Title page for ETD etd-04072008-184811

Type of Document Dissertation
Author Agnew, Jeffrey Guy
Author's Email Address jagnew@centenary.edu
URN etd-04072008-184811
Title Dactyls Reveal Evolutionary Patterns in Decapod Crustaceans
Degree Doctor of Philosophy (Ph.D.)
Department Geology & Geophysics
Advisory Committee
Advisor Name Title
Anderson, Laurie C Committee Chair
Ellwood, Brooks B Committee Member
Engel, Annette S Committee Member
Fleeger, John W Committee Member
Fry, Brian D Dean's Representative
  • diet
  • predation
  • xanthoid
  • evolution
  • crab
  • claw
Date of Defense 2007-12-17
Availability unrestricted
Because of their high preservation potential and uses in foraging and defense, decapod crustacean dactyls (movable fingers of claws) are potentially excellent test subjects for an ongoing debate concerning the relative importance of top-down (predators) and bottom-up (prey) controls on morphologic diversification and evolution. The utility of dactyls for inferring evolutionary patterns were evaluated using living and subfossil xanthoid crabs sampled from the southeast U.S. Atlantic and Gulf of Mexico coasts, and were used to: (1) distinguish the roles of selection and constraint on dactyl morphology through allometric and shape comparisons in the context of the crab’s phylogenetic relationships and inferred ecological similarities; (2) evaluate whether wear patterns can serve as proxies for diet and claw function; and (3) develop and evaluate a proxy for predation intensity on crabs that combines handedness reversal and predatory fracture frequencies. Relationships among shapes, allometries, and wear patterns of dactyl outlines were quantitatively described by principal component analyses of elliptical Fourier descriptor coefficients. Frequencies of dactyls with predatory fractures and handedness reversals were analyzed using logistic regression models.

The results of this dissertation establish a means by which dactyls can be used in detailed evolutionary studies of predator-prey interactions in the fossil record. Dactyls of xanthoid crabs were found to be shaped by recent selective pressures, as their shapes and allometries correspond more closely to their inferred ecological similarities than their phylogenetic relationships. In addition, wear patterns along the occlusal surface of dactyls can be quantitatively described by outline-based morphometric techniques and used to infer both claw function and the degree of durophagy in crabs. Predation intensities in subfossil and fossil crab assemblages also may be inferred by using frequencies of dactyl handedness reversals as proxies for nonlethal attacks and predatory fractures as proxies for total attacks (both lethal and nonlethal). Thus, the relative influence of top-down and bottom-up controls on dactyl evolution can be identified by correlating dactyl morphologies with evidence of predation either by crabs (wear patterns) or on crabs (handedness reversals and predatory fractures) using the most commonly preserved remains of living and/or fossil taxa.

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