Model of phylomorphogeny of P1 elements of the Tournaisian (Mississippian) siphonodellids

Abstract

Conodonts are a group of extinct marine organisms widely used in Paleozoic and Triassic biostratigraphy. The genus Siphonodella comprises ozarkodinid conodonts from the latest Famennian to the middle Tournaisian. The phylogenetic model of this genus plays an important role in biostratigraphy. This study is based on the authors' and published data on 31 species of siphonodellids of cosmopolitan and East European groups. A new phylomorphogeny of Siphonodella is proposed on the basis of known mechanisms of morphological transformations of siphonodellid P1 elements during speciation processes, presence of interspecies transitional forms, and data on ontogeny. The main morphogenetic pathways in the evolution of siphonodellids are the following: formation and transformation of the rostrum, transformation of the platform outline, transformation of the platform ornamentation, transformation of the keel, and histological transformations. In the process of modeling the phylomorphogeny of Siphonodella, some taxonomic problems are solved: a new species S. thompfelli is described, a new variant of the division of S. lobata (Branson and Mehl) into morphotypes is proposed, the validity of S. plana (Huddle), S. praeplana Bardasheva, Bardashev, Weddige and Ziegler, and S. gladia Zhuravlev and Plotitsyn is clarified, and the knowledge of the morphospace of most representatives of the genus is extended. The phylogeny of siphonodellids provides a basis for the new phylogenetic zonal sequences based on the conodont phylogenetic-zone concept.