The taxonomy and phylomorphogenesis of conodont pectiniform elements around the Carnian/Norian boundary is rather problematic for the apparent provincialism that affects many conodont species (Kozur, 2003; Mazza & Rigo, 2008a) and for the large number of morphotypes and transitional forms occurring during this stratigraphical interval. The mass presence of transitional forms among conodont species is explained by the rapid recovery and evolution of conodonts after the Carnian crisis (Julian/Tuvalian boundary), related to the Carnian Pluvial Event (Rigo et al, 2007). In order to resolve the problems concerning evolutionary patterns and processes of the Upper Triassic conodonts, we conducted a detailed taxonomic study of the species belonging to the five most widespread genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella) from the Pizzo Mondello section (Sicani Mountains, Western Sicily, Italy), candidate section as GSSP for the Carnian/Norian boundary (Nicora et al., 2007; Balini et al., 2008). The section is a 430 m thick continuous succession of marine limestones ranging from Late Carnian to Rhaetian in age (Nicora et al., 2007), characterized by almost uniform facies and a very rich conodont fauna. Taxonomic studies on conodont populations allowed us recognizing some discriminating morphological features for the separation of genera (the position of the pit, with respect both to the platform and to the keel end, the occurrence of nodes and/or denticles on the platform margins, the length of the platform and the shape of the keel end; see Mazza & Rigo, 2008b) and many other for the classification of species. Also morphometries have been applied to the measurable features (pit position with respect to the platform and to the keel end, platform length, keel width and free blade length) to obtain more precise and objective data for the analyses. The changes in the conodont platform morphology observed through the section, provided the following trends in the evolution of the pectiniform elements: a) the forward-shifting of the pit, b) the shortening of the platform and c) the appearance of nodes on the platform margins and the development of nodes into denticles (see Mazza & Rigo, 2008 and references therein). Species belonging to the five main Late Carnian-Early Norian genera have been thus assigned to two separate monophyletic lineages: i) the Paragondolella-Metapolygnathus-Norigondolella lineage and ii) the Carnepigondolella-Epigondolella lineage. Nevertheless, the phylogenetic relationships of some species remained still problematic. In order to solve these problems, a numerical cladistic analysis have been applied to the studied species. The morphological characters used to classify conodont species have been thus codified to generate numerical matrices to be analyzed by a program (PAUP, Swofford, 2002) for phylogenetic analysis under the parsimony criteria. The produced cladograms evidenced morphological relationships among genera and species, showing that the evolutionary trends listened above are supported by the most parsimoniuos character states distribution among taxa and confirming the importance of the considered characters for the conodont taxonomy. Furthermore, the consensus tree from the set of cladograms generated, allowed assigning some species to well identified sister groups, which coincides in this case with the conodont genera, and better defining the still uncertain systematic positions of some problematic species.
|Titolo:||Evolutionary patterns and phylogeny of the Carnian/Norian conodonts from the Pizzo Mondello section, GSSP candidate for the base of the Norian.|
MAZZA, MICHELE (Primo)
|Parole Chiave:||Carnian/Norian conodonts ; phylogeny ; cladistics ; Pizzo Mondello ; GSSP|
|Settore Scientifico Disciplinare:||Settore GEO/01 - Paleontologia e Paleoecologia|
|Data di pubblicazione:||giu-2009|
|Appare nelle tipologie:||01 - Articolo su periodico|