fossils. The preservation present is a mixture of silicification, which produces fossils recoverable by acid etching, and calcification, which does not. Many of the silicified fossils show plastic deformation, which is taken to indicate that they were not silicified in life, but rather as the result of some taphonomic process. Examined in thin section, the calcified specimens appear to show finer organic structures, supporting this hypothesis. This means that identification of the fossils from the silicified material only is unreliable. However, the preservation of what appear to be drill holes in some of these fossils, and the fact that these drill holes only appear to be present in examples of Cloudina and not Sinotubulites is likely to be significant, as this repeats a pattern seen in deposits of a similar age in Shaanxi Province, China, increasing the possibility that this represents some form of early predator-prey interaction, with predators able to select certain tubes as suitable targets for predation.
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