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ABSTRACT-The earliest known members of the Thais clade of rapanine muricid neogastropods comprise four species from the Cantaure Formation (early Miocene: Burdigalian) of Venezuela; three of these species are new. Neorapana rotundata Gibson-Smith et al., 1997, is most closely related to the Recent Pacific Mexican N. tuberculata (Sowerby, 1835), and represents the only known Atlantic occurrence of the genus. Thais brevicula new species is closest to T. callaoensis (Gray, 1828) from the Recent of northern Peru and the Galapagos. A review of the genus Thais indicates that the typical members of this group occur in the South Atlantic, West Africa, and eastern Pacific, but not in the Recent fauna of the southern Caribbean. Stramonita bifida new species is a large species related to the Recent S. haemastoma floridana (Conrad, 1837), which occurs throughout the Caribbean. A review of American species of Stramonita indicates that the taxon S. biserialis (de Blainville, 1832) from the Recent fauna of the eastern Pacific, and the taxon S. h. haemastoma (Linnaeus, 1767), may each be composed of more than one species despite the teleplanic dispersal of their larvae. Stramonita semiplicata new species is closely related to the Recent S. bicarinata (de Blainville, 1832) from the South Atlantic, and represents a lineage that occurred in the Caribbean region until at least the late Miocene. It may have given rise to the eastern Pacific genus Acanthais. The higher diversity and greater antipredatory specialization of eastern Pacific as compared to western Atlantic members of the Thais clade may have resulted from higher post-Miocene rates of speciation and lower extinction rates in the eastern Pacific.
INTRODUCTION
MEMBERS OF THE Thais clade of rapanine muricids are common predatory neogastropods on tropical rocky and mangrove shores worldwide. In tropical America, eleven eastern Pacific and eight western Atlantic species occur in the Recent fauna, with up to nine and four species respectively occurring sympatrically in the two regions. Some genera, such as Stramonita, Thais, and Thaisella, are represented by closely related counterparts on the Pacific and Atlantic sides of tropical America; whereas others-Acanthais, Neorapana, Tribulus, and Vasula-occur today only in the eastern Pacific. A species-level account of the ecological and biogeographic history of this clade must incorporate the origins, pattern of speciation, and geographical restriction in...