The latest Permian mass extinction in the Alborz Mountains (North Iran) LUCIA ANGIOLINI 1 * , ALESSIO CHECCONI 2 , MAURIZIO GAETANI 1 and ROBERTO RETTORI 2 1 Dipartimento di Scienze della Terra ‘A. Desio’, Milano, Italy 2 Dipartimento di Scienze della Terra, Piazza Universita ` 1, Perugia, Italy The Elikah River section in the Alborz Mountains (North Iran) is a fossiliferous, continuous, marine section spanning the Permian–Triassic (P–T) boundary interval. Sixty-eight taxa of brachiopods, foraminifers and algae have been collected bed-by-bed from the topmost 36 m of the Nesen Formation and the basal 2 m of the Elikah Formation, across the P–T boundary that is located in the basal metre of the latter formation. Forty-six of these taxa from 62 samples have been considered for statistical analyses to unravel the pattern of extinction. First, their range extensions have been determined, which should be added to the highest stratigraphic occurrence of each taxon to identify the extinction level within a certain confidence interval (95%). Then the pattern of the extinction has been analysed, plotting the stratigraphic abundance versus last occurrence below the extinction horizon. The resulting pattern is of a sudden extinction occurring 30 cm below the biostratigraphically defined base of the Triassic. The confidence interval (95%) for the mass extinction is 30 cm thick. As predicted by the ‘Signor–Lipps effect’, brachiopod last occurrences underestimate the time of extinction, as their abundance is below 15%. Therefore, also in North Iran located at equatorial palaeolatitudes along the southern shore of the Palaeotethys Ocean, the pattern of the latest Permian mass extinction is abrupt as observed in South China, in peri-Gondwana and in the Southern Alps. One of the most plausible mechanisms of the latest Permian sudden mass extinction could have been the emissions of tholeiitic magmas (Siberian Traps), which could have caused greenhouse gases emission into the atmosphere and thus global warming and ozone depletion, triggering the crisis (Svensen et al. 2008). Copyright # 2010 John Wiley & Sons, Ltd. Received 13 July 2009; accepted 16 October 2009 KEY WORDS Permian–Triassic boundary; extinction; confidence interval; pattern of extinction; brachiopods; foraminifers; Iran 1. INTRODUCTION Marine successions of Lopingian age containing a continu- ous section through the Permian–Triassic (P–T) boundary interval are spectacularly preserved along the Caspian side of the Central Alborz Mountain range in North Iran (Gaetani et al. 2009). The Lopingian is recorded by the Nesen Formation, a succession of limestone and marlstone rich in brachiopods, whereas the P–T boundary is preserved in oolitic limestone and microbialite in the basal metre of the overlying Elikah Formation. It has been shown (Gaetani et al. 2009; Angiolini and Carabelli in press) that the brachiopod distribution in the P– T boundary beds of Iran is very different from the pattern observed in the stratotype of South China (e.g. Liao 1980, 1987; Shen et al. 1992; Shen and He 1994; Xu and Grant 1994; Jin et al. 2000; He et al. 2005; Chen et al. 2006; Chen and Liao 2007; Shen and Shi 2007; Li and Shen 2008) and in the Southern Alps of Italy (Posenato 1988, 1991, 1998, 2009), as brachiopods seem to disappear much earlier in Iran than elsewhere. However, there is no major facies and palaeoenvironmental change associated with the disappear- ance of the brachiopods and the foraminiferal composition does not seem to vary significantly in correspondence to this bioevent. In this paper, we seek to understand the latest Permian biotic change in the Elikah River section of the Alborz Mountains (North Iran) calculating the extinction rates of foraminifers and brachiopods using the statistical technique elaborated by Marshall (1995) and Wang and Marshall (2004). These allow to compute extension to the range of a species and estimate rather precisely the stratigraphic position of the extinction horizon. Also, the eventual occurrence of a ‘Signor–Lipps effect’ – which makes sudden extinction appear gradual because of artificial range truncation (Signor and Lipps 1982) – can be discriminated checking the statistical distribution of the endpoints of the stratigraphic ranges (Meldahl 1990). Jin et al. (2000) and Groves et al. (2007) have applied this method, respectively, GEOLOGICAL JOURNAL Geol. J. 45: 216–229 (2010) Published online 15 February 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/gj.1203 *Correspondence to: L. Angiolini, Dipartimento di Scienze della Terra ‘A. Desio’, Via Mangiagalli 34, 20133 Milano, Italy. E-mail: lucia.angiolini@unimi.it Copyright # 2010 John Wiley & Sons, Ltd.