Phenetic classification of Kimmeridgian ammonites from the eastern Kachchh Basin, India

摘要

Thirty Kimmeridgian (Upper Jurassic) ammonite specimens of the genera Torquatisphinctes, Pachysphinctes, Katroliceras, and Indodichotomoceras collected from two horizons in the eastern part of the Kachchh Basin were phenetically classified. First, 23 morphological characters were selected for the analysis. The difference matrix was generated by treating the characters as a binary system (0 or 1) and giving scores between 0 and 2. The dendrogram, prepared using the R program, shows perfect groupings of the different taxa identified earlier through a traditional taxonomic approach. The percentage of shared characters has helped choosing significant characters to be used in distinguishing three major groups and one subgroup, which also validates the visual identification of the ammonite taxa. Categorization of the ammonites was also subjected to a Mann-Whitney U test to verify the results. Overall, the dendrogram depicted three distinct clusters, the Torquatisphinctes, Pachysphinctes, and Katroliceras groups. The specimens of Indodichotomoceras (designated the Indodichotomoceras subgroup) were clustered within the Torquatisphinctes group potentially because of their biplicate ornamentation and presence of constrictions. However, the Indodichotomoceras subgroup maintained its identity because of the thickness of the primary and secondary ribs.