BODY-SIZE VARIABILITY AND A SANGAMONIAN EXTINCTION MODEL FOR AMBLYRHIZA, A WEST-INDIAN MEGAFAUNAL RODENT

The megafaunal rodent Amblyrhiza inundata from Anguilla and St. Martin is often cited in lists of late Quaternary human-induced extinctions, but its date of disappearance has never been established. Here, we pr esent a suite of uranium-series disequilibrium dates from three indepe ndent Amblyrhiza sites in Anguilla, all of which cluster in marine iso tope Stage 5. Thus, there is no indication that Amblyrhiza survived in to the late Holocene, when islands of the northern Lesser Antilles wer e first invaded by humans. We argue that the most probable cause of th e extinction of Amblyrhiza was a failure of Island populations to adju st to catastrophic reductions in available range which accompanied las t interglacial sealevel maxima. We support this argument with quantita tive extinction probability estimates drawn from persistence time mode ls. Amblyrhiza exhibits body-size hypervariability, a common but under emphasized feature of island megafaunal species. We argue that hyperva riability is a record of morphological response to oscillating natural selection, which in turn is driven by asymmetries in the relationship of population size, body mass, and persistence time. The fate of Ambl yrhiza stands in marked contrast to that of most other West Indian lan d mammals, whose losses increasingly appear to have been anthropogenic ally mediated. (C) 1998 University of Washington.