Possible poriferan body fossils in early Neoproterozoic microbial reefs

Molecular phylogeny indicates that metazoans (animals) emerged early in the Neoproterozoic era1, but physical evidence is lacking. The search for animal fossils from the Proterozoic eon is hampered by uncertainty about what physical characteristics to expect. Sponges are the most basic known animal type2,3; it is possible that body fossils of hitherto-undiscovered Proterozoic metazoans might resemble aspect(s) of Phanerozoic fossil sponges. Vermiform microstructure4,5, a complex petrographic feature in Phanerozoic reefal and microbial carbonates, is now known to be the body fossil of nonspicular keratosan demosponges6–10. This Article presents petrographically identical vermiform microstructure from approximately 890-million-year-old reefs. The millimetric-to-centimetric vermiform-microstructured organism lived only on, in and immediately beside reefs built by calcifying cyanobacteria (photosynthesizers), and occupied microniches in which these calcimicrobes could not live. If vermiform microstructure is in fact the fossilized tissue of keratose sponges, the material described here would represent the oldest body-fossil evidence of animals known to date, and would provide the first physical evidence that animals emerged before the Neoproterozoic oxygenation event and survived through the glacial episodes of the Cryogenian period.

Molecular phylogeny indicates that metazoans (animals) emerged early in the Neoproterozoic era 1 , but physical evidence is lacking. The search for animal fossils from the Proterozoic eon is hampered by uncertainty about what physical characteristics to expect. Sponges are the most basic known animal type 2,3 ; it is possible that body fossils of hitherto-undiscovered Proterozoic metazoans might resemble aspect(s) of Phanerozoic fossil sponges. Vermiform microstructure 4,5 , a complex petrographic feature in Phanerozoic reefal and microbial carbonates, is now known to be the body fossil of nonspicular keratosan demosponges [6][7][8][9][10] . This Article presents petrographically identical vermiform microstructure from approximately 890-million-year-old reefs. The millimetric-to-centimetric vermiform-microstructured organism lived only on, in and immediately beside reefs built by calcifying cyanobacteria (photosynthesizers), and occupied microniches in which these calcimicrobes could not live. If vermiform microstructure is in fact the fossilized tissue of keratose sponges, the material described here would represent the oldest body-fossil evidence of animals known to date, and would provide the first physical evidence that animals emerged before the Neoproterozoic oxygenation event and survived through the glacial episodes of the Cryogenian period.
Benthic microbial structures (stromatolites and other microbialites) provide conspicuous evidence of pre-Phanerozoic life, but are difficult to understand because they rarely preserve recognizable evidence of the organisms involved. Stromatolitologists have struggled for over a century to decipher their microscopic laminae and clots, which are assumed to have been produced or influenced by in vivo and/or post-mortem biogeochemical activity, and to formalize the 'taxonomy' of their morphology and microstructure 5,11 .
The existence of metazoans by the Ediacaran period (the last period of the Neoproterozoic) is indicated by bilaterian 'body' and trace fossils 12 , and geochemical evidence (biomarkers) 13 provides disputed 14,15 , indirect evidence for Cryogenian poriferans. The search for definitive physical evidence of pre-Cryogenian metazoans is confounded by uncertainty about what to look for, but preserved physical evidence should be small, subtle and possibly altogether unfamiliar. Given that sponges are the most basic of known animals 2,3 , physical evidence of Neoproterozoic sponges could be sought, but effort focused on the characteristics of mineralized sponge skeletons (siliceous or calcareous spicules) [16][17][18] overlooks sponges with only proteinaceous (spongin or keratin 19,20 ) skeletons. Early metazoan evidence might instead resemble taphonomic (preservational) products of sponge soft tissue 21-23 rather than mineralized sponge skeletal components. Although molecular clock data suggest that sponges emerged in the early Neoproterozoic 1 , the oldest undisputed sponge body fossils are from the Cambrian period 15 .
Recent work 6,7 has shown that vermiform microstructure 4,5 -an unusual microscopic feature in Phanerozoic reefs and stromatolites that was initially interpreted as being related to algae 4 or protozoans 24,25 -is instead a keratose sponge body fossil comprising complexly anastomosing cement-filled microtubules enclosed in carbonate microspar. It is produced taphonomically 6,10 in nonspicular keratose demosponges through post-mortem calcification of soft tissue to produce carbonate microspar (automicrite), which surrounds the tough spongin fibres of the 'skeleton' of the sponge. Decay of the spongin then produces a network of complexly anastomosing tubular moulds that eventually become passively filled with sparry calcite cement. Although questioned 26 , the association between vermiform microstructure and sponges has been confirmed in undisputed body fossils of Phanerozoic sponges 7 . Three-dimensional reconstruction of vermiform microstructure has shown that tubule shape and branching configuration are too consistent and complex to be abiogenic (for example, compacted peloids), do not resemble the branching style of other possible organism types (microbial or fungal) 6 and are identical to the spongin meshworks of keratose sponges 6 . Although the existence of Proterozoic vermiform microstructure has been predicted 6,10,27 , published examples are rare 28,29 and difficult to understand.
The calcification of decaying sponge soft tissue has been documented in modern sponges 21,22 , and produces sponge 'mummies' 22 as well as a range of subtle carbonate sedimentary textures (such as peloid clusters) in living and Phanerozoic fossil sponges 7,9,21,22 . Taphonomic sediment textures (polymuds) that may be poriferan-related have previously been identified in the reefs that are the subject of this Article 23 . Article transmitted light) from the approximately 890-million-year-old (Ma) 30,31 Little Dal reefs (Stone Knife Formation 32 , northwestern Canada) (Fig. 1a,  b). These large (about 500 m in thickness, and kilometres in diameter) microbial reefs [33][34][35][36] were built mainly by variably preserved calcimicrobes that have been interpreted as filamentous cyanobacteria (photosynthesizers) 33,35,36 , and developed palaeotopographic relief of up to about 100 m above the surrounding subphotic, level-bottom carbonate-mud seafloor. Reef framework, which is generally not discernible in natural exposures, was documented from slabbed hand samples and thin sections 35 .
The reefs grew in five stages (Fig. 1c), each with different microbialite morphologies: anastomosing millimetre-to-centimetre-scale masses with no consistent shape (stages I-III); centimetre-scale anastomosing columns and digits (stages II, III and V); and steep sheet-like masses at a scale of decimetres to 10 m (stage IV). Stage V includes cement-rich to micritic, domical, turbinate and columnar stromatolites that generally lack calcimicrobes, with associated ooids and stromaclasts. Microbialites of stage I to IV grew predominantly in moderate-energy, illuminated palaeoenvironments 33,35,36 , but stage V records a shallow-water, high-energy environment. The microstructure that forms most of the reefal microbialites (especially in stages III and IV) comprises filaments that are about 10 μm in diameter, separated by 10-100-μm masses of marine cement that probably represents the calcified sheath polysaccharide of cyanobacteria 33,36 that was permineralized during microbialite growth. The reef framework, consisting predominantly of this microstructure and its taphonomically degraded equivalents 36 , defines primary void networks (millimetres to centimetres in size) that are commonly floored with geopetal carbonate mud and lined by isopachous, fibrous marine calcite cement. The relative timing of void-filling by marine cement precipitation versus geopetal sediment accumulation is variable, attesting to the very early timing of marine cement precipitation.

Results
Vermiform microstructure in samples from stages II, III and V of the Little Dal reefs (Fig. 1c) is identifiable only in rare thin sections, in which it forms millimetre-to-centimetre-scale masses of anastomosing tubes that are filled with calcite spar and surrounded by calcite microspar groundmass (Fig. 2a, b). The approximately 20-30-μm-wide tubules have complex, divergent branching and rejoining at a spacing of about 30-100 μm, form very irregular three-dimensional polygonal meshworks, are defined by enclosing microspar, lack walls, and are filled with clear, equant calcite crystals up to 20 μm wide (Fig. 2b). The homogeneous microspar groundmass that encloses the tubules comprises cloudy, equant, interlocking calcite crystals of about 2-8 μm wide, and differs texturally and compositionally from other fine-grained reefal carbonate in its uniform crystal size, lack of sedimentary texture, and dearth of detrital silicate impurities. Vermiform microstructure preservation is good to barely discernible.
Vermiform microstructure is present in three microfacies (i, ii and iii (the last divided into iiia and iiib subsets)), representing three palaeoenvironments (Figs. 2, 3). It is not present in calcimicrobe-dominated stromatolites or level-bottom carbonate mudstone that is distal to reefs.
In microfacies i (Fig. 2d, e, g, j, Extended Data Figs. 1, 2), vermiform microstructure is intercalated with carbonate mud (with or without larger reef-derived clasts and terrigenous impurities) in (1) synsedimentary debris flanking reef stages II and III, and (2) millimetre-to-metre-scale palaeodepressions on reef growth surfaces (stages II and III). It locally encrusts sides of reef-framework clasts in sediment, extends into crevices, and occupies shelter porosity under clasts (Fig. 2e).

Discussion
The shape, size, branching style and polygonal meshworks of the Little Dal vermiform tubules closely resemble both spongin fibre networks of modern keratosan sponges ( Fig. 2a-c) and vermiform microstructure either demonstrated or interpreted to be sponge-derived in diverse Phanerozoic microbial, reefal and non-reefal carbonate rocks [6][7][8]10,24,25,27,37 . The compositional and textural homogeneity of the microspar groundmass supports an origin through permineralization of a pre-existing biological substance 9 , rather than incremental accumulation of detrital sediment or microbial carbonate that passively incorporated complexly anastomosing tubular microfossils. Variable preservation and association with geopetal peloid accumulations are familiar aspects of Phanerozoic sponge taphonomy 9,21,22,38 . In previous work, detailed comparison of the three-dimensional characteristics of vermiform microstructure with branching cylindrical organism types yielded no convincing alternative to the sponge interpretation 6 .
The preference of Little Dal vermiform microstructure for environments that were not inhabited by photosynthetic calcimicrobes (reef flanks, depressions on active reef growth surface, and framework and shelter voids), versus its absence from filamentous calcimicrobial reef-framework components, suggests that (1) illumination may not have been a requirement and (2) the organism may have been unable to compete with reef-building photosynthesizers that grew and/or calcified rapidly. The interlayering of vermiform microstructure with calcimicrobe-free microbialite (microfacies iiib) in the high-energy, well-illuminated reef surfaces of reef stage V supports the

Article
interpretation that the vermiform-microstructured organism was not capable of competing with reef-building filamentous cyanobacteria, but instead occupied niches in which the filamentous calcimicrobes did not live owing to (1) poor illumination or (2) high hydrodynamic energy. The occupation of cryptic microniches (shelter and reef framework voids) by sponges (for example, microfacies i and ii), is well known in the Phanerozoic 21,37,39,40 . The obligatory spatial association of vermiform microstructure with reefs built by oxygen-producing cyanobacteria may indirectly support a metazoan interpretation. Prior to the Neoproterozoic oxygenation event, marine dissolved oxygen was probably low 41 except perhaps in the vicinity of photosynthesizing microbial communities; the metabolic requirements of metazoans may have limited early animals to localized, comparatively well-oxygenated (for the time) environments (oxygen 'oases'). Given the approximately 890-Ma depositional age 30,31 , the vermiform-microstructured Little Dal organism may been tolerant of 'low' oxygen (that is, relative to modern levels), which is a characteristic of some modern and fossil sponges 42 .
If the vermiform-microstructured masses in the Little Dal reefs are accepted as early sponge body fossils, their approximately 890-Ma age would imply that (1) the evolutionary emergence of metazoans was decoupled from the Neoproterozoic oxygenation event [41][42][43][44][45] and (2) early animal life was not catastrophically affected by the Neoproterozoic glacial episodes. If the Little Dal objects are truly sponge body fossils, they are older than the next-youngest undisputed sponge body fossils (Cambrian) 15 by approximately 350 million years.
It would not be surprising to find that the earliest sponges were reef-dwellers; the history of Phanerozoic reefs is rich with reef-building and reef-dwelling sponges 46 . If the masses of vermiform microstructure in the Little Dal reefs were to be accepted as an early Neoproterozoic expression of sponge tissue preservation, their age and proposed identity would be compatible with (1) evidence that the opisthokont (animal + fungus) clade was already established by the time of the Mesoproterozoic-Neoproterozoic transition 47,48 , (2) possible evidence of 1-billion-year-old multicellular holozoans 49 , (3) molecular clock estimates for the emergence of the Porifera in the early Neoproterozoic 1 and (4) a revised taxonomy of nonspiculate keratose sponges showing that they are a sister group to other demosponges 19 . The Little Dal vermiform microstructure is perhaps exactly what should be expected of the earliest metazoan body fossils: preservation through post-mortem calcification of sponge-grade soft tissue in the decaying bodies of small, shapeless, sessile, epibenthic and cryptic animals most closely affiliated with keratose sponges.

Online content
Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-021-03773-z.

Fig. 3 | Palaeoenvironments occupied by the Little Dal vermiform microstructure interpreted as possible body fossils of keratose sponges.
The organism lived (i) on poorly illuminated to non-illuminated carbonate mud surfaces in depressions on the reef surface and on debris aprons mantling reef flanks; (ii) in voids produced by the growth of the complex microbial framework of the reef; and (iii) interlayered with non-calcimicrobial microbialites (cement-rich and muddy-laminated stromatolites in high-energy reef-capping phase V; irregularly muddy-laminated to clotted microbialites in moderate-energy environments of reef stages II and III). Scale bars, 5 mm.

Methods
Field work was done on foot from two-person, backpacking-style camps placed at sites that are accessible only by helicopter. Samples were collected at various times between 1992 and 2018, under all required permits. Recording sample locations using GPS is not possible for most sites owing to the extreme topography of the exposures' cliffs, pinnacles and canyons, and so sample location was documented using photographs and sketches. Several samples are from a mineral-exploration drill-core stored on-site in the field. Owing to the homogeneous grey weathering of reef surfaces, lithofacies cannot be identified in the field. Instead, hand samples were collected and later slabbed and thin-sectioned. Vermiform microstructure was identified in a small proportion of the samples collected. Repeat visits focused primarily on resampling the rare areas in which vermiform microstructure had been identified. Standard 30-μm-thick thin sections were examined in plane-polarized transmitted light using a Nikon C-Pol binocular microscope fitted with digital camera and Luminera Infinity Analyze software (for lower-magnification images) and an Olympus BX-51 petrographic microscope equipped with Q-Imaging digital capture system (for higher-magnification images).

Reporting summary
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Data availability
All relevant data are contained with the Article and its Supplementary Information, or are available from the author upon reasonable request. The exact sample size (n) for each experimental group/condition, given as a discrete number and unit of measurement A statement on whether measurements were taken from distinct samples or whether the same sample was measured repeatedly The statistical test(s) used AND whether they are one-or two-sided Only common tests should be described solely by name; describe more complex techniques in the Methods section.
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Software and code
Policy information about availability of computer code Data collection Photomicrographs were imaged using Luminera Infinity Analyze v.6.5 and Qcapture 2.98.0.

Data analysis
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Data
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Study description
Describes petrographic evidence of possible body fossils of sponges that are approximately 890 million years old.

Research sample
Thin sections (30-micon-thick rock slices) from fossil reef rock in the Stone Knife Formation ("Little Dal reefs") in Northwest Territories, Canada.

Sampling strategy
Over a thousand approximately fist-sized rock samples were separated from natural rock exposures using a rock hammer. The samples were later slabbed and thin-sectioned. Initially, samples had been collected for an unrelated purpose (documenting reef microbialites in a separate, published study); sample distribution was randomly dispersed throughout all exposed parts of selected reefs in an attempt capture spatial variability in microbialites, which are not discernible on natural rock exposures. Sample field locations were documented using photographs and diagrams; obtaining accurate GPS points for sample locations is not possible given the extreme topography (limits satellite access) and the small size of the samples relative to GPS error. Areas where the thin sections contained vermiform microstructure were later revisited and resampled. Sample size is considerably larger than the masses of vermiform microstructure. Sample distribution in the reefs is dense enough and reef facies well enough understood (previously published study) for the reefal subenvironments in which vermiform microstructure is preserved to be characterised.

Data collection
Rock samples were separated from natural rock exposures using a rock hammer. Samples were shipped to the lab, sawed, polished, and thin-sectioned using standard petrographic preparation.
Timing and spatial scale Rock samples were collected during field work between 1992 and 2018. Sample size is considerably larger than the masses of vermiform microstructure that are the subject of the study.

Data exclusions
No data were excluded.

Reproducibility
Reproducibility depends on locating the exact field locations and places on exposure surfaces from which samples containing vermiform microstructure were collected. Field locations of rock samples were recorded in detail using photographs and diagrams.
Revisiting and resampling these locations in the years following the initial collection successfully yielded more material containing vermiform microstructure in thin section.

Randomization
This was not an experimental study.

Blinding
This was not an experimental study.
Did the study involve field work?

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Field work, collection and transport

Field conditions
Remote alpine-subarctic locations in the Mackenzie Mountains, Northwest Territories, Canada, that are accessible only by helicopter. Field work is possible only in the summer months when snow cover is minimal (mid-June to mid-August).
Access and import/export Field work was conducted under science licencing from the Aurora Research Institute (2016 Licence #15888; 2017, 2018 Licence #15993), and associated permissions from land use, water board, renewable resource, community, land claim, band council, and hunting-trapping organisations.

Disturbance
No temporary nor long-term disturbances were introduced.

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