ORIENTATION AND ANATOMICAL NOTATION IN CONODONTS

Abstract

All aspects of conodont paleontology rely on the identification and description of homologous anatomical units or elements. But the current schemes of anatomical notation and terms for orientation were formulated at a time when little was known of conodont anatomy or skeletal architecture, resulting in some confusion and difficulties in their application. With improving knowledge of cono- donts, these problems are becoming increasingly acute. In an attempt to address current problems, we introduce new terms for orientation in conodonts and their elements, and a modified scheme of anatomical notation. The principal axes of the conodont body are identified as rostrocaudal, dorsoventral, and mediolateral, with opposite lateral sides designated dextral and sinistral. Anatomical notation is defined according to topological relationships between elements with reference to the principal axes of the body and takes the form of letters with numeric subscripts (e.g., P 1 ,P 2 ,S 0-S4). The ozarkodinid apparatus serves as a standard, but the Pn-Sn scheme can be applied rigorously to all taxa that are known from natural assemblages or where an hypothesis of topological homology can be inferred from secondary morphological criteria. homology as an hypothesis of similarity that is based on topo- logical relations and which contains potential phylogenetic in- formation (see Rieppel, 1994 for discussion). Topology refers to the numbers of, and the relative spatial relationships between, recognizable anatomical units. Similarity, in this context, does not mean morphological similarity of the units under consider- ation. We should also point out that homology is hierarchical, operating at different levels including genes, development, and structure. It is also contextual as illustrated by the classic ex- ample of batwings and birdwings; these structures are homolo- gous as tetrapod forelimbs but not as wings. In this paper we deal only with homology of whole elements within the conodont apparatus (not homology of the apparatus, nor homology of parts of elements).