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 » I. Research System
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 » A. Phylogenetics
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 » C. SEM
 » D. X-Radiology
 » E. Development Experiments
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The Nehm lab uses living and fossil marginellid snails (Phylum Mollusca; Class Gastropoda; Family Marginellidae) of the monophyletic group Prunum+Volvarina to study the evolution of development (see photos above). Living Prunum and Volvarina species are most diverse and abundant in the Caribbean region. Fossil Prunum species are a prominent and well-preserved element of the Neogene fauna of the Dominican Republic and many other regions of the Caribbean . Marginellid shells are taphonomically durable and well preserved in these deposits due to their thickness, extensive callusing, and reinforced aperture margin (fossil shells are shown above right). Furthermore, in portions of the Rio Gurabo and Rio Cana sections, very thin and fragile juvenile shells co-occur with robust adult shells in frequencies similar to living populations (Nehm, 1998). Color patterns, many quite intricate, are also preserved in Dominican fossil marginellids. For these reasons, gastropods are an excellent source of developmental and evolutionary data.

In addition to their abundance and excellent preservation in the Dominican Republic Neogene, Prunum gastropods are an excellent research system for the investigation of developmental evolution because they have a well-established phylogenetic hypothesis, a complete preservation of ontogeny, clear morphological markers of sexual maturity, and morphologies amenable to quantitative analysis and morphogenetic study. Some specific traits are noteworthy relative to evolutionary-developmental research questions: (1). Prunum has determinate growth, which permits the recognition of juvenile and adult shells. (2). Adult shells preserve a complete record of ontogeny, which is easily examined quantitatively using x-radiographic images, and compositionally using Scanning Electron Microscopy (SEM) and hard-tissue histology. (3). Stratophenetic and maximum-parsimony phylogenetic analyses of morphological and molecular data have established the evolutionary relationships of the Marginellidae in general and Prunum+Volvarina in particular (Nehm, 1996; Coovert, 1987, 1989; Coovert and Coovert 1990, 1995; Nehm and Tran, 1997). (4). There are detailed data on the geographic, stratigraphic, and bathymetric distributions of Prunum species, and their relationship to ontogenetic and morphologic variation (Nehm and Geary, 1994; Nehm and Hickman, 1995; Nehm 1998; Nehm, 2001a).

Multiple lines of evidence demonstrate that the thousands of fossil Prunum snails from the Dominican Republic represent several separate biological species: (1). The magnitude of morphological differences between fossil species is as great as the magnitude of morphological differences between genetically related living species in the same genus (Nehm and Geary, 1994; Nehm, 2001b). (2) Morphological features used to diagnose fossil species do not vary with environmental, lithological, or bathymetric conditions in living species from the same genus, strongly suggesting they were not environmentally induced (Nehm and Geary, 1991; Nehm, in revision). (3) Morphometric data indicate clear separation among fossil species regardless of the lithology, paleoenvironment, or geographic location of the fossil samples (Nehm and Geary, 1994; Nehm 1998; 2001a). Maximum-parsimony phylogenetic analyses of molecular and morphological (shell and soft part) data and stratophenetic analysis have established a solid phylogenetic framework for this study (Nehm and Tran, 1997; Nehm, 1998; Nehm, 2001a).