Quantitatively Comparing Morphological Trends to Environment in the Fossil Record (Cincinnatian Series; Upper Ordovician)

Abstract

Determining whether morphological trends in fossil species represent evolution within a lineage or lateral shifts in morphologically variable populations through time requires a thorough examination of the details of both morphology and paleoenvironment in time and space. The purpose of this study is to explore at high resolution the relationship between morphology of the trilobite Flexicalymene granulosa and paleoenvironmental conditions in Upper Ordovician deposits of southwestern Ohio and northernKentucky. This is achieved by using geometric morphometrics to measure high-resolution morphological changes and by using gradient analysis to capture environmental gradients underlying faunal distribution patterns. Quantitatively comparing the outcomes of these two techniques provides an assessment of whether shape changes relates to environment. Results indicate that a significant amount of shape change, seen as an anteromedial movement of the eye region over time, corresponds to ordination scores. This suggests a relationship between certain aspects of morphology and environment. The combination of these quantitative techniques has provided the foundation for determining whether morphological trends within F. granulosa represent evolution or temporal shifts in geographic variation. Future work will involve examining this relationship in greater detail both geographically and stratigraphically.

Decoupling of taxonomic diversity and morphological disparity during decline of the Cambrian trilobite family Pterocephaliidae

“…I estimated the degree of variation (across time and space) for species where 10 or more specimens were available ( N = 17; Table ) by finding the average Procrustes distance (MD of Zelditch et al ., ; after Foote, ). Because of the presence of log‐linear allometric growth in many of the species (Hopkins, ), I ‘size‐standardized’ variation by applying a multiple regression of the partial and uniform warps against the natural log of centroid size using the program Standard6 (Sheets, ); the variation in the sample not due to allometry is preserved in the residuals from the regression (Zelditch et al ., ; Webber & Hunda, ; Hopkins & Webster, ; Kim et al ., ; Hopkins & Thurman, ). I calculated confidence intervals by resampling with replacement (1600 bootstraps) using the program DisparityBox 6i (Sheets, ).…”

Developmental bias in the fossil record

“…Patterns of intraspecific variation in fossil species across a geographic and putative environmental range are by no means uncommon (e.g. Cisne, Chandlee, Rabe, & Cohen, 1982;Scarponi & Kowalewski, 2004;Webber & Hunda, 2007). Such patterns have been studied through the F I G U R E 3 Sheldon's (1997) Plus ca Change model.…”

How Species Longevity, Intraspecific Morphological Variation, and Geographic Range Size Are Related: A Comparison Using Late Cambrian Trilobites

“…Thus, greater variation within some trilobite species in this clade could simply reflect more distinct geographic or temporal structure of that variation. For example, the placement and orientation of the eyes relative to the rest of the cranidium has already been shown to vary with water depth in some trilobites (Webber and Hunda 2007; Hopkins and Webster 2008) and the mean number of pygidial axial rings varies in time in others (Cisne et al 1980; Sheldon 1987). Divergence between populations may occur because of adaptation to local selection pressures or because populations are able to respond plastically to different environmental cues in either a spatial or temporal context, or both.…”

“…First, multiple regression of the partial and uniform warps against ln centroid size was used to predict the shape of an individual at the mean size for each species. The variation in the sample not due to allometry was preserved in the residuals from the regression (Zelditch et al 2004; Webber and Hunda 2007; Hopkins and Webster 2009; Kim et al 2009; Hopkins and Thurman 2010). Using this method to account for allometry assumes that the allometric trajectories estimated from different collections belonging to the same species are the same, that is, that different populations are growing in the same way.…”