EVOLUTION AND PHYLOGENETIC RELATIONSHIPS OF NEOGENE CORBULIDAE (BIVALVIA; MYOIDEA) OF TROPICAL AMERICA

Abstract

We used 24 fossil and Recent species to construct character states of both composite and exemplar taxa for phylogenetic analyses of Neogene genera and subgenera of the Corbulidae from tropical America. All characters were conchologic and two matrices, which differed in the manner that commarginal-rib characters were coded, were analyzed using branch and bound searches and maximum parsimony. Character polarity was determined using Corbula sensu strictu as an outgroup. These analyses produced a limited number of robust and well-resolved cladograms that require only one ghost lineage. Such stable results indicate a high level of congruency among characters, and demonstrate that conchologic data sets can yield highly resolved cladograms.Tropical American corbulids are not monophyletic, and include two major clades. Crown groups within these clades are endemic, and all genera endemic to tropical America first appear in the Miocene. In fact, generic diversity and body size peak in the upper Miocene of the Caribbean/western Atlantic. Range restrictions and extinctions of large-bodied genera from both corbulid clades contribute to a post-Miocene decline in body size in this region. The eastern Pacific does not experience a similar decline in diversity and body size. Diversity and morphologic trends in Caribbean corbulids coincide with regional environmental changes, in particular decreases in seasonality and productivity. Except for the extinction of Bothrocorbula, however, corbulid extinctions apparently predate faunal turnover reported for other molluscs.Associated with these extinctions, we found evidence of geographic range restriction, but not range shifts, in corbulid genera, indicating that the geologic development of environmental refugia contributed more to survival than eurytopy. Large-bodied genera of the southern Caribbean Gatunian Province (Tenuicorbula, Panamicorbula, and Hexacorbula) became restricted to the eastern Pacific. Range restriction to this high productivity refugium (i.e., paciphilic genera), rather than origination of new taxa, produced several corbulid genera now endemic to the region. Large-bodied genera from the Caloosahatchian Province of the southeastern U.S. (Bothrocorbula and Bicorbula), however, underwent global extinction. These subtropical and warm temperate taxa are presumably more eurytopic than their tropical counterparts, but do not exhibit range shifts in response to Neogene environmental change.