Cerion revisited

Some months ago Jerry Harasewych published a new paper on Cerion (Harasewych, 2012), in which he described a new fossil species and presented a hypothesis about the phylogeography of the family.

Cerion petuchi, new species, the first record of the genus from the Pleistocene of Florida, is described from Loxahatchee, Florida, from deposits of the Loxahatchee Member of the Bermont Formation (Aftonian Pleistocene). This new species is more similar to Cerion agassizii from the Pleistocene of the
Great Bahamas Bank, and to the Recent Cerion incanum saccharimeta, from the Florida Keys, than to either of the species from the Late Oligocene—Early Miocene Ballast Point Silex Beds of Tampa, Florida. Data on the geographic distribution and geological age of all known cerionids is compiled and, together with models of the geologic and tectonic history of the Caribbean region, used to construct a hypothesis of the origins, ages, and relationships among the various lineages within the family since the Upper Cretaceous. The early distribution of the family was governed primarily by overland dispersal and vicariance. It is suggested that the significant proliferation of diversity that began during the Pleistocene is due to the increased prevalence of stochastic dispersal of small propagules (either by rafting or hurricane-born) among the islands of the Cuban and Bahamian archipelagos. Sea level changes caused by Pleistocene glaciations amplified diversity by repeatedly and sequentially recombining / hybridizing and isolating neighboring
populations. Amplification of diversity would have been greatest on the Great Bahama Bank, as it had the highest number of islands that were isolated during interglacial periods and conjoined during glaciations.

This hypothesis may be interesting to work with in families or groups that show a similar distribution in Florida, the Caribbean and Central America, and (northern) South America. However, it must be noted that this model draws heavily on the work of Iturralde-Vinent, and among geologists there is still debate about the different geological hypotheses of the Caribbean region.

Recently, he presented also research on populations of Cerionids in the Florida Keys (Harasewych & Shrestha, 2013). This work is partially underpinning the paper mentioned above, and was used to determine the spatial scale and patterns of genetic diversity across the entire range of Cerion incanum, and to correlate these patterns to the geology and origin of the Florida Keys.

References:
Harasewich, M.G., 2012. The fossil record and phylogeography of the family Cerionidae (Gastropoda: Pulmonata), with the description of a new species from the Pleistocene of Florida. – The Nautilus 126: 119–126.
Harasewich, M.G. & Shrestha, Y., 2013. Patterns of genetic relatedness among populations of the genus Cerion (Cerionidae: Gastropoda: Mollusca) in the Florida Keys. In: Frias Martins, A. de et al. (eds.) World Congress of Malacology 2013. Book of Abstracts: 83.

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