Application of geometric morphometrics to late Katian and early Hirnantian-aged climacograptid populations: An investigation of graptoloid shape evolution at the end-Ordovician mass extinction
Robinson, Daniel E.
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The Hirnantian extinction at the end of the Ordovician Period resulted in the disappearance of a large fraction of marine fauna. Compared to the impact this event had on most organisms, graptoloids experienced particularly dramatic changes in diversity and species richness. Aside from the normalograptids, the Graptoloida experienced a near complete extinction, coinciding with a rapid series of climate changes. Only a few species of the once-dominant late-Katian Diplograptina survived the onset of the large-scale glacial event at the beginning of the Hirnantian Stage. Surviving graptoloid clades, primarily in low paleolatitude regions, diversified during the glacial period. The species that endured the environmental changes formed the base of the post-extinction radiation that began at the beginning of the Hirnantian. The species S. mississippiensis and S. tatianae are common elements of the late Katian low latitude graptolite fauna. In addition, colonies produced by these animals closely resemble those of species of the Normalograptidae that replaced them in the early Hirnantian. Thus, one might expect these graptolites to exhibit directional change in the face of deteriorating Katian climate and the immigration of their replacement by homeomorphic normalograptids. The purpose of the present study, therefore, is to investigate whether the populations under investigation displayed a directional pattern of shape change in response to the late Katian climate deterioration. In this study, I applied geometric morphometrics to distinguish S. mississippiensis and S. tatianae specimens, testing previous interpretations that these taxa represent distinct species. Following these results, I then assessed shape change in S. mississippiensis populations from strata of the late Katian and early Hirnantian stages, as recorded in a section of the Vinini Formation. The section, located near Vinini Creek, in the Roberts Mountains of Nevada, was selected for this study based on the fact that the entirety of it has been sampled in detail, and styracograptids are common in most of the collection horizons. A total of 12 levels from Vinini Creek section provided samples for the time series analysis of S. mississippiensis. In addition to specimens measured at Vinini Creek, populations of similar age were measured from two intervals within the Blackstone River section in Yukon, Canada and three intervals within the Jieling section from the Yangtze platform, China Although the data available for these two other localities was not sufficient to perform separate time series analyses, the intervals measured provided the opportunity to conduct preliminary morphological comparisons on a global scale and to compare the magnitude of geographic variation in colony shape to that observed within the Vinini Creek time series. Despite the massive faunal turnover experienced by graptoloids during the mass extinction, results of the shape change analysis for the lineages of interest at Vinini Creek were strongly consistent with morphological stasis in nearly all of the statistical tests conducted. Furthermore, analysis of the Blackstone River and Jieling populations suggest similar shape evolution patterns within the species group may have occurred on a wide geographic scale.