Diverse Assemblage of Ediacaran fossils from Central Iran

Reinvestigation of the Kushk and Chahmir areas (Bafq and Behabad regions) of central Iran has yielded a diverse assemblage of Ediacaran fossils, including several new species, just prior to the Cambrian explosion of complex animals. The Kushk series consists mainly of shallow marine carbonate deposits followed by deep-water calcareous marine shales. Ediacaran fossils occur commonly in the shale deposits and include biostratigraphically-important taxa Cloudina and Corumbella, which confirms a latest Ediacaran age for these deposits, the youngest examples of Kimberellomorphs (stem-group molluscs) that helps bridge the gap between their first occurrence in the middle-Ediacaran and the crown diversification in the Cambrian, and likely sponges, which are rare prior to the Cambrian.


Geological Setting
The Kushk Series outcrops widely along the western margin of the Lut Block and the eastern margin of the Central Iranian Yazd Block. Sediments accumulated in the Zarigan-Chahmir Basin, which is bounded by the Kuhbanan Fault to the east and the Posht-e-Badam Fault to the west 22,23 . The Kushk Series reaches a maximum thickness of ~520 m in our sections, although the lower boundary is not exposed and the upper boundary is faulted with the Hashem Formation (Lower Cambrian). The Kushk Series comprises 13 sub-units consisting of volcanic rhyolites, microdiorites, tuffs, dolomitic limestones, dolomite, shales, and sandstones ( Fig. 1b) interpreted as a deep, open-marine facies shallowing upwards into a carbonate platform 24 resulting from a transgression-regression sequence. Of interest to this study is the fossiliferous sub-unit 6 (30-35 m), which is exposed in multiple areas (Kushk mine site, Chahgaz, Wedge, Chahmir, and Darehdehu). It is comprised of grey, thin-bedded argillaceous shales (containing trace fossils) with intercalation of grey, medium-bedded sandstones and massive sulfide mineralization containing pyritic intercalations, and green tuffs. These grey argillaceous shales can often weather to green (as seen in much of the study area). In the Wedge locality, sub-unit 6 is composed of grey argillaceous shales with intercalation of sandstones, tuff and dolomitic tuffs, while in the Chahmir area it consists of grey, thin-bedded siltstone and argillaceous shales with intercalation of tuff and very thin-bedded mudstone. This sub-unit can be correlated with the Lower Shale Member (Chopoqlu Shale) of the Soltanieh Formation 25 in northern Iran. The extensive accumulations of volcanic material and tuffs, combined with the rapid perceived deepening of the platform, suggests that the Kushk basin may have represented an extensional rift basin related to opening of the Proto-Paleotethys sea 24 in northeastern Gondwana. Geochronological constraints in the region are limited, with four 207 Pb/ 206 Pb dates of 595, 690 and 715 Ma (all ± 120 Ma) 26 and 581 ± 8.6 Ma 27 from the Kushk area, and a single 207 Pb/ 206 Pb date of 540.7 ± 4.8 Ma 27 from the Chahmir area. Preliminary sampling of ash beds was unfortunately inconclusive; however, the discovery of classic Precambrian (Chuaria Fig. 2a) and terminal-Ediacaran index fossils (Cloudina, and Corumbella; Fig. 2b,c) support Hamdi and Jiang Zhiwen 28 in assigning a latest Ediacaran (to earliest Cambrian) age. Of the three index fossils, Chuaria (n = 1) spans the greatest temporal interval (Tonian to Ediacaran) however it remains restricted to the Precambrian. The lightly-calcifying metazoan Cloudina (n = 37; Fig. 2b) is represented in these sections by external molds morphologically identical to Cloudina molds from Namibia 29 . Given the taxonomic difficulty of assigning species to dissolution impressions, the range of "cloudinids" is latest Ediacaran 30 to possibly the basal Cambrian 31 . Finally, Corumbella (n = 108, Fig. 2c) is, to the best of our knowledge, restricted to the terminal Ediacaran 32,33 . As such, the only likely timeframe for the overlap of these three index fossils is latest Ediacaran.

Results
Our new field sites represent the most diverse assemblage of late Ediacaran (Nama-Assemblage ~545-541 Ma) fossils known 6,10,12 , and includes the previously endemic Ediacaran fossil Persimedusites chahgazensis (n = 41, Fig. 2d;), which has been subsequently described (albeit identified as "Ediacaran discs") from Argentina 34 . In addition to Cloudina, and Corumbella, Ediacaran fossils include the recently described Kuckaraukia 35   different sediment heights are found within the impression, implying differences in tissue lability and structural rigidity. Kimberella persii n.sp. shares the implied tissue differentiation and segmentation typical of Kimberella quadrata, however lacks the organic frill that outlines the periphery of the implied organic dorsal shell 36 (Fig. 3). These characters suggest a bilaterian (stem mollusc) affinity 36   of a small, goblet-to oval-shaped form preserved in negative epirelief with distinct rows of round protrusions, which originally represented external openings later infilled with sediment 37,38 . The overall columnar shape with distinct incurrent pores is suggestive of a poriferan-grade organism capable of effective filter feeding, however in the absence of siliceous or carbonaceous spicules, it is difficult to assign this species to an existing clade.

Discussion
Recent studies of late-Ediacaran sections from Namibia 10,11 and southwestern USA 8, 14 have demonstrated a distinctly depauperate global diversity of classic Ediacara biota (consisting almost exclusively of Erniettomorpha and Rangeomorpha), combined with an increase in diversity and abundance of mineralizing and organic tubular organisms 17 . The discovery of a high-diversity terminal Ediacaran population from central Iran is pivotal in assessing the nature of the end-Ediacaran extinction, in addition to a possible post White-Sea extinction of Ediacara biota 10,12 .
What is most striking about this end-Ediacaran assemblage is that it consists almost exclusively of animals. The rare and diminutive (in size) Ernitettomorpha (Fig. 2g) and Rangeomorpha (Fig. 2h,i) in this assemblage are dwarfed by the abundance of metazoans such as Corumbella 32 , Cloudina 39 , Kimberella, and Gibbavasis. The discovery of Kimberella persii extends the range of this group up to the Ediacaran-Cambrian boundary, filling the stratigraphic gap between the diverse White-Sea assemblage 6 and the diverse crown molluscs known from many Cambrian Lagerstätte 40,41 . As such, the terminal Ediacaran of Iran showcases a changing Ediacaran ecosystem with a thriving metazoan community.

Systematic Paleontology. Phylum Mollusca Linnaeus, 1758
Genus Kimberella Glaessner and Wade 1966 Kimberella persii n.sp. (Figs 2j, 3) Type species. Kimberella quadata Glaessner and Wade 1966. Description. Small (9-25 mm in length and 5-15 mm in width) oval (rarely cylindrical) fossil with anterior-posterior differentiation and prominent central segmented ridge (7-25 mm) consisting of 7-25 segments along the major axis. Adjacent to the internal segmented ridge are two parallel-sided smooth furrows 0.5-2 mm in width, surrounded by a slightly elevated and segmented 1-6 mm outer margin that strongly terminates at a distinct marginal rim.
Occurrence. Kimberella is known from the Ediacara Hills, Flinders Ranges (South Australia) 42   Diagnosis. Cylindrical to vase-shaped fossils with prominent positive nubs evenly spaced along parallel tracts around circumference.
Description. Small (4-14 mm long, 2-7 mm wide) vase-shaped, negative-relief impression consisting of regularly spaced positive relief and hemispherical bumps that run longitudinally as seven to nine rows across the width of the vase. Rows are also apparent latitudinally, resulting in a lattice-like appearance.
Remarks. Prominent positive-relief bosses likely represent a taphonomic inversion of an originally negative pit infilled with sediment. Pits may have served as an entryway for water currents. Specimens similar in construciton to the terminal Ediacaran fossil Ausia (from the Nama Assemblage), however Gibbavasis is noticeably smaller and vase-shaped rather than fan-shaped. The affinities of Ausia are unknown.
Locality. From gray argillaceous shales (sub-unit 6) of the Kushk Series in the Kushk area (Chahgaz (Dargazin) locale in the Bafq region and Chahmir area in the Behabad region, Central Iran.

Methods
More than 250 Ediacaran specimens were collected (in situ and also from float) from the bed tops of the argillaceous shales (sub-unit 6; Fig. 1b) of the Kushk Series in Kushk area (Chahgaz (Dargazin) and Wedge locales in the Bafq region, and Chahmir and Darehdehu areas in the Behabad region, Central Iran. 95 specimens from the Kushk area and 150 specimens from the Chahmir area were investigated and identified. All specimens were washed and photographed, after which detailed morphological data were collected for all specimens.