Monday 21 June 2021

Trying to explain the formation of the Ediacaran fossil Cloudina.

Cloudina, and similar fossils such as Conotubus, Saarina, Multiconotubus, Costatubus, Zuunia, and Rajatubulus (collectively Cloudinomorphs) first appeared in the Latest Ediacaran, around 550 million years ago, and persisted into the Early Cambrian, vanishing around 520 million years ago. These fossils have a funnel-in-funnel organisation without transverse structures, and may be straight or sinuous. Some of these fossils are purely organic, others are mineralised, preserved as calcite, phosphate, limonite/pyrite, or silica. Many (but not all) of these fossils are phosphatised, with there being no clear relationship between phosphatisation and mineralisation.

The relationship between Cloudinomorphs and modern Animal groups are unclear. Some specimens from Nevada have recently been shown to have a pyritised central tubular structure, combined with the nested-funnel morphology and a laminar ultrastructure, which is very reminiscent of modern Annelids. However, many Cloudinomorphs show branching, which would be incompatible with an Annalid affinity, but plausible for a Cnidarian. Thus, Cloudinomorphs may represent an entirely extinct Animal group, or possibly the convergent evolution of species from several different Animal lineages, in response to some (unknown) Ediacaran-Early Cambrian ecological niche.

Cloudina is found in a wide range of environments, including a variety of carbonats, microbial mats, and thrombolites (shallow water sediments with a clotted structure, formed by sediment particles in biofilms), and in places appear to have been reef forming organisms. They are therefore thought to have been fairly adaptable in their ecological requirements. In the Nama Group of Namibia Cloudina fossils are found in association with shallow, hydrodynamically-energetic reefs, and show the largest diameters known for this fossil, but in low-energy, microbial mat settings, smaller specimens dominate.

Cloudina is thought to have laid down a biomineral skeleton in life (rather than having minerals accumulate on tissues after it died), with either an aragonite or a high-magnesium mineralogy. Raman spectroscopy suggests that the biominerals were laid down on pre-existing organic laminae, 1–10 μm thick, and often paired, with up to eight laminae within a wall. These walls have a granular, micritic microstructure, even in phosphatised specimens, with crystal sizes of about 1 μm. This suggests a crystal growth pattern similar to that seen in modern Echinoderms, Molluscs and Cnidarians, in which growth starts with the initial formation of amorphous nanoparticles of calcium carbonate.

The extent to which the skeleton of Cloudinomorphs was mineralised in life is debatable. Cloudina fossils sometimes show brittle fracturing, which suggests a strongly calcified structure, and ductile deformation, which suggests at most weak mineralisation of a primarily organic structure. The peeling apart of laminae within the walls of the fossil Zuunia, as a well as a general prevailence of plastic deformation in Cloudinomorphs, has been cited as evidence that calcification in fact took place post-mortem, as a result of some diagenetic process. In the Mooifontein Member in the Witputs Basin of the Nama Group, Cloudina tubes have infillings of large sparry calcite crystals, which again has been taken as evidence of post-mortem mineralisation. Specimens of Cloudina from Brazil, which have high organic contents, have been found to have acicular, pseudomorphed aragonitic cements between the laminae, which are often neomorphosed to a fibrous or sparry calcite, possibly indicating pre- and post-mortem phases of mineralisation.

Determining whether a mineralisation process was directed by biological processes, rather than environmental factors, is difficult, particularly when dealing with an extinct group of organisms, This is made harder by the fact that the extent to which biological actions actually result in biomineralisation are often influenced heavily by environmental factors, and the highly ordered microstructures that are sometimes produced by the interaction of different minerals, which on their own produce highly disorganised structures. 

In a paper published in the journal Scientific Reports on 11 June 2021, Amy Shore and Rachel Wood of the School of GeoSciences at the University of Edinburgh, attempt to understand the relationship between environmental and biological controls of biomineralisation in Cloudina, by coeval assemblages from the Upper Omkyk Member of the Nama Group, Namibia, which were deposited in a range of different environmental settings, along an inner shelf to reef. Shore and Wood try to establish a relationship between the extent of mineralisation in Cloudina fossils and the environments in which they were deposited, and to quantify the degree of tube sinuosity, laminae thickness, and total tube thickness, in order to determine how they vary with the environment, and thus whether they are under environmental or biological control. The sinuosity of Cloudina fossils is known to vary considerably across the Nama Group, and therefore potentially represents an environmental indicator, possibly reflecting hydrodynamic activity or feeding potential. Shore and Wood also consider variations in diagenetic processes across the different environments represented in the Nama Group.

The Nama Group of Namibia is a fossiliferous carbonate-siliciclastic succession laid down in a beach-to-outer-shelf series of environmental settings, in the Late Ediacaran, between 550 and 539 million years ago. The sequence is split into two sub-basins, the Zaris and Witputs, which are separated by the tectonic Osis Arch, but which have been correlated by both sequence and chemostratigraphy. Shore and Wood examined fossils assigned to both Cloudina riemkeae and Cloudina hartmannae, collected from four localities assigned to the Upper Omkyk Member of the Kuibis Subgroup in the Zaris Subbasin. These exposures represent a transect of the ramp setting, and comprise the Driedoornvlakte (mid-ramp, high-energy reef), Zebra River (inner mid-ramp, thrombolitic-stromatolitic reefs), Omkyk (inner-ramp, low-energy), to Zwartmodder (proximal inner-ramp, very low-energy) locations. Each setting records a different lithology, reflecting the different hydrodynamic settings and early diagenetic environments in which the deposits were laid down. The Upper Omkyk Member lies immediately below the Hoogland Member, which contains an ash bed that has been dated to 547.32 million years ago, using uranium-lead dating.

Geological map and study site locations (1: Driedoornvlakte; 2: Zebra River; 3: Omkyk Farm; 4: Zwartmodder) within the Nama Group, Namibia, with bedding surface images of Cloudina from different coeval communities from the Upper Omkyk Member. (A) Localities. (B) Stratigraphy of the Nama Group with the Upper Omkyk Member highlighted by a star.  (C) Schematic of the Zaris Subbasin with relative position of localities, and dominant hydrodynamic regime and lithologies, and early diagenetic setting. (D) Driedoornvlakte, preserved as white or grey calcite cement surrounded by darker calcite cements, predominantly pseudomorphed aragonitic botryoids. (E) Zebra River, infilled with light sparry calcite cement surrounded by dolomitised micrite. (F) Omkyk Farm, Cloudinomorphs, probably Cloudina, including potential branching individuals (arrowed) as branching can only be proven through the presence of a shared cavity, preserved as black sparry calcite surrounded by dolomitised wackestone. (G) Zwartmodder, preserved as black sparry calcite surrounded by dolomitised micrite which preserves the fine annulated structure and phlanges (white arrowed). Possible fragments of Corumbella may also be present (red arrow). Shore & Wood (2021).

The measurement of sinuosity was derived from methods used to assess river morphology, and was defined by dividing the length of an object by the length of the straight-line distance from bedding plane surfaces, where values less than 1.1 represent a straight line and higher values increasing levels of sinuosity. Shore and Wood also determined the percentage shortening of the fossils, an estimate of the amount of shortening of the tube in comparison to the original and assumed straight Cloudina tube.

The 156 Cloudina specimens from Driedoornvlakte, on the mid-ramp reef, had sinuosities of between 1.0 and 1.7 (with a mean value of 1.04), and a shortening of between 0.00 and 41.29% (with a mean value of 3.23%). The 99 Cloudina specimens from Zebra River and have tube sinuosities of between 1.00 and 2.98 (with a mean value of 1.21), and show a shortening of between 0.00 and 66.69% (with a mean value of 13.01%). The 144 Cloudina specimens from Omkyk have sinuosities of between 1.00 and 2.42 (with a mean value of 1.07), and show a shortening of between 0.00 and 58.67% (with a mean value of 5.22%). Finally, the 99 specimens from Zwartmodder, laid down on a very low-energy shoreline behind a reef, have sinuosity values of between 1.00 and 1.36 (with a mean value of 1.04), and shortening of between 1.00 and 26.25% (with a mean value of 3.27%). Statistical tests carried out by Shore and Wood suggest that the differences in sinuosity between the sites is significant.

Cloudina terminology and sinuosity data. (A) Schematic of two Cloudina tubes with terminology (black text) and measurements (red text) derived from 2D surfaces. Sinuosity is defined as the length of the midline over the straight line distance between the two ends of the tube in longitudinal section. Lamina thickness is the thickness of an individual lamina and paired laminae thickness the thickness of two laminae including the area between. Wall thickness is the sum of all components that form the complete tube wall in transverse section from outer wall to inner wall defining the central cavity. Cements formed between paired laminae are termed inter-lamina cement, between sets of paired laminae inter-funnel cement, between different tubes inter-Cloudinomorph cement, and that which forms within the central cavity is termed intra-Cloudinomorph cement. Inorganic pseudomorphed aragonitic botryoidal cements and sediment have also been labelled in the figure. (B) Sinuosity distribution. (C) Percentage of Cloudina with different sinuosities, based on 2D bedding plane measurements. Shore & Wood (2021).

The means by which the Cloudina fossils are preserved also vary from site to site. 

At Driedoornvlakte these fossils are typically surrounded by pseudomorphed aragonitic botryoids, and show brittle deformation. The walls of the specimens are preserved as light grey calcite, or sometimes light brown to yellow crusts (indicating partial dolomitisation). Under a cathodoluminescent light this dolomite shows as bright yellow to red. The walls are made of paired laminae, separated by a non-luminescent cement, but the cement separating pairs is patchily luminescent. A dull, patchily luminescent cement is found inside the tubes, and between different tubes. The fossils are also infilled with a later dolomitised geopetal micrite with red luminescence, and pseudomorphed aragonitic botryoids, with blunt crystal terminations, which nucleate from the cements. 

Photomicrographs of Cloudina hartmannae and Cloudina riemkeae from Driedoornvlakte, Upper Omkyk Member, Nama Group. (A) Plane polarised light image of attached Cloudina with a dolomitised wall (CW) and filled with dolomitised sediment (DS), followed by an inclusion-rich sparry calcite (ISC) and a later clear burial spar (BS). (B) Cathodoluminescence image of (A), intra-Cloudinomorph cement (ICC1) nucleated from the inner Cloudina wall and inter-Cloudinomorph cement (ICC2) formed between the Cloudina tubes and nucleated from the outer wall and including the mutual cement (MC) which has the same luminescence as CW. Non-luminescent pseudomorphed aragonitic botryoids (AB) nucleate from ICC1. These are proceeded by ISC and BS. (C) Inset of (B), showing ICC1 and ICC2-cements with same luminescence. (D) Inset of (C), showing bright luminescent CW with multiple nonluminescent laminae (white arrows) with small aragonitic needles (orange arrows) nucleating from laminae. (E) Plane polarised light image of Cloudina with a dolomitised wall (CW) composed of laminae (arrowed) with thin sparry calcite infill between the two laminae, surrounded by inclusion-rich sparry calcite (ISC). (F) Transverse plane polarised light image of dolomitised individual. (G) Cathodoluminescence image of (C). (H) Inset of (F), of broken, dolomitised Cloudina wall (arrowed). (I) Inset of (G) in cathodoluminescent light, of broken bright luminescent Cloudina wall (arrowed) exposing the inter-funnel cement (IFC). The rest of the tube is infilled with non-luminescent spar (Sp). (J) Cathodoluminescence image of (E), with dull luminescent cements (arrowed) highlighting cements between the laminae and is surrounded by ISC. AB nucleate from the CW. (K) Polished slab highlighting inter-Cloudinomorph cements (arrowed) between multiple Cloudina individuals. (L) Plane polarised light image of inter-Cloudinomorph cements (arrowed) situated between Cloudina individuals from which botryoids nucleate. Shore & Wood (2021).

Cloudina specimens from Zebra River are preserved in a wackestone matrix as calcite. Detailed ornamentation is absent, and the tube is infilled with a calcite spar with a dull luminescence, with specimens showing brittle and ductile deformation. Paired lamellae can be observed, with the spaces between them infilled with acicular crystals 9.4-15.5 μm in length and 2.1-4 μm in width. A patchily luminescent cement is present between the funnels, overlain by a dull luminescent, pore-lining patchy cement. 

At Omkyk the Cloudinamorphs are dark in colour and composed of large (0.1–1 mm) calcite spar crystals. Little detail is preserved, although in some specimens the stacked funnel-in-funnel strucure can still be seen. The specimens show brittle and ductile deformation. Under a cathodoluminescent light three generations of cement can be seen within the tubes. The first of these is a thin, non-continuous acicular cement, reaching about 200 μm in thickness. This is followed by an isopachous cement with patchy or dull-luminescence with limited zonation. Finally the tubes are infilled with zoned sparry cement, which nucleates from the isopachous cement.

The Zwartmodder Cloudina specimens are again preserved as coarse calcite spar, although here fine details of the outer surfaces are generally retained, and there is little evidence of brittle deformation. These fossils are preserved as moulds, and infilled with a centripetal, sparry calcite cement with dull luminescence that becomes brighter towards the centre. For the most part the individual laminae are not visible, but a sparry calcite cement appears to be present both between laminae and within the tube. In places the sparry cement is not present; here a micrite infill separates the laminae, and both individual and paired laminae can be observed.

Shore and Wood used Electron Microprobe analysis to determine strontium concentrations and magnesium/calcium ratios in Cloudina specimens and associated cements from Driedoornvlakte and Zebra River. They found a statistically significant difference in the strontium content of the inter-laminae cements and both laminae and inter-funnel cements, and between the micritic matrix and both inter-laminae cement and inter-funnel cement of the specimens from Zebra River, but no similar variation in magnesium/calcium ratios. In the Driedoornvlakte specimens they also found statistical differences in strontium concentrations between the inter-laminae and the inter-funnel cements, and between inter-laminae and the inter-funnel cements compared to the Cloudina wall. The strontium content of the inter-laminae cement and wall are statistically different to the inclusion-rich spar, but this is not the case for the inter-funnel cement which is recrystallised as an inclusion-rich spar. There is also a significant difference between the Cloudina wall and other Cloudina-associated cements, probably as a result of the dolomitisation. Strontium content within internal cements associated with Cloudina hartmannae show no statistical differences, but there is a significant difference between the inter-Cloudinomorph cement and botryoidal cements. As at Zebra River, there is no significant  variation in  magnesium/calcium ratios between any measured features at Driedoornvlakte, with the exception of the Cloudina wall of the Cloudina riemkeae, which is presumably due to selective dolomitisation.

Electron Microprobe analysis data from Cloudina and associated cements from Driedoornvlakte and Zebra River, Upper Omkyk Member, Nama Group. (A) Strontium concentration (parts per million). (B) Magnesium/calcium ratio of Cloudina riemkeae from Zebra River. (C) Strontium concentration (parts per million) of Cloudina hartmannae from Driedoornvlakte. (D) Magnesium/calcium ratio of Cloudina hartmannae from Driedoornvlakte. (E) Strontium concentration (parts per million) of Cloudina riemkeae from Zebra River. (F) Magnesium/calcium ratio of Cloudina riemkeae from Zebra River. Detection limits for elements are shown as horizontal dashed lines (DL). Shore & Wood (2021).

No organic laminae are found in any of the specimens from any site studied. At Zebra River and Zwartmodder the laminae are preserved as moulds, while at Driedoornvlakte they have been dolomitised, and at Omkyk laminae are not detectable at all. The laminae thickness of specimens from Zebra River and Driedoornvlakte was measured using both cathodoluminescence and plane polarised light images, while those from Zwartmodder were measured using only cathodoluminescence images. The specimens Driedoornvlakte and Zebra River have laminae of similar thicknesses, with Driedoornvlakte laminae thicknesses ranging from 2.1 to 9.6 μm, and Zebra River Cloudina laminae ranging from 3.3 to 10.8 μm in thickness. However, paired laminae thickness varies between the sites; at Driedoornvlakte this ranges from 10.6 to 55.3 μm, while at Zebra River it ranges from 5.9 to 40.3 μm, and at Zwartmodder from 11 to 23.7 μm. The spalled features seen on some Cloudina specimens at Zwartmodder are now preserved as centripetal calcite cement, which consists of the inter-lamina cement and two laminae. The total thickness of these features is similar to the paired laminae at Driedoornvlakte and Zebra River. A statistical analysis showed that paired laminae thickness from Driedoornvlakte, Zebra River and Zwartmodder, are significantly different.

Photomicrographs of Cloudina from Zebra River, Upper Omkyk Member, Nama Group. (A) Plane polarised light image of Cloudina infilled with sparry calcite surrounded by micritic sediment (S). (B) Cathodoluminescence image of (A), with inter-lamina cements (ILC) forming between outer laminae (highlighted in yellow). Pseudomorphed aragonitic botryoids (AB) grow from the laminae, and the inter-funnel cements (IFC) pre-date dull luminescent burial cements with inter-lamina cements forming between paired laminae. Aragonitic botryoids infilled the Cloudina tube and micritic sediment surrounds the tube. (C) Inset of (B), in plane polarised light, showing laminae with a wavy form (yellow arrows). (D) Cathodoluminescence image of (E), laminae highlighted by yellow arrows with same form as plane polarised light image. Nucleation of inter-funnel cement indicates location of laminae. (E) Inset of (B),  inter-lamina cement located between two outer laminae (white arrows). Inter-lamina cements nucleate from inner laminae (yellow arrow). (F) Inset of (B) in plane polarised light with lamina visible (arrowed). (G) Cathodoluminescence image of (F), where outer laminae shows evidence of ‘spalling’ (arrowed) and infilled by micritic sediment. Inter-lamina cement situated between laminae and areas of patchy bright luminescence indicate inter-funnel cement. Aragonitic botryoids infill the rest of the tube. (H) Plane polarised light image of Cloudina. (I) Pseudomorphed aragonitic botryoids (AB) outside Cloudina tube. Botryoids grow from the outer tube wall into the surrounding sediment (S), and have a different luminescence to those inside the tube. Early inter-funnel cement grows from the inner Cloudina laminae (arrowed). (J) Plane polarised light image of a transverse section of a Cloudina, which is infilled by dolomitised sediment (S), and have a different luminescence to those inside the tube. Early inter-funnel cement grows from the inner Cloudina laminae (arrowed). (J) Plane polarised light of a transverse section of a Cloudina, which is infilled by dolomitised sediment (S) and two sets of broken paired laminae (arrowed) (K) Cathodoluminescence image of (J), bright luminescent dolomitised sediment infills the central area of the broken tube. Dull luminescent botryoids (AB) outside the tube. Dull luminescent inter-funnel cement forms the Cloudina skeleton. (L) Inset of (K), with evidence of breakage of the non-luminescent inter-lamina cement (white arrows) and inter-funnel cement (black arrows). (M) Plane polarised light of Cloudina tube infilled with sparry calcite (SC) with preserved laminae (yellow arrowed). (N) Cathodoluminescence image of (M), with laminae observed in plane polarised light (yellow arrows), laminae shown in cathodoluminescence (white arrows). Laminae are preserved with the same luminescence as the sparry calcite infill with a dull cement inter-lamina cement between the laminae. Shore & Wood (2021).

The wall thickness of Cloudina is defined by Shore and Wood as the width between the outer tube wall and the inner tube wall forming the internal cavity. This was measured in specimens from Driedoornvlakte, Zebra River and Zwartmodder. At Driedoornvlakte, maximum wall thickness for any individual Cloudina tube ranges from 0.25 to 1.88 mm, while at Zebra River and Zwartmodder, they ranged from 0.16 to 1.43 mm, and from 0.12 to 1.38 mm, respectively. The wall of the Cloudina specimens forms the greatest proportion of their total width at Driedoornvlakte, with ratios ranging from 0.077 to 0.505, with those at Zwartmodder ranging from 0.079 to 0.434, and those at Zebra River showing the smallest range from 0.065 to 0.375. There is a correlation between the wall thickness and the total thickness of the tube, but not a strong one.

Photomicrographs of Cloudina from Zwartmodder, Upper Omkyk Member, Nama Group. (A) Plane polarised light image of compacted Cloudina skeleton (CS) surrounded by dolomitised micritic sediment (S). (B) Cathodoluminescence image of (A), featuring dull cement which becomes well-zoned with bright luminescence during later growth. The cement infill is centripetal cement (CC) which nucleates from the wall and grows into the mould formed through dissolution. (C) Plane polarised light image of broken and spalled laminae. (D) Cathodoluminescence image of (C), the skeletal tube consists of a dull luminescent centripetal cement, which in thicker areas is brightly luminescent. Dolomitised micritic sediment surrounds the `spalling’ skeleton. (E) Inset of (B), brightly luminescent cement nucleates from sediment grains, which protrude into the Cloudina mould (white arrows) and are overgrown by a non-luminescent cement (yellow arrows) that formed before the centripetal cement. (F) Inset of (D), sediment infills areas between spalling potential inter-laminae (I-L?), spalling occurs where the centripetal cement has not formed between the laminae. Bright luminescent cements on micritic sediment grains which protrude into the Cloudina mould which is overgrown by a non-luminescent cement (yellow arrows). (G) Sparry calcite (SC) infill of the Cloudina skeleton, where cement crystals protrude from surrounding sediment (white arrows). Cube-shaped holes in the sparry calcite may represent plucked micro-dolomite or pyrite crystals. Shore and Wood (2021).

The lowest sinuosity is seen in Cloudina specimens from Driedoornvlakte and Zwartmodder, with the highest sinuosity seen at Zebra River. The consistency of this pattern strongly suggests that this feature relates to the environment in which the organisms lived, with mineralisation occurring on a flexible organic template, which enabled the developing Cloudina to adapt to the setting in which they found themselves. Unfortunately, there have been few studies of sinuosity in modern benthic organisms, but Animals such as calcareous tubed Polychaetes are thought to orientate themselves to maximise feeding efficiency, which can vary with ambient currents and local sedimentation regimes.  It is therefore difficult to theorise as to why in would have been advantageous for Cloudina to be more sinuos in some environments than others, although it is likely that this was a response to multiple factors, such as substrate type and morphology, vertical or horizontal growth, competition for space, nutrient regime, water depth, and response to hydrodynamic energy and water flow.

Photomicrographs of Cloudinomorphs, probably Cloudina, from Omkyk Farm, Upper Omkyk Member, Nama Group. (A) Plane polarised light image image of Cloudinomorph tube. (B) Cathodoluminescence image of (A). (C) Inset of (A). (D) Cathodoluminescence image of (A), with early acicular cements (AC) which vary between bright and non-luminescence. Acicular cement is followed by poorly-zoned acicular bladed calcite (AB). A well zoned blocky calcite (BS) infills the remaining tube cavity. Shore & Wood (2021).

Shore and Wood note that their observations are all based upon two dimensional measurements taken from bedding planes, and that more information might be obtained from a study of three dimensional sinuosity in Cloudina specimens.

The method of preservation seen in the Cloudina fossils of the Nama Group varies from site to site, and is apparently driven by local conditions, although in all cases the laminae thickness is consistent, suggesting that the original form is being preserved. Shore and Wood were unable to see the micritic microstructure described for the walls of Cloudina from other locations, but all methods of preservation are consistent with originally organic laminae which became calcified.

The specimens at Omkyk and Zwartmodder are preserved as moulds, with no preserved skeletal walls. At Omkyk the acicular isopachous cement initially grew from the Cloudinomorph walls into the surrounding sediment, witht he subsequent loss of the skeleton and the infilling of the subsequent void with well-zoned clear sparry burial calcite. At Zwartmodder the original skeleton is replaced by a sparry calcite, which probably represents dissolved paired laminae together with inter-lamina cement and inter-funnel cement. This suggests that the dissolution of the skeletons occurred before cements developed in the surrounding sediments, and that when these sediments did begin to form, they grew into the moulds.

Dissolution at these shallow, inner ramp localities was probably driven by the influence of freshwater, which is far more efficient at dissolving most minerals than seawater, and which, when combined with the decomposition of organic material, is particularly efficient at removing aragonite. In contrast, the mid-ramp sites Driedoornvlakte and Zebra River show a preservation mode driven by the reformation of aragonite in a pseudomorphed botryoidal phase, something likely to have happened in a phreatic zone (sediment in which relatively all pores and fractures are saturated with water) following burriel in a marine environment.

The Cloudina specimens from Zebra River show undulose laminae, with some evidence of both brittle fracturing and ductile deformation, while those at Driedoornvlakte show evidence of brittle deformation only. At both Zebra River and Zwartmodder, some specimens show signs of compaction breakage, where the resultant area was infilled by sediment. In these cases, inter-lamina and inter-funnel cements formed prior to the breakage. The dull luminescent inter-funnel cement seen at some locations, particularly Zebra River, occurred before compaction and lithification of the surrounding dolomitised sediment. This can be demonstrated by the sharp fracture of the inter-funnel cement where sediment has encroached into the tube. All of the cements predate the formation of the botryoidal aragonite seen inside and outside some tubes. Where these botryoids exist, those outside the tube show a brighter luminescence than those inside, suggesting there was some degree of diagenetic compartmentalisation. At Zebra River these have been replaced in the outermost inter-lamina cements by non-luminescent neomorphic cements later in diagenesis.

A variety of cements seen in the specimens are formed from fine, acicular crystal bundles were deposited prior to transport and breakage of the tubes, and also pre-dated the cement botryoids, so can be inferred to be very early syn-sedimentary. The sparry calcite seen in some specimens formed after the replacement or dissolution of these original cements.

The inter-funnel cements seen in the Nama Group Cloudina have also been recorded in specimens from Brazil, Paraguay, and Spain, indicating that this feature is widespread, and not dependent on local diagenetic conditions. Shore and Wood suggest these were formed when the Animal was alive, and provided mechanical strength and rigidity to the tube.

Similar cements have been described in the extant Sphinctozoan Sponge Vaceletia, which some biologists have suggested has a mode of biomineralisation retained from the earliest Animals. In Vaceletia the skeleton is secreted upon a non-collagenous organic template, which becomes substituted by crystalline aragonite deposited as tangled crystal bundles. The organic framework consists of proteins and polysaccharides rich in galactose, glucose and fucose, the latter suggesting that bacterial exopolymeric substances may be involved in calcification. In these Sponges the base of the skeleton is free from living tissue, and often by a micritic granular secondary deposit. Cements in Cloudina have been shown to contain organic material, which supports the idea of microbial involvement in cement precipitation. Secondary deposition of such cements in areas where aragonite crystals persist after the loss of soft tissue from a region has been demonstrated in a range of organisms, including Scleractinian Corals and the algae Halimeda. If the same was true for Cloudina, then these cements could have been laid down pre- or post-mortem.

The different diagenetic histories at the different localities make it impossible to compare elemental signatures directly, but there are consistent statistically significant differences between phases at different localities. Shore and Wood note the consistent difference in strontium concentration between various Cloudina-associated cements and botryoidal cements, which may suggest different origins. However, botryoidal cements within the tubes of Cloudina show no difference in strontium concentration to other intra-tube sediments, and botryoidal cements external to the tubes show similar strontium levels to other external sediments, which may suggest that cements within Cloudina specimens show a general tendency to retain strontium better than the surrounding matrix. A similar strontium distribution has been documented in Cloudina specimens from the Tamengo Formation of the Corumbá Group, Brazil. Shore and Wood therefore conclude that internal cements form early in the preservation of Cloudina fossils, possibly while the organism was alive, but that no clear elemental signature can be determined which might indicate either a diagenetic origin from a different pore fluid or biological fractionation.

The Cloudina specimens do show differences in paired lamina thicknesses between localities, but this might be due to local diagenetic differences, particularly at Zebra River, where the laminae can be seen to be flexible. However, it is also possible that the differences occur due to the different methods used to measure thickness on specimens from different locations: laminae at Zebra River and Driedoornvlakte were measured using both cathodoluminescence and plane polarised light images, but the cathodoluminescence images show thinner laminae compared to their plane polarised light counterparts. When comparing data of laminae thickness collected from plane polarised light images only, the data sets are not statistically different, supporting the hypothesis that this is an artifact of the measuring system, but, unfortunately, this sub-set of the data is to small to be considered reliable, and when only cathodoluminescence data is used, the differences remain, particularly if spalled lamenae are excluded from the calculations. 

The thicknesses of the moulds of paired laminae seen at Zwartmodder fall in the range of paired laminae at other sites. This suggests that the assumed moulded laminae are paired laminae combined with inter-lamina cements, as observed at Driedoornvlakte and Zebra River. These presumed Cloudina laminae from Zwartmodder actually have a narrower range of thicknesses than those from Zebra River or Driedoornvlakte, despite the sparry calcite moulds being thicker than the laminae from those locations. Shore and Wood suggest this is because the moulds formed by the dissolution of both the paired laminae and the inter-lamina cement.

Cloudina laminae from the Mooifontein Member (and those from Paragauy) get slightly thicker than those from the other locations, up to 5 μm (8 μm in Paragauy) bur Shore and Wood consider that this is likely to be a result of the dolomitisation in these specimens.

The thickness of the walls of the Cloudina specimens and the thickness of those walls as a proportion of the total thickness of the tubes varies across the Zaris Subbasin, with the walls being significantly thicker at Driedoornvlakte than the other localities. Driedoornvlakte is thought to have been the most hydrodynamically energetic of the localities analysed, making it likely that the thickness of the walls was environmentally-controlled, with possible direct causes including higher rates of carbonate precipitation at Driedoornvlakte than at the other locations, a selective advantage in being able to resist strong currents, and better food sources.

Features of Cloudina walls, including laminae thickness, paired laminae thickness and ratio between the maximum Cloudina wall width and maximum tube width of Cloudina of the Upper Omkyk Member, Zaris Subbasin, Namibia. (A) Lamina thickness. (B) Paired laminae thickness. (C) Distribution of ratios of wall thickness: tube thickness. (D) Relationship between wall thickness: tube thickness. (E) Summary of quantitative features of Cloudina at the four coeval sites of the Upper Omkyk Member. Shore & Wood (2021).

Across the Zaris Subbasin shelf setting Cloudina specimens have consistent lamina thicknesses, which suggests this is a feature under biological control. The living Cloudina appear to have precipitated a calcitic skeleton onto an organic matrix, producing a structure which was quickly replaced after burial. The nature of the cements which replaced the original calcitic skeleton varies from location to location, but the essential process appears similar.

The sinuosity of the Cloudina tubes varies across the Zaris Subbasin ramp setting, which appears to indicate this feature was under environmental control, possibly as a way of maximising feeding efficiency in different settings. The diameter of the Cloudina tubes, and the thickness of the tube walls also varies across the basin, again implying this was under environmental control.

See also...

Online courses in Palaeontology. 

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