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Patterns of morphological diversity among and within arcid bivalve species pairs separated by the Isthmus of Panama
- Peter B. Marko, Jeremy B. C. Jackson
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- Journal:
- Journal of Paleontology / Volume 75 / Issue 3 / May 2001
- Published online by Cambridge University Press:
- 20 May 2016, pp. 590-606
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Geminate species are morphologically similar sister-species found on either side of the Isthmus of Panama. The existence of all geminates in the tropical Eastern Pacific ocean and the Caribbean Sea is most often explained by vicariance: closure of the Central American Seaway 3.1 to 3.5 Ma simultaneously isolated populations of species with amphi-American distributions. In this paper, we test the potential of morphological measurements for discriminating between Recent geminate species pairs from three genera (Arca, Arcopsis, and Barbatia) in the bivalve family Arcidae and examine the prospects for distinguishing nominal species in the fossil record. Fourteen morphological variables were used to characterize shell shape and multivariate methods were used to discriminate between five Recent species pairs. Collection sites were also used as a priori groups for discrimination to describe patterns of intraspecific morphological variation and to evaluate differences among samples from different geographic regions.
On average, 84 percent of specimens within geminate pairs are classified correctly following five separate discriminant analyses with nominal species as the grouping variable. Although all but one arcid species pair are discriminated with high statistical significance, some collection sites within species are highly morphologically distinct. Overall, a large proportion of specimens from each collection locality (79 percent on average) can be classified correctly to site although no single site possessed a multivariate centroid that was significantly different from all other conspecific centroids. The distinctiveness of some collection sites, however, raises the possibility that some nominal species may harbor cryptic species, indicating the need for wider geographic surveys of both molecular and morphological variation within geminate species pairs.
The eigenvalue coefficients derived from the Recent samples of one geminate pair (Arca mutabilis and A. imbricata) were used to assess the potential for identifying arcid species in the fossil record. Discriminant analyses of fossil Arca indicate that the forms that characterize Recent A. mutabilis and A. imbricata are present in the fossil record as far back as the Late Early Miocene, in the Cantaure Formation of Venezuela. Because a deep water connection between the Eastern Pacific and Western Atlantic existed until the Middle Miocene, the morphological differences associated with Recent A. mutabilis and A. imbricata likely existed well before the rising Isthmus affected ocean circulation patterns in tropical America. Therefore, despite great overall morphological similarity, these putative geminate species likely have a time of divergence that is at least four times older than final seaway closure. The geographic distribution of fossils also suggests that morphological forms associated with each Recent species had amphi-American distributions both before and after isthmus formation but are now geographically restricted to either side of the isthmus in the Recent fauna.
Contributors
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- By Basem Abdelmalak, Joseph Abdelmalak, Alaa A. Abd-Elsayed, David L. Adams, Eric E. Adelman, Maged Argalious, Endrit Bala, Gene H. Barnett, Sheron Beltran, Andrew Bielaczyc, William Bingaman, James M. Blum, Alina Bodas, Vera Borzova, Richard Bowers, Adam Brown, Chad M. Brummett, Alexandra S. Bullough, James F. Burke, Juan P. Cata, Neeraj Chaudhary, Michael J. Claybon, Miguel Cruz, Milind Deogaonkar, Vikram Dhawan, Thomas Didier, D. John Doyle, Zeyd Ebrahim, Hesham Elsharkawy, Wael Ali Sakr Esa, Ehab Farag, Ryen D. Fons, Joseph J. Gemmete, Matt Giles, Phil Gillen, Goodarz Golmirzaie, Marcos Gomes, Lisa Grilly, Maged Guirguis, David W. Healy, Heather Hervey-Jumper, Shawn L. Hervey-Jumper, Paul E. Hilliard, Samuel A. Irefin, George K. Istaphanous, Teresa L. Jacobs, Ellen Janke, Greta Jo, James W. Jones, Rami Karroum, Allen Keebler, Stephen J. Kimatian, Colleen G. Koch, Robert Scott Kriss, Andrea Kurz, Jia Lin, Michael D. Maile, Negmeldeen F. Mamoun, Mariel Manlapaz, Edward Manno, Donn Marciniak, Piyush Mathur, Nicholas F. Marko, Matthew Martin, George A. Mashour, Marco Maurtua, Scott T. McCardle, Julie McClelland, Uma Menon, Paul S. Moor, Laurel E. Moore, Ruairi Moulding, Dileep R. Nair, Todd Nelson, Julie Niezgoda, Edward Noguera, Jerome O’Hara, Aditya S. Pandey, Mauricio Perilla, Paul Picton, Marc J. Popovich, J. Javier Provencio, Venkatakrishna Rajajee, Mohit Rastogi, Stacy Ritzman, Lauryn R. Rochlen, Leif Saager, Vivek Sabharwal, Oren Sagher, Kenneth Saliba, Milad Sharifpour, Lesli E. Skolarus, Paul Smythe, Wolf H. Stapelfeldt, William R. Stetler, Peter Stiles, Vijay Tarnal, Khoi D. Than, B. Gregory Thompson, Alparslan Turan, Christopher R. Turner, Justin Upp, Sumeet Vadera, Jennifer Vance, Anthony C. Wang, Robert J. Weil, Marnie B. Welch, Karen K. Wilkins, Erin S. Williams, George N. Youssef, Asma Zakaria, Sherif S. Zaky, Andrew Zura
- Edited by George A. Mashour, Ehab Farag
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- Book:
- Case Studies in Neuroanesthesia and Neurocritical Care
- Published online:
- 03 May 2011
- Print publication:
- 03 February 2011, pp x-xvi
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