Taxonomic Evaluation of Various Morphological Characters in the Late Cretaceous Desmoceratine Polyphyletic Genus “Damesites” from the Yezo Group in Hokkaido and Sakhalin

Abstract

Intra- and interspecific variation and ontogenetic changes in various shell characters of the Late Cretaceous desmoceratine ammonoid “Damesites” are described, and their taxonomic implications are discussed based on specimens from the Cretaceous Yezo Group in Hokkaido and Sakhalin. Our study reveals that many “diagnostic” features (e. g., appearance of longitudinal striations, height of ribbing, regularity of ribbing as well as constriction curvature) and early internal shell structures, are in fact inappropriate as diagnostic features of “Damesites” morphotypes. In contrast, ontogenetic changes in shell ornament, curvature of growth lines and whorl expansion ratio are herein demonstrated to be key characters for species recognition and reconstructing the phylogenetic relationships of the taxa of the subfamily Desmoceratinae. Based on these results, previously described “Damesites” species from the uppermost Turonian-lower Campanian interval should be reclassified into three groups. “Damesites damesi,” “D. damesi intermedius,” “D. semicostatus,” and “D. laticarinatus” are assigned to the first group. “D. ainuanus” and “Damesites sp.” are assigned to the second group. “D. sugata” from the Yezo Group represents the third group. Furthermore, analysis of ontogenetic changes in shell ornament, curvature of growth lines, and whorl expansion ratio suggests that the second and third groups together belong to a different evolutionary lineage from the first group.