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Abstract
We analyzed the morphological relationship between Echinolampas appendiculata, Rhyncholampas carolinensis and Eurhodia rugosa from the middle Eocene Castle Hayne Limestone in southeastern North Carolina. We also investigated the heterochronic relationship between R. carolinensis and E. rugosa. Specimens were analyzed quantitatively using a truss consisting of 18 distinct morphological measurements. Principle component analysis (PCA) of these data indicated that the three groups are morphologically distinct. The analysis also showed that the groups shared parallel growth trajectories. Together with the PCA analysis, bivariate analyses showed a strong heterochronic relationship between R. carolinensis and E. rugosa. Additionally, statistical analysis of the data indicated Echinolampas appendiculata exhibited the most morphological variability among the three groups. Extinct and extant members of the three genera were found to inhabit fairly distinct water depths and substrates, thus the groups may have divided up their environment according to water depth. In light of the morphological results and phylogenetic data we speculate that Rhyncholampas carolinensis and Eurhodia rugosa initially diverged from a Rhyncholampid ancestor. Competition between the variable E. appendiculata and a morphologically similar Rhyncholampid species may have provided the selective pressure for the evolution of the relatively deep-water cassiduloid Eurhodia.
Acknowledgements
We would like to graciously thank Burt Carter, Phil Novack-Gottshall, and two anonymous reviewers for comments and criticisms on early drafts of this paper. Our thanks also go out to Dan McShea for his consistent advice and support as well as Patricia Weaver for her help with field work and supplying specimens from the North Carolina Museum of Natural Sciences.