APPLICATION OF GEOMETRIC MORPHOMETRICS TO NON-ISOLATED GRAPTOLOID POPULATIONS ACROSS THE HIRNANTIAN EXTINCTION INTERVAL

Morphometrics provide a robust set of tools for quantifying biological shape change in taxonomic, ontogenetic and phylogenetic studies. Landmark-based methods are particularly powerful due to the explicit utilization of homologous features in analysis. Such methods have traditionally been applied to three-dimensional specimens, including recent research on isolated graptoloids. Here, we discuss methodology used to measure flattened specimens from Katian-aged sections in China, Nevada and the Yukon, and address some concerns regarding shape analysis of two-dimensional samples. Homeomorphic species included in the analysis are Climacograptus mississippiensis, C. miserabilis, C. putillus, C. tatianae, and Normalograptus extraordinarius; all of which possess climacograptid thecae and lack spines aside from the virgella. Coordinates are collected for eight homologous landmarks at the proximal end of the rhabdosome, and for six landmarks on alternating pairs of thecae. In order to increase sample size, sets of landmarks can be treated independently by allowing intact portions of incomplete rhabdosomes to be measured. Differences in the degree of preservation and orientation between specimens introduce complications such as obstruction or degradation of thecae and spines, potentially resulting in misidentification. Furthermore, differential degrees of collapse and compaction result in small dimensional variations that can introduce bias within the data set. Such complications demand the use of caution when selecting individuals for analysis, and when digitizing landmarks.