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Earth & Life (http://www.geofinds.com), (2006-10-1), 1(1): 1--6.
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The above questions depend on our understanding on the evolution of conodonts during the PTB. Many researchers have dealt with the evolution of conodonts across the PTB (Sweet, 1970; Clark et al., 1986; Tian, 1993; Ding et al, 1996; Mei et al, 1999; Yang et al., 1999; Lai et al., 2000.). Based on conodonts in Meishan sections, Clark et al. proposed that there was no mass extinction but reduction in abundance of conodonts across the PTB (Clark et al., 1986.). Erwin (1993) believed that conodonts are not influenced by the biotic crisis near the P/T boundary. After their work, however, more conodonts were described from the PTB sections in Meishan, Shangsi, and Kashmir (Zhang et al., 1995, 1996; Wang, 1995; Mei et al., 1998.), and their conclusions need updating. Ding et al. (1996) determined the lineage from Hindeodus latidentatus to Hindeodus parvus, which serves as the basis for definition of the beginning of the Triassic Period with the first appearance of the conodont Hindeodus parvus (Yin et al., 2001.). Kozur (1998) recognized three extinctions in conodonts in the PTB and Early Triassic: the first is the disappearance of warm- water adapted Clarkina components somewhat below the H. parvus zone, the second is the extinction of those adapted to both warm water and cool water at the base of Isarcicella isarcica zone, and the third is the disappearance of cool-water adapted Hindeodus conodonts in an above horizon. Any studies of biotic evolution need reliable taxonomic data. During a recent study, we found that many conodonts from these PTB sections were incorrectly identified (Wu, 2005). For example, many specimens previously placed into H. latidentatus and H. typicalis (Zhang et al., 1996.) actually should be placed in other taxa. Incorrect identification may artificially elongate or shorten the ranges of some taxa and lead to bias in understanding of conodont evolution. To avoid such a bias and improve accuracy of evolutionary study and biostratigraphic studies, a revision on the incorrectly assigned conodonts is needed. Comparison of enlarged photo involved specimen with involved holotype shows more than 60 previously described conodonts from these PTB sections were incorrectly identified. We corrected all of them (Wu, 2005). This revision causes changes in geological ranges of many taxa (Fig. 1), which would lead to some different interpretation of the evolution of conodonts across the PTB. 2. Evolution of conodonts across the PTBThe PTB interval of Meishan is lithologically divided as 8 beds in ascending order: 24e, 25, 26, 27a, 27b, 27c, 27d, and 28 (Zhang et al., 1995.). Each bed has a thickness less than 10 cm. The following discussions on the evolution of PTB conodonts are based on all after-revision conodonts from Meishan, Shangsi, and Kashmir sections, but not Selong section in Xizang, since the conodonts of Selong section (Yao et al., 1987; Mei, 1996; Jin et al., 1996; Orchard et al., 1994, 1998) seems to belong to a different paleobiogeographic region.
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