Doctoral Dissertations

Date of Award

12-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

Hazel Delcourt, Paul Delcourt, Michael McKinney, Gary McCracken

Abstract

Members of the family Succineidae, like many other pulmonate landsnails of North America, are thought to have remained morphologically static during the Pleistocene. In order to evaluate this claim, it is necessary to (1) have the ability to identify the various succineid species from their shells, (2) grasp the functional significance of morphological change if it occurs, (3) understand the extent of succineid morphological variation and how it is partitioned in space, and (4) apply knowledge gained from modern species to their fossil representatives. Succineids are karyotypically diverse and conchologically conservative. Among and within related genera, it is often difficult to identify species from shell-based observations. More confidence is usually placed on soft tissue, especially reproductive features. The extent to which three common succineid species, Succinea ovalis Say, 1817, S. grosvenori Lea, 1857, and Catinella avara (Say, 1824) can be discriminated is based on specimens from museum collections (N = 648). Factor analytical and discriminant function techniques, when applied to an appropriate set of traits, segregated species much more clearly than univariate and bivariate approaches. Orientation of the shell during measurement should be based upon the orientation of the protoconch and not upon the aperture. The biometric consequences of orienting the shell by the aperture are substantial enough to cause invalid conclusions. Proper orientation of shells, accurate measurement of spire whorls, and multivariate analysis discriminate S. ovalis, S. grosvenori, and C. avara. Differences among species are produced by two fundamental allometries. C. avara and S. grosvenori are ontogenetically scaled, occupying the same multivariate growth trajectory at differing sizes. They would be considered progenetic and hypermorphic, respectively, relative to one another. S. ovalis shows a more divergent multivariate growth pattern, being displaced with respect to the C. avara and S. grosvenori trajectory. The geometry and functional biology of S. ovalis. were analyzed in terms of six features of the skeleton: Shell shape, shell size, aperture shape, aperture size, shell composition, whorl overlap, and carbonate efficiency. The size of the skeleton in S. ovalis is quite variable. When conditions are poor, size reduction is the most efficient way of controlling dehydration by reducing the absolute size of the aperture. Large size may reflect favorable moisture and calcium conditions and reduce the relative energy devoted to the shell. Independent of initial size, whorl expansion (W = 3.0) implies high animal growth rates. The spire index (SI) or translation rate, T, (== 1.6) reflects the preference S. ovalis for steepened substrates. Aperture eccentricity (AI = 1.53) reflects low spire inclination. The thin and organic rich shell may serve several functions. Besides minimizing shell weight, thin shells do not impede the growth and expansion of the body mass. It is argued that the ordinarily expensive high organic fraction of the shell does not detract from the energy savings associated with a thin shell, but is symptomatic of the rapid growth rate of S. ovalis. Low carbonate content, thin shell, and low overlap characterize S. ovalis. This implies that S. ovalis diverts much of its energy growing body mass while minimizing the energy devoted to shell. Overall, the structure of the shell is consistent with a rapid growth rate and preference for steep surfaces. Combined use of factor, principal component, and nested ANOVA techniques revealed that most morphological variation in S. ovalis is (1) related to size and (2) concentrated at local scales. Although secondary mechanisms (e.g., time, age structure and gene flow) are discussed, high local variability and low regional differentiation is thought to reflect phenotypic plasticity. It is speculated that this phenotypic plasticity is a product of the S. ovalis genetic structure, itself a function of a life history profile consistent with a colonizer. The impedance of broader scale differentiation and the high local variability reflects scale dependent variation in the physical environment. Factor analysis and discriminant functions were performed using modem (N = 494), and fossil specimens (N = 49) of S. ovalis. Modern specimens sample a large portion of the present range of S. ovalis. Two fossil samples were used: (1) the Robert Local Fauna (N = 24) from southwestern Kansas and (2) specimens collected by the author from Cass County, Illinois (N = 25). The Robert Local Fauna has a radiocarbon date of 11.1 ky BP. The location of the Cass County specimens below the modem soil, within the Peoria loess, suggest an approximate age of 12 ky BP. Results suggest that the fossil samples are morphologically marginal to the modern samples. That is, the Mahalanobis distances separating the fossil from the modern provinces are higher than those among the modern samples. Although no associations are found between phenetic variation and biome type, the strongest statistical associations with ecology suggest influence by the dynamics of ecological transitions in space and time. Among the various reasons discussed to explain the morphological variability, a combination of phenotypic plasticity and stabilizing selection associated with the last glacial-interglacial transition are hypothesized are thought to be most probable.

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