EVOLUTIONARY SHAPE TRENDS IN LATE KATIAN AND EARLY HIRNANTIAN-AGED CLIMACOGRAPTIDS: AN EXAMPLE OF MORPHOLOGICAL STASIS ACROSS THE END-ORDOVICIAN MASS EXTINCTION INTERVAL

The Hirnantian extinction resulted in a dramatic faunal turnover in graptoloid communities. During this interval of rapid climate change, clades that had previously dominated low paleolatitude regions in the Katian Stage became extinct as a large radiation occurred simultaneously within the normalograptid clade. The species Styracograptus mississippiensis was a common component of the low-latitude fauna that was replaced by the Normalograptidae during the Hirnantian glaciation. Furthermore, S. mississippiensis colonies bear a close resemblance to those constructed by the lineage that replaced them. Therefore, it could be hypothesized that S. mississippiensis demonstrated directional shape change in response to climate deterioration and the immigration of the Normalograptidae. Using geometric morphometrics, this hypothesis was tested on Late Katian to Early Hirnantian-aged specimens of S. mississippiensis that were collected from a section of the Vinini Formation in the Roberts Mountains of Nevada. Despite the prominent species turnover that occurred during the extinction interval, a range of multivariate analyses of Procrustes shape data from the Vinini time series were all strongly consistent with morphological stasis. Size change, in contrast, exhibits a dynamic consistent with random walks. Nevertheless, based on partial least-squares analysis of geochemical proxies, shape and size data, size change is significantly correlated with measures of habitat quality. Our results imply that these organisms did not alter colony shape in response to the changing environment even as they entered mass extinction and despite the fact that colony size did respond to changes in late Katian oceanographic conditions.