1190


Journal of Paleontology 91(6):1178–1198


trail, namely Helminthoidichnites tenuis and Helminthopsis tenuis, whereas the second group consists of the grazing trail Cochlichnus anguineus, and the bilobate trail Cruziana isp. The latter occurs on the same bed as the branching burrow Treptichnus pedum. The Upper Shale Member contains fifteen ichnospecies, including Cochlichnus anguineus, Cruziana pro- blematica, Curvolithus isp., Didymaulichnus miettensis, Gordia marina, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, Phycodes isp., Planolites montanus, Psammichnites gigas, Rusophycus avalonensis, Treptichnus pedum, Treptichnus pollardi, and Treptichnus isp. Our study indicates that four distinctive trace-fossil zones


can be recognized in the Soltanieh Formation, all belonging to the early Cambrian (Fig. 6). Ichnozone 1 characterizes the middle interval of the Lower Shale Member (157–171m above its base). This ichnozone is of low ichnodiversity, containing only three ichnotaxa of simple horizontal trails, namely Hel- minthoidichnites tenuis, Helminthopsis tenuis, and Cochlichnus anguineus, collected 157m above the base of the Lower Shale Member (Fig. 7). A wrinkled surface associated with these specimens suggests the presence of microbial mats (Fig. 2.1), which allowed the superb preservation of these delicate struc- tures. Cochlichnus anguineus, Helminthoidichnites tenuis, and Helminthopsis tenuis commonly occur in direct association with the microbial mat and record microbial grazing as one of the most widespread feeding strategies across the Ediacaran- Cambrian boundary (Buatois et al., 2014). Although this ichnofauna shows similarities with that of the


Ediacaran (Fedonkin, 1985; Narbonne et al., 1987; Jensen, 1997; MacNaughton and Narbonne, 1999; Gehling et al., 2005;


Buatois and Mángano, 2011; Mángano et al., 2012), the presence of the Anabarites trisulcatus-Protohertzina anabarica Zone (Hamdi et al., 1989) in the dolomites below and above the Lower Shale indicates a Fortunian age. Also, uncontroversial exmples of Cochlichnus have not been recorded in the Ediacaran (Buatois and Mángano, 2016). The most likely interpretation is to consider this ichnofauna as the distal expression of those that characterize the Treptichnus pedum Zone (see Discussion). Previous reports of Chuaria sp. from the lower interval of the Lower Shale Member at Vali-Abad and Soltanieh Mountains (Hamdi et al., 1989; CiabeGhodsi, 2007) indicate that Chuaria persisted into the lowermost Cambrian. A similar situation has been observed in the Heziao and Jijiapo sections, Hubei province, southern China, where Chuaria sp. has been reported from siltstone and chert from the lower Cambrian Yanjahe Formation (Steiner, 1994). Amard (1997) also suggested an early Cambrian age for Chuaria from the Pendjari Formation of West Africa. These findings question the utility of Chuaria for global correlation of Proterozoic succes- sions. If this is the case, then either the whole Soltanieh Formation is of early Cambrian age, or the Ediacaran-Cambrian boundary is placed within the Lower Dolomite Member below the oldest occurrence of SSF. Ichnozone 2 represents the uppermost interval of the Lower


Shale Member, the Middle Dolomite Member, and the lower interval of the Upper Shale Member (171m above the base of the Lower Shale Member to 80m above the base of the Upper Shale Member). A total of six ichnospecies occur in this zone including, Cruziana isp., Gordia arcuata, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, and


Figure 6. Stratigraphic column of the Soltanieh Formation, showing distribution of trace fossils, small shelly fossils, biozonation based on trace fossils and small shelly fossils, and correlation in relation to the Yangtze platform, China. Small shelly fossil data based on Hamdi et al. (1989), CiabeGhodsi (2007), and Tashayoee et al. (2012).


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