A radical reinterpretation of the growth and form of the stromatolite Conophyton lituus(Maslov) from evidence of syngenetic biofilm mineralisation

Robert V. Burne

Research School of Earth Sciences, The Australian National University Canberra, ACT 0200, Australia

School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia

Abstract Conophyton (Maslov) is a cylindroidal stromatolite form-genus characterized by nested conical laminae. Well-preserved Conophyton, up to 4 m tall and with basal diameters of up to 50 cm, are exposed in the Proterozoic Atar Formation of Mauritania, where many occur together, in growth position, as fields of individual columns spaced between 5 and 70 cm apart. The uniformity of these forms and their regular distribution suggest that they grew in quiet-water environments below wave base. Evidence for their penecontemporaneous organomineralization is indicated by nearby toppled examples of undeformed Conophyton forms alongside eroded lithified Conophyton fragments in carbonate breccias. Two characteristics of Conophyton have been used to classify the structures - the form of the lamination and the nature of its axial structure. A mathematical/physical model provides an explanation for the growth pattern of Conophyton. It predicts that coniform structures with thickened axial zones form when upward organic growth of a biofilm moderately exceeds the rate of its mineralization. The varying characteristics of these features between different forms of Conophyton are thought to reflect biomineralization of the decaying biofilm rather than differences in the composition of microbial communities. A modern example of a syngenetic mineralization process capable of producing similar structures has been observed in the contemporary sediments of Lake Preston,Western Australia,where benthic microbial mats are being transformed into coniform lithified crusts.The initial biomineralization of the coniform mat forms magnesium silicate that first coats and permineralizes web-like microbial extracellular polymeric substances (EPS) and then coalesces into a uniform mass that provides mechanical strength to the cones. At a later stage, massive carbonate crystal growth occurs that over-prints much of this texture, leaving only small, remnant areas of the magnesium silicate phase. Many fossil Conophyton are composed of dolomite, and the remains of the microbial communities responsible for their construction are rarely found, except in areas of chert within the Conophyton. It is suggested that Proterozoic Conophyton were constructed in a tranquil environment through the accretion of microbial mats that were syngenetically permineralized by a magnesium silicate such as a smectite. Later, much of the unstable smectite would be susceptible to diagenetic replacement by either dolomite, or chert in which remnants of microbes that had been coated or permineralized could, potentially, be preserved.

Keywords Conophyton, Morphogenesis, Proterozoic, Stromatolite, Microbialite, Mineralisation, Biofilm,Lake Clifton, Lake Preston, Maslov

1.Introduction

Conophyton (Fig. 1) is a well documented stromatolite form-genus (Komar et al., 1965; Semikhatov and Raaben, 2000). Examples of Conophyton are known from the Archean,but they become widespread in the Proterozoic only to virtually disappear in the late Riphean (Tonian) (Krylov and Semikhatov, 1976).Some forms were particularly abundant in certain periods of the Proterozoic (Preiss, 1976) and have been successfully used as markers for regional stratigraphic correlation (Preiss, 1971; Semikhatov, 1976). Only sporadic occurrences have been recorded in the Phanerozoic e.g. from the Messinian of Spain(Feldmann and McKenzie, 1997) and the Aptian of Brazil (Varej～ao et al., 2019). Ancient Conophyton are generally composed of dolomite or calcite. The fossilised remains of the microbial communities are only rarely found, frequently preserved in locally silicified parts of the Conophyton (e.g., Schopf and Sovietov,1976; Cao et al., 2001; Cao and Yuan, 2006),although the majority of Conophyton are composed of dolomite or calcite. It is important to understand the factors influencing the growth and form of Conophyton in order to explain why, at times during the Proterozoic,they dominated relatively deep,tranquil areas of the sea floor. This paper clarifies the Russian type description of Conophyton, assesses the environmental significance of some of its well-documented Proterozoic occurrences, considers the mathematcal modelling of Conophyton growth and form, and advances a novel explanation for the genesis of the definitive Conophyton form based on the results of a study of geobiological processes influencing the present day growth and form of coniform microbialites in Lake Preston, Western Australia, where small, coniform microbial mats are syngenetically mineralized by magnesium silcate. It is concluded that these enigmatic stromatolites were constructed by syndepsositional biofilm organomineralization in relatively deep water environments that were marked by the virtual absence of detrital sedimentation.

2.The definition of the form-genus Conophyton

Vladimir Maslov's definitive descriptions of Conophyton were published in two Russian papers(Maslov,1937, 1938). These papers included very brief summaries in English and the full texts of these works have been rarely cited outside of the Russian literature and many important aspects of Maslov's observations of the type material have been overlooked or, even worse,misquoted. For example, Rezak (1957) published the following misrepresentation of Maslov's Russian text whereby the growth position of Conophyton was inverted placing the apex of the cone at the base of the structure! -

Genotype:Conophyton lituus Maslov(1937).

Generic diagnosis.-Cylindroidal colonies composed of nested conical laminae. Apex of basal cone usually attached to the substratum;long axis of cone inclined at some angle to bedding surfaces(Rezak,1957,page 135).

To avoid further confusion,new English translations of the essential elements of Maslov's two publications are reproduced, in full, in this paper.

The name “Conophyton” was first proposed in Maslov(1937)along with the formal description of the type specimen, Conophyton lituus (φυτ?ν Greek-a plant, κ?νο? Greek - cone, lituus Latin - bugle). This specimen (Figs. 2 and 3) had been collected by L. M.Shorokhov from red-coloured limestones on the banks of the lower Tunguska River between the town of Turukhansk and its confluence with the Severnaya River,Siberia.The specimen was initially thought to be an inorganic concretion but closer inspection revealed“a stromatolite growing from down upwards and forming a kind of paling” resulting in a “tapering and cylindrical lamellar concretion”.In a cross-section cut through the generally red-coloured cone(Fig.3)it was seldom possible to distinguish the microstructure of the original laminations as they are disrupted by lighter-coloured spots and lenses.The origins of these spots and lenses were not clear to Maslov, but he concluded that they could be of secondary origin,since the supposedly primary laminations are distorted by them. This led Maslov (1937, 1938) to the conclusion that the original microscopic structure of this stromatolite was almost always obscured by a high degree of recrystallization. Maslov (1938) identified features in thin sections that seemed to be a result of natural,syngenetic alterations occurring during the growth of the stromatolite. He astutely observed that “during the life of the blue-green algae evidently a large amount of clayey ferruginous ooze was being settled.Due to this fact the internal structure of the deposited carbonate became much more distinct. The latter circumstance permits us to suppose with a fair degree of certainty that in this case traces of cells of these plants are present”.Microscopic analysis showed that the type-specimen was composed of various sizes of carbonate grains (a term that was unfortunately mistranslated in the English summary of the paper as“carbonaceous” grains) along with very fine-grained ferruginous clay. Some large carbonate grains were darker in the centre due to the carbonate's “vitiation by the argillaceous ferruginous material”, and this was accompanied by an abrupt decrease in the size of carbonate grains (Fig. 3). There are rows of globular and lenticular carbonate bodies cemented by opaque ferruginous clay “additions” and other fine-grained material. Small tubes, presumed to represent original organic matter, occur in some layers as dark features. He concluded that “We can elucidate - with some degree of difficulty … ….. the mode of growth and conditions of life of these old lower plants that,primitive as they were, erected complicated structures in their struggle for existence.” Maslov (1938,page 328).

Fig.1 Silicified Conophyton in the 1.7 Ga Dungaminnie Formation,NT,Australia(grid ref.53 K NB 0578973 8154545).Australian 50c cent coin(28 mm) for scale. Note fenestrae and thickening of laminae in the axial zone.M.J. (Jim)Jackson in the background is～168 cm tall and the depth of field creates a misleading illusion of a towering Conophyton.

Fig. 2 Holotype of Conophyton (Conophyton lituus). Viewed from above(top)and lateral view(Length～9 cm)(bottom).Collection of Leontovich. Modified from Maslov (1937).

The Conophyton name was rapidly adopted(although the details of Maslov's definitive description were largely overlooked)and many other examples of Conophyton were identified during the course of the systematic geological mapping of the USSR, where they were used as an aid for regional geological correlation. Komar et al. (1965) proposed that, to assist this stratigraphical analysis,the variety of Conophyton types observed might be subdivided on the basis of a supposed “taxonomic rank”, using variations in the nature of the axial zone,nature of lamination,relative thicknesses of alternating dark and light laminae,and other morphometrics, to define seven different forms in the hope that each would have stratigraphic significance.

3.Environmental significance

Fig. 3 A) Transverse section through Conophyton lituus, showing lenses and layers separated from each other by dark material(width 1.5 cm); B) Enlarged view of the same specimen, showing differentiation of the lenses and layers.Note darker centres of some light lenses (Width 4 mm). Compare with Fig. 10. Modified from Maslov(1938).

Serebryakov (1976) concluded that stromatolites with different forms but similar microstructures were a product of differing depositional environments and that specific unrepeated associations with other forms represent depositional parasequences with little significance for inter-basinal stratigraphic correlation.Indeed, detailed studies of stromatolite-bearing sequences have provided compelling evidence for a restricted depositional setting of Conophyton. For example, Donaldson (1976), studied the remarkable Conophyton of the Dismal Lakes Group in the Bear Province of the Canadian Shield. These Conophyton are composed of dolomite, have diameters generally between 10 and 50 cm, have conical accretionary surfaces inclined at greater than 70°, and a depositional (as opposed to synoptic) relief that can exceed 10 m. They occur in beds that can be traced for hundreds of kilometres. Donaldson considered that it would be impossible for such forms to accrete by sediment entrapment. The lack of sedimentary structures of waves, strong currents or subaerial exposure suggested that they formed in tranquil subtidal environments. He concluded that the purity of the carbonate, the scarcity of terrigenous detritus and the steeply inclined conical laminations implied that the structures formed from a process of mineral precipitation. He noted “Lamination textures show a great diversity, ranging from uniform to discontinuous and lumpy. These textures, resembling those regarded as characteristic of several named “forms”, appear to represent various stages of diagenetic recrystallization of originally smooth and continuous laminae”.

Other well-exposed examples of Conophyton have been described from the Proterozoic Atar Formation of Mauritania (Bertrand-Sarfati and Moussine-Pouchkine 1985, 1998; Kah et al., 2009). These are up to 4 m tall, and many occur in growth position, in fields of columns with basal diameters of up to 50 cm that are spaced 5-70 cm apart. Three particularly interesting features are displayed in these outcrops (Bertrand-Sarfati and Moussine-Pouchkine 1998, p33):

(i) Evidence suggests that living Conophyton columns were free of interspace filling during growth, although remains of later crinkled microbial mats are locally found draped between the Conophyton;

(ii) Conophyton and the related form, Jacutophyton, can be found growing at the same level,the difference in forms may be a question of the space available and a slight change in the behaviour of the microrganisms responsible for their creation,

(iii) There is an absence of any evidence of allochthonous sediment supply which suggests that they grew under very quiet conditions,and that, when their growth ceased, the spaces between columns were filled by either microbial growth or cement.

The form and distribution of these Conophyton indicate that they grew in quiet-water environments below wave base. Localized toppling of Conophyton occurred at certain horizons (Fig. 4), however the structures retain their form after toppling. These toppled structures, along with penecontemporaneous breccias containing clasts of lithified Conophyton,demonstrate that organomineralization of Conophyton occurred as they grew. Although some plastic deformation of surface layers is apparent in these disrupted specimens,clearly they were essentially both rigid and strong when they were toppled.

4.Morphogenesis

How could these rigid, steep and tall conical structures form?This is an important question,since at certain times during the Proterozoic, Conophyton dominated relatively deep, tranquil areas of the sea floor in monotonous fields that may have extended for hundreds of square kilometres (Fig. 5).

Fig. 4 Conophyton from the Atar Group, Islamic Republic of Mauritania. A) Toppled Conophyton; oblique bedding plane view;Jacob staff marked in 10 cm intervals; B) Pavement of broken Conophyton and stromatolitic breccia;bedding plane view.Width of image 80 cm. Modified from Kah et al. (2009).

Fig. 5 Vologdin's impression of a living field of Conophyton lituus on a Proterozoic sea-bed. Vologdin proposed to rename the form Tschichatschevia lituus in honour of Pyotr Aleksandrovich Chikhachov who he claimed was the first to recognize stromatolites. Reproduced with modification from Vologdin (1962).

Vologdin(1962),who considered Conophyton to be a form of marine alga, suggested that “It is possible that this group of seaweed favoured the formation of stromatolites with a narrow-conical form, similar to the rostra of belemnites,…..It is necessary to concede the possibility of the existence of a group of algae,of which almost no record of the cellular structure is preserved in extractable state, in which there was a characteristic mass formation of mucus-like compounds capable of sliding along an inclined substrate surface uphill until the highest point was reached,partly smearing along the way. This can be said from the form and size of fossilized clots of mucus, their position in the composition of elemental layering and frequently observed squashing of the clots in the upper points of the layers”.

Bertrand-Sarfati and Moussine-Pouchkine (1998)have summarized the essential growth characteristics of the Conophyton occurrences at Lekhleigate,Mauritania,in this way-“As soon as the shape of the laminae become conical (30-40 cm above the beginning of the buildup), the trunks rapidly acquire a central (i.e. axial) zone and a cylindrical shape that rapidly reaches a high synoptic relief(more than 2m of elevation of the trunk above the sediment surface).Based on three characteristics,i.e.i)the total height of the individual columns (up to 4m); ii) the high synoptic relief above the surrounding sediment; iii)the vertical dip of the laminae on the surface of the conophyton trunks, it can be supposed that the living part of the Conophyton-Jacutophyton was free of interspace filling”(rephrased from op.cit p.33).

5.The search for a modern analogue

It may be concluded that this remarkable ecosystem could only have flourished in the absence of biological diversity since no comparable occurrences have been observed on modern sea floors. In the absence of convincing modern analogues, small coniform microbial structures that are found in restricted modern ecosystems such as peritidal, lacustrine and hot-spring environments have been used by a number of authors as proxy analogues for understanding morphogenesis of ancient Conophyton, e.g. Walter et al. (1976), Petroff et al. (2010), and Bosak et al.(2010). Although these models and experiments have aroused interest e.g.from Mei et al.(2021)they have actually contributed very little to the understanding of the origin of the magnificent Proterozoic Conophyton.

Fig.6 The axial zone of Conophyton,A)Silicified Conophyton from the Dungaminie Formation-locality close to that shown in Fig.1(width of image 11 cm);B)Detail of crestal zone of Conophyton in 6a-note irregular expansion of apical area of laminae and contrasting colour of expanded apical fill material (height of image 1.5 cm); C) Modified reproduction of Fig. 5 from Komar et al. (1965) showing three of the variations in apical laminations used by the authors in the classification of Conophyton; D) A reproduction, with modification, from Bosak et al. (2009) of their Fig. 2(C) showing a thin section of an experimentally-grown unconsolidated cone showing disrupted fabrics, large voids (former blisters), and bubbles (Scale bar 1 mm). Refer to the original references for details.

Perhaps the most often cited model for Conophyton morphogenesis is that proposed by Walter et al. (1976) to explain the growth of the conical modern stromatolites that occur in Yellowstone National Park. They proposed that these forms were produced through a five-step morphogenetic sequence.The initial step involved the accentuation of tangled knots of filaments that developed on smooth microbial mats as a result of filament gliding,and that this perturbation became gradually exaggerated until a cone formed. Cao et al. (2001) and Cao and Yuan(2006) have proposed a similar model for the growth of the Conophyton form developing from “buds” of filamentous bundles on an initial microbial mat.Walter et al. (1976) noted that “silicification occurs continually during the growth process”, though the effects of this on morphogenesis were not explained. This morphogenetic model does not account for the regular spacing of cones that is typically observed in ancient examples, and Donaldson (1976) considered that the material described by Walter et al. (1976) from Yellowstone did not provide“an adequate environmental analogue for beds of laterally linked Conophyton that can be traced for hundreds of kilometres”.

Fig. 7 Mathematical modeling of Conophyton laminations with examples from the Dunganninie Formation. A) Model of the evolution of a vertical axial section through two contiguous cones; B) Contiguous Conophyton, the larger one is ～80 cm tall; C) Transverse section of an evolving model of a parabola field; D) Cross sections of a Conophyton field (image width ～150 cm). Modified from Batchelor et al. (2004a).

Bosak et al. (2010) expanded on the Walter et al.(1976) analysis of the Yellowstone structures by undertaking laboratory experiments that involved coneforming microbial communities cultured from samples taken from ponds in Yellowstone National Park comparable to the localities studied by Walter et al.(1976). They found that the filamentous cyanobacteria generated gas bubbles that became trapped in the crestal zones of the coniform laminae, and suggested that this might account for the expanded and,in places, fenestrate character of the axial zone(Fig. 6) of ancient Conophyton (Komar et al., 1965).Flannery and Walter (2012) found this explanation unsatisfactory.

By contrast, Petroff et al. (2010) have suggested that conical stromatolites are created when carbonate minerals precipitate beneath а living microЬial mat covering the surface of the cone.They considered that this mineral precipitation was limited bу diffusion of mineralizing ions through the microЬial mat, and therefore the rate of precipitation would bе faster in regions of high curvature. Petroff et al. (2010)considered that competition for nutrients helped determine the commonly observed regular spacing between small coniform structures of mats in hydrologically quiet environments, and suggested that this was a consequence of nutrients being delivered to the mats by diffusion. They made the obscure suggestion that approximately centimeter-scale spacing between vertical structures might record the length of day.However, Bosak et al. (2013) pointed out that the suggestions of Petroff et al. (2010) do not explain the size or the spacing of the large cones of the Late Proterozoic that grew in relatively deep environments where mixing would have destroyed strong diffusive gradients in the fluid.

Fig. 8 Horizontal cross sections through Conophyton from the Dungaminie Formation at a locality close to that shown in Fig.1.A)Cross section showing variations in lenticular form of laminations and development of horizontal axial zones on either side of the circular core of the specimen.It is hypothesised that these patterns reflect the nature of gentle persistent current flow around the evolving structure. Scale in mm; B) Five evolving growth stages of the horizontal cross-section of another Conophyton from the same locality showing the variation of form during the development of the structure, thought to represent the evolution of the ambient current.

Flannery and Walter(2012)reviewed the record of modern lacustrine and peritidal benthic microbial mats with coniform morphology,including the silicified examples from Yellowstone National Park that were given the formal name Conophyton weedii Walter (in Walter et al., 1976). Implicit in this nomenclature is the assumption that these modern coniform structures are comparable to large Precambrian Conophyton.However, despite superficial similarities, these hotspring structures differ in many ways from the holotypic Conophyton, most significantly in scale.

6.Morphogenetic modelling

Batchelor et al. (2004a, b) adopted a theoretical approach to understanding Conophyton morphogenesis by modelling the growth and form of microbialites from the perspective of the statistical physics of evolving surfaces.

They concluded that the evidence for an almost complete covering by a benthic microbial community(BMC) and infinitesimal sediment deposition during growth suggests that the Conophyton form is determined by only two factors;the availability of light and the rate of mineral accretion.They further concluded that vertical phototropic or phototactic microbial growth combined with surface-normal mineral accretion are sufficient variables to account for the growth and form of many ancient stromatolities. Their model predicts that simple domical stromatolites form when upward growth of the biofilm is less than or comparable to the rate of mineralization. However, coniform structures with thickened axial zones, typical of Conophyton,only form when upward growth of the biofilm exceeds the rate of mineralization. Batchelor et al.(2004a) demonstrated that these latter forms show close similarity to Conophyton exposed in the 1.7 Ga Dungaminnie Formation in the Northern Territory,Australia (Figs. 7 and 8). Moreover, this model shows that, when the rate of biofilm accretion greatly exceeds that of mineralization, angular cones form that have no distinct axial rounding of axial zones that are similar to the “egg-carton” stromatolites that have been claimed to be among of the earliest macroscopic evidence of life (Allwood et al., 2007).

7.Horizontal equivalents to the crestal axial zone

The modelling described in the previous section assumed a circular cross-section for the evolving cone.Structures similar to those that typify the crestal axial zone can be observed traversing the horizontal axial planes of many Conophyton with lenticular or teardrop cross-sections. These have been described by Komar et al. (1965, their Fig. 3 on p.21) as “ribs located radially in cross-section …. .the diagnostic value of the axial planes is still unclear”,and,on page 27, they note that some Conophyton have “radial ridges-‘a(chǎn)xial planes’the number of which can reach 4

Fig. 9 A) Horizontal section through a field of Conophyton, Atar Group, Islamic Republic of Mauritania. Note the horizontal axial structures and lenticular form of individual stromatolites, and their organized pattern into“chains”;B)Diagrammatic representation of two adjacent“chains”of lenticular Conophyton in the upper part of the previous photograph, showing the sinuous relationship of adjacent axial zones and the interference pattern of extrapolated surfaces from adjacent chains.Note that convex patterns of one chain correspond to concave patterns in the adjacent chain. Do these features reflect the ancient flow-patterns of a current?

-6”.Bertrand-Sarfati and Moussine-Pouchkine(1998)described similar features from Lekhleigate,Mauritania (Fig. 9). They illustrate a surface showing Conophyton in elliptical cross-section that all show“elongation of the individuals following the pinching of laminae along sinuous axis, sometimes continuous from one section to the other”. Conophyton from other localities show similar cross-sections, for example the examples from near“Heartbreak Hotel”,NT,Australia,studied by Batchelor et al.(2004a)(Figs.1 and 8). Starting from a core with a circular crosssection, the form develops either an asymmetric“tear-drop” or symmetrical lenticular coniform elongations with the elongations characterised by axial zones(Fig.8).Bruce Henry(Personal Communication)has drawn attention to the resemblance of these forms to horizontal expressions of fluid flow patterns of gentle, persistent currents around a cylindrical rod.The rapid lithification of Conophyton therefore preserves a translation of process into form, with the structure of its cross-section fossilising the flow patterns of the ambient current operating during growth of the structure, with rates of accretion inversely proportional to flow pressure.It might even be possible to use outcrop evidence to interpret the variation in these patterns across a group of adjacent Conophyton and thereby reconstruct flow patterns through the evolving Conophyton field (Fig. 9). It seems Conophyton grew in gentle,but persistent water-currents that maintained flow velocity and direction over very long periods.

8.Internal structure and mineralization

Some authors have ascribed the genesis of other types of coniform structures to bacterial filament motility, for example Bartley et al. (2015) described small (<1 m) coniform (though not Conophyton) stromatolites that they suggested were formed “by the upward growth of motile filaments and the rapid lithification of microbial laminae”. Filament motility was also proposed by Walter et al. (1976) for the Yellowstone material(which he considered to be modern examples of Conophyton). Grey et al. (2012)concluded that “The morphogenetic mechanism responsible for the formation of axial zones has been observed in living stromatolites, making axially zoned stromatolites one of the few Precambrian fossils for which a robust modern analogue is known”. They suggested that continued growth of the axial zone is dependent upon gliding motility in filamentous microorganisms, a role played by cyanobacteria in all modern examples. The identification of axially zoned conical stromatolites in the rock record thus implies the presence of filamentous microbes capable of gliding motility and is circumstantial evidence for the presence of cyanobacteria. They report examples of axially zoned stromatolites from the 2.72 Ga Tumbiana Formation and the c.3.43 Ga Strelley Pool Formation,both in Western Australia, and suggest that these microbially mediated structures are part of a morphological continuum that stretches from the present day to the early Archean.

Fig. 10 Detail of laminae of a specimen of Conophyton garganicum, modified from Fig. 5 of Riding (2008). The fabric has been interpreted as a“hybrid stromatolite”(i.e.composed of alternating laminae of biotic and abiotic origin). In this case, there is alternation of submillimetric layers of fine-grained lithified microbial mat and light-coloured, essentially abiogenic sparry layers. The discontinuity and textural variation in the light layers suggest that these are better interpreted as evidence for carbonate precipitation within the original primary dark laminations. Specimen from the Middle Riphean of the former USSR, stratigraphic unit and locality not known, and donated to Geological Survey of Canada by M.A.Semikhatov. Photograph: Hans Hofmann. Width of view 8 mm.

Fig.11 A sub-aqueous coniform thrombolite～25 cm tall,Lake Clifton,Yalgorup Lakes,Western Australia.Charophytes grow adjacent to the thrombolite.At the time this photograph was taken(1988)the Lake was hyposaline.Polished impregnated section through a similar coniform thrombolite from the same locality. Note the clotted structure - the form is composed of a complex fabric of stevensite and aragonite.

However, filament motility cannot account for the morphogenesis of the tall,rigid larger-scale cones that typify true Conophyton that attained heights of up to 4 and possibly even 10 m, though at any oe time the synoptic relief rarely exceeded 2 m.This indicates that they grew by the accumulation of strong rigid layers(Fig.1).What can be deduced about the nature of the early organomineralization that produced this?Despite evidence of “pieces of disrupted mat peeling off the structures”(Bertrand-SarfatiandMoussine-Pouchkine, 1998, p33), the preserved integrity of toppled and eroded Conophyton of Mauritania(Fig.4)provides clear evidence that they became mineralised as microbial communities were creating them(Bertrand-Sarfati and Moussine-Pouchkine, 1998).Indeed, Riding (2011) suggested that uniform spacing of lamination in some Conophyton is a result of alternating submillimetric layers of fine-grained lithified microbial mats and light-coloured, essentially abiogenic,sparry layers.He therefore referred to them as“Hybrid Stromatolites” (Fig. 10). However, Petroff et al. (2010) had earlier suggested the more plausible theory that conical stromatolites form when carbonate minerals precipitate beneath the living microbial mat that covered the cone surface, an explanation consistent with the fabric shown in Fig. 10. They suggested mineral precipitation was limited by diffusion of mineralizing ions through the microbial mat,and that the rate of precipitation would be faster in regions of high curvature.

Komar et al.(1965)classified the microstructure of Conophyton laminae into three types: (i) Ribboned(typified by Conophyton cylindricum) (ii) “Strokes of the Atom”, (typified by Conophyton garganicum) and(iii) Clotted (typified by Conophyton lituus). Grey and Awramik (2020) reimagined these three catagories as“l(fā)aminar architectures” with (i) Conophyton jacqueti displaying filmy laminar architecture, (ii) Conophyton garganicum australe displaying striated laminar architecture, and (iii) Conophyton new form (Pingandy type) displaying Streaky Laminar Architecture. It has already been noted that Donaldson (1976) regarded these textures of Conophyton laminations to be of diagenetic origin. This is consistent with Maslov's observation that the microscopical structures of Conophyton were almost always obscured by a high degree of recrystallization. Maslov (1937, 1938) also noted that the holotype contained significant amounts of argillacous material that served to preserve microstructure, and that some lenticular carbonate bodies in Conophyton laminae were darker at their centre due to clay inclusions. Maslov concluded that the structures grew “in a shallow sea that abounded in argillaceous ooze”, with fine colloidal material suspended in the water.However,the absence of any trace of fine detrital sediment in the sediments surrounding these Conophyton provides incontravertable evidence that Conophyton generally grew in clear,quiet waters free of suspended sediment.It is more likely that the origin of this apparently argillaceous material lay with the Conophyton-forming microbiota creating favourable local conditions for its formation,and inclusions of this clay within nodules of carbonate demonstrate that the clay predated the carbonate.

9.The Yalgorup Lakes - a model for Conophyton morphogenesis?

The Yalgorup Lake system in Western Australia(Brearley and Hodgkin, 2005) contains several microbialite-bearing localities that have provided plausible explanations for the formation of the enigmatic, ancient Conophyton which is provided by mechanisms of microbialite biomineralisation that have been revealed by research undertaken on the modern thrombolites of these lakes that are formed by rapidly lithifying benthic microbial communities with no trapping and binding of detrital sediment.It is clear that these lakes do not provide analogues for the geographical environments in which ancient Conophyton flourished, but they contain examples of microbially-influenced lithogenesis that, taken together, provide a plausible explanation for the formation of Conophyton in ancient sub-aqueous environments.Burne et al.(2014)have demonstrated that the well-described modern thrombolites of Lake Clifton (Fig. 11) were syndepositionally mineralised immediately below the living surface biofilm in a zone where decaying benthic microbial biofilms become permineralised by a magnesium silicate mineral, stevensite. In places this stevensite becomes syngenetically altered to aragonite that overprints the primary microbial structures with crystalline spots, lenses and irregular patches(Fig. 12).

At a nearby locality,ephemeral,benthic microbial mats, characterised by small pinnacles, have been observed (Fig. 13A, C) (Moore, 1992 unpublished,quoted in Burne, 2016). When examined in 1992 the pinnacles were subcylindrical, erect. 1-5 cm high,1-2 cm diameter,spaced about 1-2 cm apart,more or less contiguous and had sub-circular transverse sections. They were constructed by a benthic microbial community dominated by the cyanobacterium Phormidium,but also including Spirolina,Aphanothece and Chroococcus as well as diatoms,principally Brachysira(Moore, 1992 unpublished). Lithified morphological equivalents of comparable size to the living microbial pinnacles in Pamelup Pond (Fig. 12B, D) are observed on the lake floor of the adjacent Lake Preston. They contain clear evidence of their microbial origins.Though they have subsequently undergone four distict phases of mineralisation(Figs.13 and 14).Initially(i)a magnesium silicate first permineralised the decaying microbial biomass,retaining some of the original weblike microbial textures,then ii)the magnesium silicate coalesced into a uniform mass that in places became transformed to a serpentine mineral(chrysotile and/or lizardite), then (iii) crystalline aragonite over-printed much of the original texture, forming spots, clots and lenses of carbonate,finally(iv)high magnesium calcit grew as rims around the aragonite clots and lenses and overprinted some of the remaining areas of magnesium silicate.

Fig. 12 QEMSCAN? (www.fei.com/products/sem/qemscan) mineral map overlain on co-registered BSE image of a mesoclot in a subaquous Lake Clifton conical thrombolite 86,635,078.The colours have been enhanced for clarity. Stevensite (green) coats and permineralises the original cyanobacterial web of the microbialite.Areas of aragonite (white) overgrow the stevensite thereby eliminating traces of the original microbial fabric. The red colour identifies mixtures of stevensite and aragonite, highlighting the boundaries of the mineral phases. Grey areas are organic material,e.g. in the filament remnant extending diagonally up from the centre of the bottom of the image. Black areas are cavities. Scale bar is 1 mm.

Fig. 13 Comparison of A) living pinnacle mat from Pamelup Pond (photo credit - L.S.Moore) and, B) lithified hardground from Preston Lakebed with coniform surface structure.Note cavity rich cut cross section of sample.Scale in cm;C)sectioned pinnacle form Pamelup Pond mat (Scale 1 mm) (photo credit - L.S. Moore) to be compared with D) polished vertical section of a lithified pinnacle from the indurated lakebed crust. Scale 1 mm.

It is suggested that the syngenetic mineralization of the Proterozoic Conophyton biofilms occurred through processes similar to those described above from Lake Clifton and Lake Preston.Very soon after the formation of the ancient Conophyton's biofilm it could have been rapidly syngenetically transformed into a rigid,lithified structure through early permineralization of microbial organic matter by a magnesium silicate phase. The magnesium silicate would then coalesce into a uniform mass, imparting significant mechanical strength to the Conophyton. This suggestion of a former magnesium silicate phase is supported in the type specimen of Conophyton (Maslov, 1937) by its significant argillaceous content, despite the lack of any evidence of argillacoeus material in the surrounding sediment.The frequent occurrence of silicified specimens of Conophyton at other localities adds further support to this suggestion. Subsequent to the early siliceious mineralisation of the type specimen,carbonate crystal growth overprints much or all of the initial texture to give the secondary texture of “spots” and “l(fā)enses”(Fig.2)as mentioned and illustrated by Maslov(1937).Recent examples of this sequence of events can be clearly seen in the modern microbialites form Lake Clifton (Fig. 13) (Burne et al., 2014) and Lake Preston(Fig. 14) (Burne, 2016). If the carbonate overprinting extended through the whole structure, as may have frequently been the case, it might leave only small relict areas of the magnesium silicate phase or obliterate it entirely, hence the concept of a mineralogical“Missing Link” suggested by Burne et al. (2014). It is therefore concluded the type specimen of Conophyton lituus(Maslov)could plausibly have been constructed in a tranquil environment by the accretion of biofilms that became syngenetically permineralized by a mineral such as smectite which became,in places,replaced by carbonate,silicate minerals or silica.

Fig. 14 SEM Image of sample from lithified pinnacle from Lake Preston, similar to those shown in Fig. 9B. (a) Mg silicate phase reflecting microbial filament web;(b)Massive Mg silicate phase;(c)Aragonite crystal aggregate; (d) Late high-Mg calcite crystal aggregate.

10. Conclusions

Maslov (1937,1938) in his description of the type material of Conophyton,concluded that the form grew“in a shallow sea that abounded in argillaceous ooze”,with fine colloidal material suspended in the water.However, the lack of fine detrital sediments accumulating between the Conophyton clearly indicates that Conophyton generally grew in clear,quiet waters free of suspended sediment. It is more likely that the argillaceous material formed as a consequence of the microbiota that constructed the Conophyton having created favourable local conditions for clay formation.Maslov's observation that inclusions of clay within nodules of carbonate in Conophyton seems to confirm that the clay predates the carbonate. Indeed, the features noted by Maslov might equally be taken to reflect the precipitation of an authigenic clay that permineralized organic cells, thus preserving their structure. Analagous processes of early permineralisation by magnesium silicate,followed by overprinting by calcium carbonate has been well-described in microbialites from Lake Clifton (Fig. 12, Burne et al.,2014) and Lake Preston (Fig. 14, Burne, 2016).

The careful study of contemporary geobiological processessprovides important keystothe understanding ofthe evolutionand significanceofancientmicrobialites(see, for example, Burne and Moore, 1987). If Conophyton did,in reality,form as predicted by the model proposed by Batchelor et al.(2004a),then they werethe product of a simple but effective growth strategy involving a relatively homogeneous, extensive benthic microbial community. Such a community would be extremely vulnerable to predation and competition(Lowe, 1994) and would therefore be able to flourish only where these ecological pressures were minimal.This morphogenetic explanation for the formation of Conophyton might also explain the decline of the form in the Neoproterozoic (Komar et al., 1965; Krylov and Semikhatov,1976)as a logical consequence of the evolution of greater biological diversity in the quiet marine environments that they had dominated for so long.

We conclude that syngenetic and early diagenetic carbonate mineralization of microbialites may effectively obscure all traces of the original microbial communities,leaving only morphogenetic evidence for their organosedimentary origin. This process rapidly imparts significant mechanical strength that would enable the construction of the large rigid cones.If the carbonate overprinting extends through the whole structure,as may have been common,it would either leave only small relict areas of the Mg silicate phase or obliterate it entirely. The Proterozoic Conophyton could plausibly be constructed in a tranquil environment by the accretion of microbial mats that were syngenetically permineralized by carbonate.

List of Abbreviations

IGCP - International Geological Correlation Program

SEM - Scanning Electron Microscope

Declarations

The samples used and analyzed during this study are available from the author. The Baas Becking Geobiological Laboratory, The International Geological Correlation Program, the Australian National University, the University of Western Australia and the University of Queensland made contributions toward the funding of this study.

Acknowledgements

Inspiration for the research described here came from the realization that superficially similar modern coniform mats such as those described from Yellowstone and assigned to Conophyton by Walter et al.(1976), and the various modern conform and tufted mats that I encountered during the Baas Becking Geological Laboratory's studies of the intertidal microbial mats of Fisherman Bay, Spencer Gulf, and Hamelin Pool, Shark Bay, simply could not provide a satisfactory explanation for the construction of the ancient, tall, and rigid, stromatolite form-genus Conophyton. I examined classic examples of Proterozoic Conophyton in the field during an IGCP 261 sponsored field trip to Mauritania (guided by Janine Bertrand-Sarfati) and during a field trip to the Dungaminnie Formation of the Northern Territory,Australia with Murray Batchelor and Jim Jackson.Field work in the modern environments of Lake Clifton and Lake Preston in the Yalgorup Lake system of Western Australia was undertaken with Linda Moore,and the sampling of the pinnacle mats of Pamelup Pond was undertaken by Linda Moore and Robert Hilliard. Petrological preparation, examination, and XRD analyses were undertaken in the laboratories of the Research School of Earth Sciences, and SEM examinations were made at the Centre for Advanced Miscroscopy, Australian National University, and at the University of Queensland. Permission to reproduce information from the late Linda Moore's unpublished 1992 manuscript was kindly granted by her sisters,R.and J.Moore.Chaojia Mei assisted with the preparation of Figs. 8 and 9 and, along with Gavin Young,Norma Burne,Linda Kah,Stephen Kershaw and an anonymous reviewer,provided valuable criticisms,insights and thought-provoking discusions that added much to this paper.Yuan Wang and the editorial staff of JoP are thanked for their advice, patience and editorial assistance.