Contributions to Zoology, 78 (3) – 2009Geerat J. Vermeij; Han Raven: Southeast Asia as the birthplace of unusual traits: the Melongenidae (Gastropoda) of northwest Borneo

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Discussion

The melongenids we have described in this paper exhibit two features that are otherwise unknown in the family, namely, an adapical septal wall in the aperture (Melongena murifactor) and an erect inner lip (Pugilina erecta and P. ickei). Although the latter feature is common in many gastropod families (see above under P. erecta), it is unknown in any other living or fossil melongenids. The septum of M. murifactor is unique among gastropods.

Phylogenetic and geological studies (Wilson and Rosen, 1998; Meyer, 2003; Wilson and Vecsei, 2005; Frey and Vermeij, 2008; Renema et al., 2008; Williams and Duda, 2008) indicate that the shallow-water marine faunas of southeast Asia underwent dramatic diversification during the early Neogene, as the area of shallow habitat increased by a factor ten and as productivity due to upwelling and runoff increased. Much of the distinctive character of shallow-water Indo-West Pacific faunas, including the high incidence of shell defenses in molluscs, dates from this expansive phase. These conditions — intense predation and competition, high productivity, and extensive habitats — are ideal for the evolution of adaptive novelties (Vermeij, 2003). Similar favourable conditions may have existed elsewhere during this time interval as well, but the adaptive departures and specializations seen in central Indo-West Pacific faunas of the Neogene are unrivaled in other regions. We therefore suggest that southeast Asia was exceptionally favourable to the establishment of traits that expanded the phenotypic range of the clades in which they arose. Vermeij (2001) already pointed to this time interval in southeast Asia as a time and place in which many gastropod lineages (at least four in greater southeast Asia) independently evolved a labral tooth, a structure on the outer shell lip that speeds up predation on hard-shelled prey.

Among species of the gastropod genus Nerita, a partially enveloped shell (subgenus Amphinerita) and the sand-dwelling habit (subgenus Linnerita) are unique to the Indo- West Pacific, with envelopment being confined to its inner portions (Frey and Vermeij, 2008). Preliminary data from a survey of Neogene clade-specific innovations of molluscs by one of us (GJV) indicate that 32 ecologically or morphologically unique innovations evolved in the Indo-West Pacific, of which at least 15 are restricted to greater southeast Asia. Only ten comparable innovations have been identified for the Neogene to Recent time interval in the tropical Atlantic and eastern Pacific. Further work on this topic is in progress and will be presented in a future paper.