Contrasting morphology and growth habits of Frutexites in Late Devonian reef complexes of the Canning Basin, northwestern Australia

Frutexites‐like microstructures are described from the exhumed Late Devonian reef complexes of the northern Canning Basin, Western Australia. Several high‐resolution imaging techniques, including X‐ray microcomputerised tomography, scanning electron microscopy and X‐ray fluorescence microscopy, were used to investigate morphology and composition in two samples. Three types of Frutexites‐like microstructures (Types I–III) have been identified. Type I, found lining an early marine cement‐filled cavity in fore‐reef grainstone facies, consists of dendritic structures formed primarily of coccoid bacteria with filamentous bacteria embedded in sheets of amorphous extracellular polymeric substances (EPS). These ferromanganiferous dendrites have laminated to spheroidal textures. Types II and III are from a toe‐of‐slope hardground. Type II grew in a crypt between two corals, is also dendritic and composed of bacilliform and filamentous bacteria embedded in an amorphous EPS sheet. The opaqueness of these ferriferous dendrites precludes more detailed description of textures. Type III grew as branching columnar microstromatolites and is composed of entwined filaments of Girvanella, Rothpletzella and Wetheredella with Fe‐enriched outer walls that generate Frutexites‐like microstructures. Types I and II resemble Frutexites sensu stricto as defined by Maslov (Stromatolites, Trudy Instituta geologicheskikh nauk Akademiya nauk SSR, 1960) and are the result of the consecutive growth and permineralisation of biofilms composed of mixed bacterial communities growing in cryptic habitats. Type III superficially resembles Frutexites sensu stricto based on macroscopic field observations, however, detailed microscopic analysis reveals that it is composed of Fe‐enriched tubular walls surrounded by Mn‐enriched calcite.


Despite these marine examples, modern marine analogues
for Frutexites have not been described (Jakubowicz et al., 2014).
Frutexites-like microstructures display a variety of morphologies including dendritic, pillar-shaped, club-shaped and branching or unbranching columns.Diverse growth models have been proposed to explain the different morphologies and various states of preservation.For example, Böhm and Brachert (1993) suggested that the ferromanganesian mineralogy of Frutexites-like microstructures replaced primary carbonate.In contrast, Guido et al. (2016) proposed that Fe-and Mn-enriched fluids promoted the dissolution of original micritic material during a syndepositional diagenetic stage to create pores and discontinuities within the micrite suitable for the colonisation by Fe-and Mn-oxidising bacterial communities.
Alternatively, Myrow and Coniglio (1991) suggested that growth was linked to microbial activity through organically influenced precipitation of Mg-calcite subsequently altered to haematite.
Much of the controversy over the definition of Frutexites in current use stems from the difficulty in determining its origin.
This study uses a suite of high-resolution imaging techniques such as conventional optical microscopy, scanning electron microscopy combined with energy-dispersive spectroscopy (SEM-EDS), X-ray microcomputerised tomography (micro-CT) as well as X-ray fluorescence microscopy (XFM) to morphologically and geochemically characterise dendritic microstructures attributed to Frutexites to determine whether they represent Frutexites sensu stricto.
In this study, Frutexites-like microstructures from two samples were examined in detail with the aim of characterising their composition and growth habits in comparison with the holotype of  Becker & House (1997).Modified from Becker & House (1997).Maslov (1960) and published examples of varying ages and environments elsewhere.The two samples are from a cement cavity fill within a fore-reef facies of the Napier Range at Dingo Gap (Figure 1b) and the McWhae Ridge hardground (Figure 1c), which represent two distinct depositional settings that are well constrained by biostratigraphic ages.In addition, Frutexites has been observed in breccias and Neptunian dykes elsewhere in the Napier Range (Figure 1b; George unpub.data).

| G EOLOG IC AL S E T TING
The study area is located along the northern margin of the large Canning Basin.Late middle Devonian rifting along this northern margin formed the northwest-trending Fitzroy Trough (Figure 1a).
McWhae Ridge is located at the southeastern end of the exhumed reef complexes (Figure 1a).The Frutexites Bed (named in Playford et al., 1984) is well exposed on the western flank of McWhae Ridge (Figure 1c).The Frutexites Bed is located near the top of Section 371 of Becker et al. (1991) above several marker beds and is exposed as a ledge overlying the dolomitic Upper Marker Bed (Figure 2a).It is exposed for ~40 m at this locality although it is known to extend locally at least 18 km 2 (Nicoll & Playford, 1993).This package of red siltstones and limestones of the Virgin Hills Formation thins northward and its base represents a major downlap surface on the early Frasnian steeply dipping, coarse fore-reef facies (Sadler Formation) and coeval inter-reef basinal mudstones of the Gogo Formation (George et al., 2009).
At Dingo Gap, and elsewhere along the Napier Range, the forereef succession ranges from coarse breccia-dominated facies (locally abutting eroded platform margins) to mixed carbonate debrites and turbidites that thin basinward into an inter-reef basin or open ocean (George et al., 1997;George & Chow, 2002).Most of the facies record redeposition of carbonate sediments formed on the shallow water platforms with local evidence for in situ slope deposition (e.g.deep water stromatolites) and intraclastic breccias from reworking slope facies (George, 1999;George et al., 1997).
Within this depositional setting, Frutexites-like microstructures were sampled from an early marine cement-filled cavity (Figure 2c) in red grainstones and near a biohermal deep-water stromatolite (see figure 6d in George et al., 1997).There are also common fibrous cements related to early lithification of the slope, such as bedding-parallel cement sheets.The age of the slope facies at this location is Famennian (Upper crepida to no younger than Lower marginifera conodont zones; George et al., 1997).
Other Frutexites-like microstructures have been observed within Neptunian dykes closely associated with deep-water stromatolites (e.g. Figure 2d from the edge of the Frasnian South Behn platform, Figure 1b) and similar in style to the condensed depositional setting of the McWhae Ridge Frutexites Bed.Frutexites-like microstructures have also been observed growing cryptically in a calcite cement-filled cavity in coarse breccias at the eastern entrance to Tunnel Creek (Figure 2e).

| Microscale computerised tomography (micro-CT)
A semi-correlative imaging approach was used to target regions and stitched using XMReconstructor® software (v11.1.5707.17179,Zeiss) using a 0.7 kernel size recon filter setting.The visualisation and analysis of data generated from μCT scans were performed using Avizo (v 2021 and 2022) software.A detailed workflow to extract dendrites from the plug volume is provided in Supplementary Material (Table S1).

| Scanning electron microscopy (SEM)
After computerised tomography and thin section production, a hammer was used to dry fracture fresh surfaces from the plugs resulting in small rock chips (~3 mm across).Dry working conditions were maintained in order to avoid contemporary contamination.
Subsequently, these rock chips were set on SEM mounts using copper tape to ensure ground contact between sample and stub holder.
These SEM mounts were air gunned to remove any dust before gold coating.Acid etching was not utilised to avoid dissolution of clays and carbonate phases.Gold coating (10 nm thick) was applied prior to SEM analysis to increase sample conductivity.Gold-coated samples were stored in an SEM pin mount storage box that was kept in an airtight sealed cabinet.SEM focused on the identification and 3D distribution of microbial relics (bacterial remnants and extracellular polymeric substances).High-resolution imaging was undertaken on the JEOL JCM-7000 NeoScope Benchtop SEM at the CMCA, UWA.
Information was also gathered about the spatial relationships of the relics with inorganic phases including carbonate cement, Fe-Mn oxides, and clays.Both secondary electron (SE) and backscattered electron (BSE) images were gathered under high vacuum.Samplespecific SEM beam conditions and working distance were required for the irregular rock chip surfaces.

| Energy-dispersive spectroscopy (EDS)
Energy-dispersive spectroscopy (EDS) available on the JEOL JCM 7000 Neoscope Benchtop SEM was used for spot analysis of microbial components observed in rock chips to obtain geochemical compositional data and differentiate organic (microbial relics) from inorganic phases (carbonate and clays).Only qualitative chemical data could be obtained by EDS spot analysis because of the irregularly fractured surface of the mounted rock chips.
Quantitative elemental compositions were obtained via SEM-EDS from the three gold-coated polished thin sections using the TESCAN VEGA3 SEM, at the CMCA, UWA.Aztec® Version 3.0 (Oxford Instruments NanoAnalysis) software was used to conduct point analyses and produce elemental maps presented in Supplementary Material.

| Synchrotron X-ray fluorescence microscopy (XFM)
Two polished thin sections (one from each sample) were scanned with the X-ray fluorescence microscopy (XFM) beamline at the Australian Synchrotron ANSTO, Melbourne (Howard et al., 2020).
XFM generates half-quantitative elemental maps with combined morphological and geochemical information about the microbial communities and associated mineralisation.A Kirkpatrick-Baez mirror microprobe was used to analyse the polished thin sections at 18,500 eV with 1-2 μm step size with a dwell of 0.7 ms.Synchrotron elemental maps were produced for Ca, Fe, Mn, Co, Cu, Cr, Ti, Zn, Pb, Ni, Sr, V, Y, Ir, Au and Pt using GEOPIXE software on the Australian Synchrotron Computing Infrastructure (ASCI) (Kirkham et al., 2010;Ryan, 2000;Ryan et al., 2014).Elemental maps for respective structures and normalised elemental distributions are presented in Supplementary Material.

| Terminology
The two analysed samples display dendritic shapes formed by divergently branched microcolumns.The terminology used to refer to parts of the microstructures in this study is based on Myrow and Coniglio (1991).For general description, without genetic implications regarding biotic or abiotic origin of the features, we use the term 'Frutexites-like microstructure' following Awramik & Grey (2020)'s original 'Frutexites microstructure'.To distinguish different microstructures in the samples, three types, Type I-III, have been identified.In describing morphology, the terms dendritic and arborescent are considered equal.In discussing the internal microstructure of the dendrites, terms such as laminated, peloidal and spheroidal are used.

| Morphology, growth features and mineralogical composition
Frutexites-like microstructures in the Dingo Gap sample show dendritic columns that grow inwards from all sides of the cavity (Figure 3a).These well-developed, slender dendritic columns (Figure 4a) with microstromatolitic (Figure 4c) to spheroidal textures (Figure 4d) are termed Type I.The columns are ~0.5-1mm wide, up to 7 mm long and are composed of subhedral-to-anhedral finely crystalline calcite of 30-50 μm across (Figure 4a).Columns are typically opaque-very dark reddish brown at the base (Figure 4e) becoming semitranslucent light brown towards the top (Figure 4a).
Small spheroidal projections typically appear at the tops of dendritic columns (Figure 4c,d).They initiated the onset of a new branch during column growth.An individual branch is commonly ~0.05-0.3mm wide and up to 1.5 mm long and composed of multiple chambers stacked on top of each other (Figure 4c).Their walls may be thin, opaque, dark reddish brown with a clear outline or semitranslucent rusty brown with a vague outline in thin sections.In cross-section, a single chamber is 50-100 μm across on average and topped by convex upward laminae in microstromatolitic dendrites (Figure 4f).
The laminae are dark reddish brown and typically associated with very fine particles.At high magnification, the dendrites incorporate small cell-like spheroids (~20-50 μm) (Figure 4g,h).

The McWhae Ridge sample displays two types of microstruc-
tures.Type II is associated with a calcareous fossil embedded in a silty-micritic matrix.Although the fossil has been recrystallised, its tubular morphology suggests that it is likely to be an encrusting coral (Figure 5a).Type II nucleated on and grew out from the fossil as dense dendritic shrubs with fanning branches surrounded by microcrystalline (micrite) cement (Figure 5a,c).The shrub has a broad column base, 1 mm across and 1.5 mm long.A singular branch is ~0.05-0.1 mm wide and up to 1 mm long (Figure 5d).Iron impregnation is pervasive and has resulted in a micropeloidal texture that conceals internal detail as well as blurring the outline of branches (Figure 5d,e).In cross-section, the branches are ~150 μm across and a single chamber is on average 50 μm across (Figure 5e).
Type III is distinguished by its shape, size and composition.Its microstromatolitic columns typically have narrow bases (~0.4 mm wide) that gradually widen to ~5 mm with limited branching (Figure 6a).The columns may be up to ~5 mm long and are surrounded by a layered silty-micritic matrix (Figure 6a).Successful branching is rare, resulting in a predominantly columnar appearance.Some microstromatolites form on skeletal grains or, more commonly, on cup-like upright skeletons embedded within the hardground (Figure 6a).These have been interpreted as Aulopora corallites based on their composition, dimensions and in situ appearance.The internal fabric of the microstromatolites is laminated (Figure 6b) to irregular where branches formed (Figure 6c).
Sheets of these combined Girvanella, Rothpletzella and Wetheredella tubes are found encrusting small Aulopora corallites (Figure 7c,d) as well as larger corals embedded in the hardground.
Tubular filaments encrust the calyx (Figure 7d).Some corallite margins show tubular perforations with the same dimensions as the Girvanella filaments encrusting them.However, it remains unclear whether these perforations are the result of microboring by the filamentous microbial community or natural pores.There are multiple thin siltstone layers (~10-30 mm thick) with scattered detrital grains within the hardground (Figure 7e).
Locally, they evolved into larger microcolumns with discrete laminated fabrics similar to those observed in the top section of the Frutexites Bed.The surrounding matrix is predominantly siltstone with minor micrite.The margins of both smaller and larger microstromatolitic columns show calcification (Figure 7f).

| Microbial relics and geochemical composition
A wide variety of microbial elements ranging from coccoid, ba- Locally, chains of bacilli-type cells can be discerned within Type I chambers (Figure 8b).These chains are typically coated by a very thin bacterial sheath composed of C, O, Ca and Fe.In some places, collapsed filamentous bacterial sheaths are coated with very-fine-granular Fe oxide that also coats chambers (Figure 8c).At high magnification, this Fe-coating consists of abundant nano-scale discs with apparent holes in their centre (Figure 8c).A second type of coating on Type I chambers features closely packed spheres composed of Fe-Al silicate platelets (Figure 8d).
Type II chamber walls contain deflated filamentous bacterial sheaths embedded within mineralised planar EPS (Figure 8e).At high magnification, this EPS sheet also embeds short and long rodlike bacilli and a few nanocrystals (Figure 8f).The short rods are In contrast, Type III is primarily composed of large tangled filaments that have grown columnar microstromatolites (Figure 8g).
The filaments are fully calcified internally and have a Fe-Mn-Al-Si coating (Figure 8h), composed of C, O, Ca, Fe, Mn, Al and Si with local K and Ti.The coating is generally 3 μm thick, however, where well developed, can be as much as 15 μm thick.In size, shape and composition, the filaments resemble the calcimicrobe Girvanella.
The microstromatolitic columns are surrounded by crudely laminated micrite.S2 and S3).The ferromanganese dendrites are distinct within the surrounding Sr-bearing calcite cement.The margin of the spheroids within the dendrites is composed primarily of Fe without any other metals present.

| Elemental distribution
In contrast, both Type II and III display high concentrations of Fe.The Type II shrub contains Mn distributed diffusely within its dendrites.Based on XFM and EDS, the Fe-enriched dendrites have moderate concentrations of Mn, Ni, Co, Si, Al and Cu (Figure 5b), as well as minor quantities of V, Ti, Cr, Zn, Pb and Mg (Figures S2, S3 and S5b; Tables S2 and S3).Micrite cement is dispersed within Type II dendrites (Figure 5f) and is not enriched in the range of trace elements measured in Type II (Tables S2 and S3).S2 and S3) and are concentrated in the areas of elevated Fe (Figure 6d).These trace elements are absent in surrounding silty-micritic matrix.Iron concentrated in the outer walls of the Girvanella, Rothpletzella and Wetheredella filaments enables their tubular outlines to be distinguished.Mn is moderately distributed within the micrite between closely packed filamentous tubes (Figure 6d).Ca is abundant in the interfilament micrite as well as in the matrix between microstromatolites.Detrital grains within the matrix show elevated concentrations of Fe and Ti, moderate amounts of Mn, Si and Al and minor Y, Zr, V, Cr, Zn, Pb, K, Zn, Ni, Mn and K based on combined EDS and XFM analyses.
The Frutexites-like microstructures at McWhae Ridge have been associated with an iridium anomaly with Ir and Pt elevated with respect to background (Nicoll & Playford, 1993;Playford et al., 1984;Wallace et al., 1991).Biological and/or biochemical processes were proposed as the mechanism for concentrating these heavy metals in the filaments (e.g.Dyer et al., 1989;Hurley & van der Voo, 1990;Nicoll & Playford, 1993;Playford et al., 1984).

| Three-dimensional growth patterns
Imaging Frutexites-like microstructure Type I enabled the colony to be distinguished as well as the individual columns within the calcite cements filling the cavity.Columns nucleated from the edges of the cavity and grew perpendicular to the cavity walls up to 5 mm long (Figure 3b).Columns commonly branch from their base leading to a bushy dendrite at the base and growing out into more slender dendrites.
Imaging Type II was challenging because of the iron impregnation of both the microfossil and surrounding micritic matrix.However, its subtle signature can be discerned below larger corals embedded in the hardground (Figure 9a,b).Type II nucleated from the edge of a coral and grew out to fill the crypt between two in situ corals attached to the hardground at an angle (~30°).One of the corals was partially overgrown by a sheet of filamentous tubes 4 mm thick (Figure 9a,b).

From macro-CT imaging, Type III forms layers within the
Frutexites bed intercalated with multiple thin siltstone layers where microcolumns are sparser in cross-section view (Figure 3c,d).In plain view, Type III is heterogeneously distributed in patches resembling small colonies (Figure 3e,f).Apparent alignment corresponds to dissolution of enlarged voids.Micro-CT imaging of a laminated section with Type III shows part of microstromatolites formed on calcitic cup-shaped features (Figure 9c,d).Their upright position suggests they grew in situ on the hardground (Figure 9d).A sheet consisting of a Girvanella-Rothpletzella-Wetheredella assemblage encrusts these cup-shaped features (Figure 9e) identified as Aulopora corallites due to matching dimensions and shape with those preserved on the top surface of the Frutexites Bed (Figure 9d).

| Origin of Frutexites
Application of high-resolution imaging has enabled the distinction of microbial communities forming the Frutexites-like microstructures analysed in this study.Type I was constructed primarily by colonies of coccoid bacteria and some filamentous bacteria collectively forming spheroidal biofilms that would appear laminated/ microstromatolitic when in a microtuft-like stacked arrangement.The coccoids are morphologically very similar to spherical forms discerned in black Frutexites within hot-water travertines in Morocco (Chafetz et al., 1998).The size of the coccoids described in this study matches the coccoid moulds of Koban and Schweigert (1993) in Frutexites crusts in synsedimentary karstic cavities.In addition, the C, Ca and O composition with Mg, K, Cl and Fe are very similar to the elemental composition of permineralised cocci analysed in modern microbialite studies (Kazmierczak et al., 2009;Perri et al., 2018).Furthermore, in our study, these spherical bodies form patches embedded within a smooth amorphous EPS sheet rather than forming a homogenous sheath the way silica spheroids are typically precipitated (Renaut et al., 1998).
In this aspect, Type I strongly resembles the slender dendritic calcimicrobe Epiphyton that has been found to be constructed by colonies of calcified coccoid cyanobacteria (Pratt, 1984;Weller, 1995;Zhang et al., 2019) or other rod and/or spheroid forms (Chafetz & Guidry, 1999).The few filaments observed resemble the filamentous and segmented structures reported for Frutexites encrusting Jurassic Neptunian dykes (Reolid, 2011), as well as filaments in Jurassic Frutexites crusts on macro-oncoids formed on a hardground (Reolid & Nieto, 2010).
Despite a strong superficial resemblance in outcrop and petrographically, high-resolution imaging has shown that Type III is morphologically, compositionally and mineralogically different from Types I and II.A Type III microstromatolite contains tangled tubular filaments and is characterised by low branching frequency.The small calcified tubes are very similar in size, shape and composition to the calcimicrobe Girvanella (Stephens & Sumner, 2003;Sun et al., 2021;Xiao et al., 2020).In addition, these microcolumns are very similar to the Girvanella columns described by Playford et al. (1976, their  The microbial relics in all three types described in this study confirm their biogenic origin.Based on observations and comparisons with published examples, we conclude that Frutexites Types I and II are Frutexites sensu stricto as described by Maslov (1960).
Due to its strong morphological and compositional differences, Type III is not considered Frutexites sensu stricto and was instead constructed by a mixed Girvanella-Rothpletzella-Wetheredella filamentous assemblage.
Our observations suggest that a mixed microbial community (filamentous, coccoid and bacilliform bacteria) was important for the development of Frutexites.A combination of spherical, rod-shaped and filamentous bacterial fossils was also found in Frutexites-like microstructures growing on Middle Devonian oncoids (Cavalazzi et al., 2007).It is possible to have mixed microbial communities including coccoid-, rod-shaped and filamentous microbes in mature biofilms compared with a colonising biofilm that is likely to display a lower-diversity microbial community (Stephens & Sumner, 2002).
The high concentration of Fe and Mn observed in Type I and II dendrites in this study supports the involvement of Fe-and Mnoxidising bacteria.The moderate-to-minor presence of other metals suggests all were available in the seawater (e.g.Fe, Mn, Ba) or in clays (e.g.Al, Ti, V, Cr) within surrounding sediment.Clays can provide nutrients to microorganisms while eliminating their waste products through adsorption on to clay surfaces (Ransom et al., 1999).
Metals adsorbed onto the negative sites of the bacterial sheaths or EPS would oxidise upon decay of these bacterial surfaces and induce local precipitation of Fe and Mn oxides (Beveridge et al., 1997;Braissant et al., 2007;Frankel & Bazylinski, 2003).Although beyond the scope of this study, metagenomic analysis of these ancient microbial communities may reveal the likely bacteria involved (e.g.et al., 2008;Casaburi et al., 2015).

| Cryptic Frutexites
The growth of Frutexites Type I in a cavity fill within a fore-reef slope facies suggests that coccoidal bacteria accumulated in local depressions on sheets of macromolecular EPS excreted by these coccoid colonies and other filamentous bacteria (Figure 10a phase 1).Negatively charged EPS adsorbs cations such as Ca 2+ , Fe 2+ , Mn 2+ and other trace elements from the surrounding environment (Arp et al., 2001;Dupraz et al., 2004) and creates a potential for biomineralisation (Beveridge et al., 1997;Cuadrado et al., 2012).Through their EPS or biofilm, bacteria set up crystallisation points where (Fe and Mn) oxidation and adsorption of metal ions available in the proximity would promote the formation of amorphous precursor phases that subsequently precipitate as Fe/Mn-(oxyhydr)oxides through further abiotic processes during degradation of these organic surfaces (e.g.Erhlich, 1975Erhlich, , 2002;;Han et al., 1997;Jiang et al., 2020;Lozano & Rossi, 2012;Mamet & Préat, 2006;Polgári et al., 2012;Reolid, 2011).The degradation of microorganisms and EPS through hydrolytic destruction results in release of cations initiating Fe-Mn-Al-Si mineralisation within the EPS sheet and generating a more rigid, nanocrystal-bearing amorphous phase (Dupraz et al., 2004;Konhauser, 1997;Perri et al., 2012Perri et al., , 2018Perri et al., , 2021)).The observed nanocrystals on microbial relics identified as EPS are precursors to more stable manganese and iron oxides.Synchronously, microbial features (coccoids and bacterial sheaths) calcify to produce an iron-manganese-organic composite (i.e.permineralised biofilm) that locally retains calcified microbial features as observed under SEM (Konhauser, 2007, p. 163; and references therein).
Subsequent growth of new biofilms produces chambered microtuft-like structures with an internal laminated or spheroidal texture (Figure 10a phase 2).This process corresponds to the apical growth model proposed by Pratt (1984) for other dendritic calcimicrobes such as Epiphyton and the Renalcis.As new biofilms grow, older degrading biofilms are being mineralised with iron and/ or manganese precipitates (Figure 10a phase 3).The very-fine-granular iron oxide precipitates, which are reddish-brown under conventional microscopy and consist of submicrometre disc-shaped plates under SEM, resemble similar features interpreted as haematite Fe 2 O 3 by Welton (1984).The transformation into haematite probably proceeds through dehydration and/or internal rearrangement (Konhauser, 1998).An abiotic Ostwald-ripening guided dissolution-regrowth process of primary MnCO 3 and FeCO 3 might have been involved further transforming nanograins into larger Fe/Mn-(oxyhydr)oxides like described for manganese carbonate deposit in China (Huang et al., 2022).Preservation of microbial components in Jurassic microbialites has been ascribed to Ostwald-ripening entombment shielding them from diagenetic alteration and destruction (Potter-McIntyre et al., 2017).Internal structural rearrangement triggered by changing physicochemical conditions in the environment could also have been involved in the abiotic formation of Fe/ Mn-(oxyhydr)oxides (Jolivet et al., 2006).Fe/Mn-oxide ageing could have further adsorbed metal cations from the environment due to their highly reactive surface sites (e.g.Ford et al., 1997).
We propose that mineralisation of the degrading biofilm forms an iron-and/or manganese-enriched Frutexites chamber wall and thus its general outline.This supports the interpretation that iron mineralisation was an early diagenetic process as proposed by Myrow and Coniglio (1991).Locally, further transformation of the permineralising biofilms resulted in spherical radiating clusters of Fe-Al silicate plates (Figure 10a phase 4).This is especially the case at the base of columns where older biofilms display a more advanced state of degradation.These areas appear dark brown under conventional microscopy and typically have a wider range of trace elements.New projections were constructed on a specific biofilm resulting in a new branch (Figure 10a phase 4).This could have resulted from locally high concentrations of coccoid or bacilli-type bacteria in the biofilm or projections may have initiated on patches of more mature bacteria that achieved cell division earlier than younger bacteria elsewhere in the biofilm.Zhang et al. (2019) proposed that Epiphyton branch formation was controlled by the growth hormones of coccoid colonies.
The smaller growths at the top of the Frutexites Type I dendrites may reflect slowed growth which may indicate that micronutrients were gradually depleted in the cavity (Figure 10a phase 5) potentially related to a reduction in nutrient supply and/or permeability being reduced by compaction and cementation.Cements in the cavity fill show that seawater did circulate with subhedral-to-anhedral finely crystalline calcite cements encasing the dendritic columns (Figure 10a phase 6).Calcite envelops entire chambered sections of Frutexites Type I dendrites, leading to the proposition that post-mortem processes might have elevated ambient alkaline levels to the supersaturated calcium carbonate point and triggered preferential calcite cementation around specific dendrite sections (Zhang et al., 2019).
The shape, internal structure and composition of Frutexites Type I observed in this study are very similar to Frutexites-like microstructures described by Böhm and Brachert (1993;their plate 36, figure 8; plate 37, figure 2), Myrow and Coniglio (1991;  The Type I dendrites grew on all sides of the cavity oriented towards its centre.This suggests that the mixed coccoid-filamentous microbial community did not depend on light for its growth and is thus non-phototropic.Ultimately, the Frutexites layer was overlain by multiple generations of early marine fibrous calcite cement filling the cavity (Figure 10a  ering is an important source of dissolved nutrients in the oceans, and especially during relative sea-level falls (e.g.Averbuch et al., 2005;Balter et al., 2008;Percival et al., 2019).Given the sub-tropical climate, Fe-dust coatings on siliciclastic detrital grains that accumulated on deltas along the northern basin margin were potentially a ready source of Fe and wind-blown silt and clay.Other trace elements could also originate from this detrital sediment supply as was concluded for elevated Ti and Cr values in the fore-reef slope successions elsewhere in the reef complexes (George et al., 2014).XFM data have shown that the base of Type I columns contains Fe, Mn and an associated suite of trace elements (e.g.Ti and Cr), whereas the younger parts of the columns mainly consist of Mn.This difference in elemental composition between base and top of the dendritic column may be related to fluctuations in oxidising fluids within the cavity with increased Mn indicating short-lived stagnating conditions.
The growth mechanism of Frutexites Type II is similar to that of Type I except that bacilliform bacteria were responsible for excretion of EPS (Figure 10b phase 1) and consecutive growth of new active biofilms on older, degrading and mineralising biofilms.The opaqueness of Type II precludes detailed interpretation of growth; however, it is likely that this type developed spheroidal and laminated chambered segments as seen in Type I dendrites.In this study, opaqueness in Types I and II is associated with an advanced stage of EPS permineralisation.XFM analysis shows that Type II shrubs consist predominantly of Fe with only minor amounts of Mn.This distribution of Mn may be explained by preferential incorporation of Mn in the calcite encasing the dendrites.The homogenous distribution of major and trace elements throughout the dendrites suggests the ongoing availability of these elements in surrounding seawater.
Although associated with a hardground, Type II grew freely in a cryptic space sheltered between two in situ corals (Figure 9b).In the absence of the usual favourable firm and stable sites for nucleation (e.g.cavity walls, lithified microbialites, hardgrounds), Frutexites-like microstructures are known to encrust skeletal material including rugose corals (Jakubowicz et al., 2014) and ammonoids (Woods & Baud, 2008).In this crypt, Type II column grew outward nucleating from the skeletal substrate (Figure 10b phase 2).
Nanocrystals grew within decaying EPS leading to its permineralisation (Figures 8f and 10a phase 3).These nanograins are interpreted as ferriferous seed crystals within the planar EPS.The silt and clay content deposited under very low energy conditions at McWhae Ridge, as seen in multiple siltstone layers in the Frutexites Bed and surrounding it, suggest that microbial communities may have scavenged metals from this fine sediment.Certain microorganisms retrieve metals, actively or passively, from their surroundings to fulfil cell functions (Konhauser, 2007, p. 114).Small differences in elemental composition between Type I and Type II may be linked to the available fine sediment in immediate surroundings.The cryptic site was filled with intergrown Type II columns that achieved a homogenous and advanced stage of EPS permineralisation prior to entombment in silty micritic cement (Figure 10b phase 4).
Type II morphological and compositional characteristics resemble Frutexites-like microstructures described by Playford et al. (1976; figure 10b) and Jakubowicz et al. (2014;figure 3a,e-Type 1 Frutexites).The style of growth is similar to that described by Woods and Baud (2008) where Frutexites-like microstructures grew perpendicular to the substrate (within microbialite laminae or ammonoids) on a hardground.

This study shows that the characteristic dendritic tuft-like
Frutexites microstructure is a result of the consecutive growth and permineralisation of biofilms (incorporating EPS and mixed bacterial communities) preserved through syndepositional calcite precipitation.This supports the degradation and calcification model proposed by Kazmierczak et al. (2009).Similarly, Stephens and Sumner (2002) proposed a fossilised biofilm model for the growth of the calcimicrobe Renalcis which would have been formed by the episodic growth, cal-

| Hardground-related Frutexites-like microstructures
The morphology, branching and ferro-manganiferous character of Type III would seem to suggest that it is a Frutexites-like microstructure based on these observations.However, findings from conventional microscopy, SEM-EDS and micro-CT indicate that Type III cannot be accommodated within Frutexites, as defined by Maslov (1960).Using high-resolution imaging techniques, different filaments were distinguished and identified as a Girvanella-Rothpletzella-Wetheredella assemblage that formed microstromatolites.In the first phase, this assemblage would encrust larger corals forming a thick sheet on the up current side of skeletal substrate to capture micronutrients from its surroundings (Figure 10c phase 1).In subsequent layers, the Girvanella-Rothpletzella-Wetheredella assemblage would grow branching columns on the hard substrate or the in situ corallites (Figure 10c phase 2).
Additional geochemical data were acquired in a previous study on the McWhae Ridge Frutexites by Hühne (2006).Analysis of Frutexites Type III and associated automicrite yielded a positive Ceanomaly, negative lanthanides and depleted LREEs.The incorporation of Ce 4+ in Fe-/Mn-oxides and hydroxides was interpreted as resulting from metabolic activity of microorganisms under variable redox conditions (Hühne, 2006).Early marine calcite cement, characterised by a negative Ce-anomaly, was interpreted as an oxidative marine environment which is consistent with the broader Lennard Shelf study of Nothdurft et al. (2004).
Girvanella, Rothpletzella and Wetheredella filaments grew together in columnar microstructures.There are two potential advantages of this arrangement: (1) a robust structure that was able to keep above surrounding low energy sediment accumulation; and (2) access to available nutrients descending through the water column and/or carried by weak currents along the sediment substrate.Mathematical modelling of hybrid biofilm structures has revealed a link between the shape of biofilms and nutrients (Picioreanu et al., 1998).As the biofilm grows, some bacterial colonies extend over more substrate than others and, with a larger surface area, can capture more nutrients.
Therefore, new biofilms will grow on these more successful patches forming positive relief on the substrate.In a nutrient-deprived environment, the voids between biofilm irregularities are not filled with new biomass and the microcolumns take shape.Opportunistic colonisation by filamentous assemblages on Aulopora corallites and other in situ corals also fits this growth model.Stability of the microstromatolitic columns was facilitated by the intergrown tubular arrangement and permineralisation of EPS coating the outer walls of the filaments.Absorption of cations by the EPS and permineralisation led to early lithification.Disintegration of these benthic filamentous microorganisms at the water-sediment interface, and EPS degradation, released ions such as Ca 2+ that was available to be precipitated as calcite within the filamentous tubes.
Release of Fe 2+ was a source of cations for precipitation of iron-(hydr) oxides on the outer bacterial wall.This process has been described by Mamet and Préat (2006) as bioparagenesis in Phanerozoic limestones This study shows that the dendritic ferro-manganiferous outline typical for Frutexites results from the consecutive growth and permineralisation of biofilms.Type I and II resemble Frutexites sensu stricto as defined by Maslov (1960) and grew in cryptic microenvironments.
Geological map showing the distribution of exhumed Devonian reef complexes along the northern margin of the Canning Basin and the Dingo Gap and McWhae Ridge study site locations.Beagle Bay-Harvey-Pinnacle Fault System (BB-H-P FS) defines the edge of the Lennard Shelf.Modified from Becker & House (1997).(b) Location of Frutexites at South Behn, Dingo Gap and Tunnel Creek locations along the Napier Range.Modified from George et al. (1997).(c) Geological map of the McWhae Ridge area.Studied specimen sampled from the same Frutexites Bed as described in Section 371 by of interest.Samples were first scanned at low resolution using the Medical X-ray Computed Tomography scanner (Medical XCT), Siemens SOMATOM definition AS®, at CSIRO-Energy, Kensington, Perth.An energy beam of 140 kV/1000 mA with a helical acquisition (0.35 mm pitch, 0.6 mm thick XCT slice, 0.1 mm slice spacing) was applied.Raw data were automatically stitched and reconstructed in high resolution using the instrument software.A 3D image of the entire sample was generated by stacking the contiguous crosssectional slices with Avizo® (V.2021) software.From this information regions of interest were obtained from which vertical plugs (~25 mm long and 15 mm diameter) were obtained from both samples and scanned at higher resolution using X-ray microcomputed tomography (micro-CT, Versa 520 XRM, Zeiss) at the Centre for Microscopy, Characterisation & Analysis (CMCA), at the University of Western Australia (UWA) running Scout and Scan software (V.11.1.5707.17179,Zeiss).The source was set to 140 kV and 71.3 μA with source-detector positions set to −45 and 100 mm respectively.An HE2 source filter was applied to screen out low-energy X-rays and mitigate beam hardening.A 0.4× objective lens was used with 2× camera binning to achieve an isotropic voxel resolution of 21.4 μm.Each scan comprised 2001 projections through 360° with an exposure of 2 s for each projection.Vertical stitching was used to capture the full length of each plug.Raw data were automatically reconstructed (including centre shift and beam hardening correction)

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I G U R E 2 (a, b) McWhae Ridge sample location.(a) View of the Frutexites bed and sample site (rectangle) and underlying bedding.(b) Close-up view of the Frutexites Bed locally composed of Type II and abundant Type III levels alternating with siltstone laminae.(c) Dark layer of Type I (arrowed) lining a cement-filled cavity in fore-reef slope succession at Dingo Gap.Sample site is shown in inset.(d) Layer of Frutexites-like microcolumns (within yellow dashed lines) overlying linked-columnar deep-water stromatolites (white outline) collectively filling Neptunian fissure in Pillara Limestone, South Behn platform.Lens cap 5 cm across in all photos.(e) Frutexiteslike microstructures (arrowed) growing cryptically from the surface of platy bioclasts (black outline) in calcite cementfilled cavities (yellow outline) in fore-reef slope breccias at the eastern entrance of Tunnel Creek.
Three polished thin sections (30 μm thick, 25 mm × 50 mm) were examined under bright-field light with a Nikon Eclipse 50i POL optical microscope.One vertically oriented thin section was produced from the Dingo Gap sample down the plug length.The two thin sections of the MWR sample were made from a horizontal slice through the top of the plug and a vertical cut down the middle of the plug.Images were acquired with a digital Nikon camera and processed using the auto-exposure image-capturing setting in the Nikon NIS-Elements® digital imaging software.Petrographic study focused on characterising the dendritic fabrics, identification of prokaryotic and/or eukaryotic features and their spatial association with bioclasts.

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I G U R E 3 Macro-and microscale CT images of collected samples.(a, b) Type I, Dingo Gap sample.(c-f) Type III, McWhae Ridge sample.(a) Sawn Dingo Gap sample face showing redcoloured dendrites forming a discrete layer (dotted lines).Cavity is further filled with calcite cements (CC).(b) Type I columns nucleate from a surface on internal sediments (dashed line) and grow perpendicular to this surface (arrowed).Columns are thickly grown at their base with common branching and grow into slender dendrites.(c) Sawn McWhae Ridge sample face showing browncoloured microcolumns forming discrete layers alternating with silt layers (within dashed lines).(d) Cross-section macro-CT scan showing internal alternation of Type III and silt (traced).Small volumes correspond to Type III columns, whereas larger volumes (circled) are dissolution enlarged voids most commonly developed at base of Frutexites Bed.(e) Top view of McWhae Ridge samples.(f) Top view macro-CT scan shows patchy distribution of Type III columns (dashed circled) and apparent alignment (dashed line) results from dissolution enlarged voids.

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I G U R E 4 Photomicrographs of Type I, Dingo Gap sample.(a) Type I growing from edge of cavity.Slender dendrites at base growing out to broader fan-like dendrites.XFM scanned area indicated by black rectangle.(b) XRF Synchrotron elemental maps of base dendrite.High amount of Mn overall, Fe concentrated at the base of column, moderate amounts of Co, Cr, Si, Al, V, Pb and Ti and minor amounts of Cu, Ni, Zn, Y, Ir and Pt in the Fe-enriched areas of dendrite.Fine-crystalline calcite cement primarily comprises Ca and minor amounts of Sr. (c) Well-developed column partially spheroidal, partially laminated (arrows) embedded in fine-crystalline calcite cement.Projections nucleating at extent of each branch (encircled).(d) Spheroidal texture at base of column, laminated middle section (rectangle), spheroidal projections at the end of column (encircled) leading to branching of column.(e) Dark reddish-brown Feoxide coating around chambers at base of column.(f) Close-up view from (e) of very fine particles along dark-reddish-brown laminae (arrows) in laminated section of the column.Light-brown blurry outline of chamber.(g) Fan-like dendrite with spheroidal texture.Large spheroids at base, and smaller spheroids towards the top of dendrite.(h) Close-up view from (g) showing an aggregation of spheroids (encircled).
cilli and filamentous bacteria, tubular bacterial sheaths as well as different types of extracellular polymeric substances (EPS) have been identified by SEM in these Frutexites-like microstructures.Type I is characterised by amorphous, smooth, planar EPS hosting a multitude of coccoidal bacteria (0.5-1 μm across) in local depressions up to 20 μm across (Figure 8a).EDS spot analyses reveal that the coccoid bacteria are composed of C, O, Ca, Fe and K with minor Mg and Cl.The EPS sheet overlies finely crystalline calcite (Figure 8a).Typical composition of the EPS is C, O, Ca, Fe, Al and Si.Filamentous EPS ~7 μm across and ~80-100 μm long overlies the planar EPS in between patches of coccoid bacteria and has almost the same elemental composition (with some Cl) as the planar EPS.
0.5 μm across and 1-2 μm long, whereas the long rods are up to 3 μm long and are composed of C, O, Ca, Fe, Mn, Al and K.The bacterial F I G U R E 5 Photomicrographs of Type II, McWhae Ridge sample.(a) Dendritic shrub nucleating from a recrystallised coral fossil (Cor) and growing perpendicular to its substrate.Some calcite-cemented voids in microcrystalline (micrite) cement (MC).XFM scanned area indicated by black rectangle.(b) XRF elemental maps of Fe-enriched shrub with moderate concentrations of Mn, Ni, Co, Si, Al and Cu, and minor concentrations of V, Ti, Cr, Zn, Pb and Mg.(c) Fan-shaped column with intense iron impregnation surrounded by micrite cement.(d) Closeup view of inset in (c) showing branch with micropeloidal texture where iron impregnation conceals internal detail.(e) Cross-section of a Type II dendrite.Internal content of chambers obscured by intense iron impregnation.(f) Closeup viewing of inset in (e).BSE image showing micrite cement dispersed within Type II dendrite cross-section (left), EDS elemental maps showing the distribution of Fe (centre) and Ca (right).Al, Mg, O and Si are present in moderate amounts within the Fe-enriched Type II chamber coating.sheaths contain Ca, O, Ca, Fe, Mn, Al and Si.The planar EPS typically consist of C, O, Ca, Fe, Mn, Al and Si with rare K and Ti signals.
Combined SEM-EDS and X-ray fluorescence microscopy images highlight major and trace elements along the Frutexites-like microstructures.Type I dendrites are primarily composed of C, O and high Mn concentrations.Fe is only highly concentrated at the base of the column (Figure 4b) and has a clear outline in contrast to Mn which has a more diffuse distribution along dendrites.Moderate amounts of Co, Cr, Si, Al, V, Pb and Ti (Figure 4b), as well as minor amounts of Cu, Ni, Zn, Y, Ir and Pt, appear in the Fe-enriched areas.These elements are especially enriched in Type I dendrites, whereas absent or present in quantities below the XFM detection limit in surrounding calcite cement (Figures S1 and S5a; Tables

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Photomicrographs of Type III, McWhae Ridge sample.(a) Type III columnar microstromatolites in a siltymicritic matrix (M).Column bases manifest along specific sedimentary horizons.In upper lamina: microstromatolite growing on Aulopora corallite (Aul) serving as anchoring substrate.XFM scanned area indicated by black rectangle.(b) Closeup view from A showing Girvanella (G) filaments within laminated internal of microstromatolite growing on Aulopora substrate.(c) Close-up view from (a) of the base of a branch showing calcified Girvanella tubes at the centre of the microstromatolite (encircled).Outer part of microstromatolite encrusted by Girvanella (G-Rothpletzella (R)-Wetheredella (W) assemblage.(d) Close-up view of dashed rectangle in (a).Left: RGB map (red = Fe, green = Mn, blue = Ti) showing elevated Fe values in coating around Girvanella-Rothpletzella-Wetheredella filaments, elevated Mn values for micrite within microstromatolites and Ti-enriched silt grains in silty-micrite matrix.Right: Elemental map of Ca highlighting the micritic matrix in between laminae of the microstromatolites and in the intercolumn matrix.The elemental composition of the Type III microstromatolites consists mainly of C, O, Fe and Mn.Within the microstromatolites, Ti, V, Cr and Zr are present in minor amounts (Figures S4 and S5c;Tables reported elevated concentrations for Ir, Pt, Ni, Co, As, Zn, Mo, V, Fe and Mn for their Frutexites Bed sample.Elemental mapping of the Frutexites Bed at McWhae Ridge has revealed different element concentrations among the dendritic Frutexites (Type II), columnar microstromatolites (Type III) and surrounding silty-micritic matrix with Fe, Ni, Co and Cu most abundant in Type II, and a broad suite of elements including Zn, V and Ni in the surrounding matrix.Iridium or platinum was not detected within Type II, Type III or the surrounding silty-micritic matrix, however, it is likely that they are below the detection limits (Figures 4c, 5c and 6d) based on the ppb concentrations previously published for the McWhae Ridge sample

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I G U R E 8 (a-d) SEM images of Type I, (e, f) Type II and (g, h) Type III.(a) Scattered coccoid bacteria (circled) in depressions on smooth amorphous sheets of extracellular polymeric substances (EPS).Planar (P-EPS), filamentous (F-EPS) and nervate type (arrow) of EPS overly finely crystalline calcite (FCC).Inset (rectangle) of patch of coccoid bacteria.(b) Chain of bacilli-type cells with distinguishable elongated cells (division between cells indicated by arrows) covered by a very thin bacterial sheath.(c) Chambers coated with very-fine-granular Fe-oxide.Inset (rectangle) of veryfine-granular Fe-oxide that consists of abundant nano-scale discs with apparent holes in their centre.(d) Chamber coated by closely packed spheres composed of Fe-Al silicate platelets.Inset (rectangle) of sphere of Fe-Al silicate platelets.(e) Internal view of a Type II chamber (outlined) surrounded by siltymicritic matrix.(f) Patchy distribution of bacilliform bacteria (encircled) on mineralised EPS sheet incorporating nanocrystals (NC).Collapsed bacterial sheaths embedded in planar EPS (arrows).(g) Columnar microstromatolite (edge outlined) surrounded by silty-micritic matrix (M) constructed by accumulation of entangled filaments.(h) Close-up view of G showing micrite-filled tube interior of Girvanella filament covered by Fe-Mn coating (traced).

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I G U R E 9 Microscale CT imagery of Types II and III, McWhae Ridge sample.(a) Type II shrub growing in crypt between two in situ corals (Cor 1 & 2).Sheet of tubular filaments (mixed Girvanella-Rothpletzella-Wetheredella assemblage) encrusts Cor 1. One-sided sheet encrustation by filamentous community and location of cryptic Type II in the shadow of Cor 1.(b) Type II and sheet of tubular filaments vaguely distinguishable with microscale CT.(c) Microscale CT scan of upper two-thirds of a vertical plug showing distribution of cup-shaped Aulopora corallites.(d) Field image of encrusting Aulopora corallites exposed on the top surface of the Frutexites Bed.Shape, dimensions and distribution of corallites in top surface match that of cup-shaped features in extracted from scanned plug.Inset image of cross-section plug in (c).(e) Two Aulopora corallites encrusted by filamentous Girvanella-Rothpletzella-Wetheredella community.Insets of cylindrical (left) and cup-shaped (right) corallites.
figure 10a) in deep-water stromatolites at McWhae Ridge.Other larger calcified tubular features intertwined with Girvanella are interpreted as Rothpletzella and Wetheredella.
1 0 Proposed growth models showing sequential phases for (a) cryptic Frutexites Type I in the Dingo Gap cavity fill; (b) cryptic Frutexites Type II associated with embedded in situ corals in the McWhae Ridge hardground; and (c) branching microstromatolites Type III on McWhae Ridge hardground.
their figure 7), Chafetz et al. (1998, their figure 5c,d) and Jakubowicz et al. (2014; their figure 5, Type 2 Frutexites).The Frutexites-like microstructures observed in cement-filled cavities at Tunnel Creek formed in a similar cryptic setting as Type I.
phase 7).The surrounding red-coloured fore-reef facies generally indicate Fe in seawater and the likely source of Fe, Mn and other trace elements in Frutexites Type I. Continental weath- cification and sediment agglutination in EPS associated with biofilm clusters.It is also similar to the development of Ephiphyton as discussed by Zhang et al. (2019) to have formed by successive growth and calcification of coccoid colonies along branches.Just like Types I and II discussed in this study, other dendritic calcimicrobes Renalcis and Epiphyton have been linked to calcified coccoid and rod-shaped bacteria with their overall morphology controlled by mineralisation of EPS (Pratt, 1984; Stephens & Sumner, 2002; Zhang et al., 2019).
characterised by red pigmentation related to iron-encrusted filaments.This EPS mineralisation can happen in vivo or post mortem and results in the early fossilisation of these filaments thus lithifying the base of the microstromatolite.It is difficult to determine at what rate EPS mineralisation processes happened, however, the well-preserved rounded character of the filaments suggests very soon after death and before tubular flattening caused by sediment accumulation.The branching microstromatolite structures observed on top of linked columnar deep-water stromatolites in the South Behn platform Neptunian dyke have a similar shape and dimension as Type III. 6 | CON CLUS ION In this study, three different Frutexites-like microstructures, Types I-III, have been distinguished based on their branching morphology and their composition.These Frutexites-like microstructures were sampled from well exposed and dated Early Famennian fore-reef slope successions at McWhae Ridge and Dingo Gap, Lennard Shelf and northern Canning Basin.Through observations using conventional microscopy and high-resolution imaging techniques, combined SEM-EDS, macro-and micro-CT and XFM beamline at the Australian Synchrotron, the characteristics and genesis of these Frutexites-like microstructures were established.• Type I grew as long and slender dendrites on the edges of a cavity fill within the fore-reef slope facies at Dingo Gap.It was constructed by predominantly spherical coccoid bacteria, with some filamentous bacteria embedded in EPS.They have laminated to spheroidal internal textures characterised by heavy iron impregnation.Two types of Fe-precipitates coated the chambered dendrites: a very-fine-granular haematite iron-oxide; and spherical radiating clusters of Fe-Al silicates platelets.•Type II formed in a small cryptic space between two in situ corals embedded in a hardground section at McWhae Ridge.The broad dendritic shrubs nucleated from a skeletal substrate and grew outward into the crypt.Type II is composed primarily of short and elongate bacilliform bacteria with some filamentous bacteria embedded in an amorphous EPS sheet.This permineralised EPS has a granular appearance due to the incorporation of nanocrystals.•Type III grew as columnar microstromatolites on a hardground in low energy, toe-of-slope conditions at McWhae Ridge.A mature column is typically enlarged upwards with limited branching.The column consists of a mixed Girvanella-Rothpletzella-Wetheredella assemblage in which the tubular filaments grew together.These filaments are up to two orders of magnitude larger than filaments observed in other Frutexites Types.Each filament has a Fe-Mn-Al-Si coating.

Frutexites
was likely constructed by non-phototrophic, Fe-and Mnoxidising coccoidal, bacilliform and filamentous bacteria.Further metagenomic studies could be valuable to reveal the bacterial phyla involved.In contrast, Type III, although morphologically very similar, is represented by microstromatolite columns composed of other calcimicrobes.This study has shown that caution is required when identifying Frutexites based on macroscopic observations only.We recommend applying high-resolution methods such as those used in this study as a basic suite of techniques to investigate morphology, composition and growth style of Frutexites and by analogy other calcimicrobes such as Girvanella and Renalcis.These higher resolution data provide insight and improved understanding of the environmental conditions in which different calcimicrobes grew and potentially substantiate their use as palaeoenvironmental indicators.