The Zuun-Arts biota is preserved in the Zuun-Arts region of Zavkhan Province, between the cities of Uliastai to the north and Altai to the south. The fossils are preserved in a thinly bedded black shale facies that is slightly less than 7 m thick (Fig. 1). The fossiliferous zone is in strata from 40 cm to 80 cm within the black shale (Fig. 1). Petrographic thin section analysis shows the dominant microfacies to be an unmetamorphosed thinly laminated quartzose siltstone (Supplementary Fig. 1). The fossils are found between two distinctive facies: a lower carbonate containing conspicuous stromatolites and an upper thin bed of cherty phosphorite (Fig. 1). These bounding facies are correlative to stratigraphic units 9 and 10 of Khomentovsky and Gibsher16 at the Ediacaran–Cambrian boundary section at Bayan Gol in adjacent Gobi-Altai Province. The biota is therefore at the base of the Zuun-Arts Formation13,14,17 in what appears to be a transgressive lag deposit. Recent sequence stratigraphic, chemostratigraphic and biostratigraphic work confirms an upper Ediacaran age for these strata13,14,17.

This biota’s age relationship to the diverse Lantian and Miaohe biotas remains unclear and it is not currently possible to determine its absolute age. The oldest convincing Ediacaran trace fossils date to about 555 Ma18 and this biota occurs below the earliest trace fossils in this region13. This may mean that it is slightly older than 555 Ma, which would put it within the younger age estimates for the Miaohe biota11.

Chinggiskhaania bifurcata has thin filaments lacking transverse longitudinal divisions or ornamentation that gently curl and rarely branch (Fig. 2a). The filaments have fine lengthwise lineations that are not always well preserved. Mean filament width is 0.47 mm (N = 100) and ranges from 0.23 mm to 0.76 mm (STDEV = 0.12 mm). There does not appear to be any consistent distal tapering in filament width. Some filaments have fluctuating widths along their length, suggestive of twisting deformation. The filament branching angle ranges from 43° to 85° with a mean of 63° and standard deviation of 15° (N = 7). Filament fragments are the most common mode of preservation and they are innumerable in the fossiliferous zone of the strata. One well-preserved specimen contains just four filaments, showing that the filaments are not densely grouped (Fig. 2b). Two specimens preserve details of the basal region of the organism, showing a narrow attachment area below a stem of tightly gathered filaments (Fig. 2d,e). Based on the branching filaments, thallus-like morphology and basal attachment structures that resemble a stipe and holdfast, C. bifurcata is interpreted as a multicellular benthic alga. C. bifurcata is most comparable to Doushantuophyton from the Miaohe biota and Huangshanophyton from the Lantian biota6,9. It differs significantly from these genera in that its filaments are not as densely assembled in the thallus as either genus, they do not branch as commonly as in Doushantuophyton and they lack the septation of the filaments of Huangshanophyton6,9.

Figure 2 Photographs and photomicrographs of exceptionally preserved multicellular algae fossils from Zuun-Arts biota. (a–h) Chinggiskhaania bifurcata. Photographs taken under cross-polarized light. Scale bars = 5 mm. (a) Specimen showing characteristic thin, rarely branching filaments (IEZAB0002). (b) Specimen of a thallus containing four filaments, one branching, that converge toward the lower right of the photograph (holotype; IEZAB0001). Base of specimen is poorly preserved. (c) Fragmentary filaments preserved in a bundle (IEZAB0003). (d) Basal portion of a thallus showing stipe comprised of closely grouped filaments above a narrow holdfast (IEZAB0004). (e) Another specimen of basal portion of a thallus from the same slab as 2d showing stipe and holdfast (IEZAB0004). (f–h) Typical branching of filaments (IEZAB0002, IEZAB0003 and IEZAB0002, respectively). (i,j) Zuunartsphyton delicatum. Photomicrographs taken under normal light. Scale bars = 1 mm. (i) Larger specimen showing shrub-like thallus with curly filaments (holotype; IEZAB0007). (j) Smaller specimen also showing shrub-like thallus with curly filaments (IEZAB0008). Full size image

The other new species of probable multicellular benthic algae in this biota, Zuunartsphyton delicatum n. gen., n. sp., is known from three individual specimens. It has a small shrub-like morphology, less than 3 mm in diameter, composed of thin tightly curling filaments (< 0.1 mm wide) that do not branch and lack transverse longitudinal divisions or ornamentation (Fig. 2i,j). Its attachment structures are unknown. Z. delicatum is interpreted as a multicellular benthic algae species because of its thallus-like morphology composed of thin filaments. This species is not closely comparable to any algal taxa from the Miaohe or Lantian biotas.

SEM-EDS analysis of two C. bifurcata filaments shows consistent high concentrations of Al and Si relative to other elements, although Si is not enriched relative to the matrix (Fig. 3). C is also locally concentrated in portions of the specimens, but is not preferentially associated with any morphological feature of the fossils (Fig. 3). These results are consistent with preservation as aluminosilicate clay mineral films with some carbon present. The fossils were likely originally preserved as carbon films and were diagenetically altered to aluminosilicate minerals. Small areas of high Fe concentration also occur in one specimen in the same zones as high C concentration (Fig. 3a). SEM examination of these zones reveals framboidal minerals consistent with pyrite (Supplementary Fig. 2). This pyrite likely precipitated as a result of the sulfate reduction during the decay of the filaments. Similar pyrite framboid concentrations are known from Lantian and Miaohe biota fossils10. These SEM-EDS results are comparable to the fossil assemblages of the middle Cambrian Burgess Shale biota, which are also preserved as aluminosilicate clay mineral films19. This preservational style contrasts with the Lantian and Miaohe biotas and most Cambrian Burgess Shale-type deposits, which primarily preserve fossils as carbon films10,20.