Application of numerical cladistic analyses to the Carnian-Norian conodonts: a new approach for phylogenetic interpretations

The high intraspecific variability of conodonts in the upper Carnian-lower Norian interval and the proliferation of numerous species in this relatively short stratigraphical time generated many problems in the understanding of the Late Triassic conodonts phylogeny, systematic and taxonomy. The abundance of synonymies per species and the absence of an established phylogenetic model contribute to increase the issues concerning the conodonts systematic. Unfortunately, being natural assemblages of the Late Triassic conodont apparatuses still unknown, a multielement approach to the problem would just introduce more biases instead of solutions. The application of cladistic methods to the platform elements, instead, may provide an extremely valid methodology in the understanding of their evolutionary relationships. Platforms are in fact the most abundant conodont elements and they have well observable evolutionary patterns and, thus, they can be considered as the primary basis for the interpretation of the Late Triassic conodont phylogeny. We applied numerical cladistic analysis to the species belonging to the five most widespread Late Triassic genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella and Norigondolella) from the Pizzo Mondello section (Sicani Mountains, Western Sicily, Italy), GSSP candidate for the Norian (Mazza et al., 2010 and references therein). This section provides several advantages for these kind of taxonomic and cladistic studies on conodonts: the section is a continuous succession of pelagic carbonate sediments (Calcari con selce or Halobia Limestone auctorum; Cherty Limestone, Muttoni et al. 2001, 2004; Guaiumi et al. 2007), characterized by uniform facies, high sedimentation rates and, more important, it has a rich conodont record which is representative of faunas spread in the entire Tethys. A taxon-character data matrix describing the distribution of 68 characters among 2 outgroup and 32 ingroup taxa was thus compiled and processed using PAUP* 4.1. The analyses confirmed the validity of a series of evolutionary trends among the platform elements, evidenced the most important morphological characters for their classification and led to a reinterpretation of the phylogenetic position of the genera considered: Metapolygnathus and Epigondolella resulted the only two monophyletic groups and, thus, true phylogenetic genera; Paragondolella a polyphyletic assemblage of basal members of the ingroup; Norigondolella a paraphyletic series of taxa bracketed by Paragondolella and Carnepigondolella a paraphyletic group including all conodonts more derived than Paragondolella and Norigondolella but outside the Metapolygnathus and Epigondolella clades. These results show the potentiality of applying cladistic methods also to parataxonomic taxa. Parsimoniously grouping by synapomorphies (shared derived characters) provides a more informative classification than using simple similarities (phenetics). A cladogram, in fact, describing more of the character state changes than a phenogram, gives a higher information content and provides a more natural classification.