Abstract

The ecology and biogeographic affinities of fossil bivalve faunas from the late Holocene of the Galápagos Islands and the Triassic of the Wallowa accreted terrane are markedly similar. These similarities provide further evidence that the Wallowa terrane originated as an oceanic island arc and that it lay in the eastern proto-Pacific during the Late Triassic, and they corroborate leomagnetic and lithologic evidence for the allochthonous, low-latitude origin of the Wallowa terrane and define the limits of its Triassic longitude relative to North America. Similarity between Galápagos and Wallowa bivalve faunas is based on (1) comparisons of living and fossil Galápagos bivalve faunas and (2) faunal comparisons between a Galápagos fossil locality and a Wallowa one. The first indicates that fossilization has not significantly biased the ecologic and biogeographic signature of the Galápagos "parent" community. This supports the validity of comparisons of fossil faunas from accreted terranes with counterparts on modern oceanic islands. The second shows that Galápagos and Wallowa fossil localities both exhibit the "hybrid" biogeographic pattern characteristic of oceanic islands. This pattern comprises a variable endemic component, coupled with an array of species from diverse faunal provinces. Quantitative ecologic comparisons of Galápagos and Wallowa faunas show that their species are generally similar in feeding habits and substrate preferences. Differences can be explained by the post-Triassic adaptive radiation of infaunal bivalves.

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