Orientation and anatomical notation in conodonts

Mark A. Purnell, Philip C. J. Donoghue, and Richard J. Aldridge

ABSTRACT-- All aspects of conodont paleontology rely on being able identify and describe 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 conodonts 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 rostro-caudal, dorso-ventral, and medio-lateral, 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., P₁, P₂, S₁-S₄). The ozarkodinid apparatus serves as a standard, but the P_n-S_n scheme can be applied rigorously to all taxa which are known from natural assemblages or where an hypothesis of topological homology can be inferred from secondary morphological criteria.

Figure 1. Biological orientation in conodonts; the new terminology as applied to the head and apparatus of an ozarkodinid conodont.

Figure 2. Biological orientation and topological anatomical notation and orientation. A. Stereo pair of the ozarkodinid apparatus viewed from dorsal to ventral. B. The ozarkodinid apparatus viewed from rostral to caudal.

Figure 3. Schematic map of the relative positions of elements in the topological scheme of notation (cf. Sweet, 1981a, fig. 13; 1988, fig. 2.10).