The Frasnian – Famennian boundary in Vietnam and evolutionary meaning of FADs and LADs
The latest Frasnian and earliest Famennian conodont assemblages at the Si Phai section in northernmost Vietnam are similar to those from coeval strata in Europe, but their taxonomic diversity is significantly lower and the order of species appearances is different. Unlike most other localities, where ‘Palmatolepis’ linguiformis is a very late Frasnian species preceding Klapperilepis ultima, it emerges much earlier at Si Phai, already co-occurring with Ancyrodella lobata. Its contribution to the assemblage is there many times higher than elsewhere. This allows tentative apparatus reconstruction proving its congenerity with Manticolepis rhenana, which at Si Phai occurs only in the lowermost portion of the M. linguiformis range. Another peculiar aspect of this Vietnamese section is the abundance of Avignathus in its lower part. In a subsequent sudden change of apparently ecological nature, a diverse set of polygnathids Ctenopolygnathus and Poly gnathus species followed by Klapperilepis ultima enriched the assemblage. The second dramatic faunal exchange event resulted in the disappearance of all the palmatolepidids except for K. ultima. The Si Phai conodont species succession does not differ from those of other regions of the world in that both the first and last occurrences were caused by ecological changes that, apart of this terminal Frasnian event, are not necessarily of global extend. The widely held assumption that most changes in distribution of conodonts were synchronous may not be true. A truly reliable age correlation should not be based on ecologically controlled appearances and disappearances but on well-documented evolutionary transitions. Unfortunately, evolutionary origin of no one of the palmatolepidid species occurring in Si Phai (with possible exception of Manticolepis winchelli) has been determined elsewhere. This makes the probably phyletic succession of Ancyrodella chronospecies a more reliable tool for age correlation.
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Document Type: Research Article
Publication date: 01 June 2018
This article was made available online on 05 December 2017 as a Fast Track article with title: "The Frasnian–Famennian boundary in Vietnam and evolutionary meaning of FADs and LADs".
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