Date of Award

January 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

First Advisor

Joseph H. Hartman

Abstract

This paper addresses the question of how strong the record of contiguity is for the 250 million-year-old Diplodon lineage by examining the geographic and temporal distribution of fossil specimens identified as Diplodon.

Diplodon (Mollusca, Bivalvia, Unionoida, Hyriidae) has a fossil record extending back to the Middle Triassic (Anisian Stage). The known distribution of fossil specimens identified as this genus occurs on four continents (North America, South America, Australasia, and Antarctica). The place of origin and pathways of range expansion through time are far from well explained. Both fossil and extant freshwater mussel taxa are subject to evolutionary and phenotypic morphological convergence, which has resulted in problems of identification and classification.

Because of the tendency of freshwater mussels to converge toward similar morphologies, project methods focused on metadata rather than the specimens themselves. The biostratigraphic ranges of specimens identified as Diplodon were determined in order to target temporal and geographic gaps in the fossil record. Without a comprehensive taxonomic review, only Diplodon taxa in current use from documented specimen locations are used in this report. This project has produced paleolandscape maps of the regions that have recorded Diplodon specimens. These first-generation maps were used to qualitatively analyze possible avenues of taxon dispersion through time. Production of paleolandscape maps was based on a new methodology that can be expanded for with other taxa on a global scale.

The evolutionary lineage represented by use of the name Diplodon is not well supported. Geographic and temporal data suggest that hard-part morphology has been an incorrect basis for classification. Five distinct temporal gaps of at least a single geologic stage in duration were identified in the Diplodon fossil record between 245 Ma (beginning of the Anisian Stage) and 5 Ma (end of the Messinian Stage). These gaps occurred during the 1) Middle Triassic (Ladinian); 2) Late Triassic–Middle Jurassic (Norian–Bathonian); 3) Early Cretaceous (Berriasian–Barremian), 4) late Early Cretaceous–Late Cretaceous (Albian–Cenomanian); and 5) early-middle Eocene (Ypresian–Bartonian) intervals.

Gaps in the record are supported by 1) the pattern of additional specimens that lack as much temporal resolution; 2) geographic distances and paleolandscape features between known fossil localities; and 3) the species names applied to these specimens. Continued study of genus-group morphological characters of fossil specimens and molecular analyses of living specimens is necessary to create a Diplodon diagnosis that takes into account morphologic variation (including convergence with other taxa) and the geologic age and geographic relationships among specimens.

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