Melanopsis from the Pleistocene site of 'Ubeidiya, Jordan Valley: direct evidence of early hybridization (Gastropoda: Cerithioidea)

'Ubeidiya is a 1.4 Myr-old Lower-Pleistocene site in the Jordan Valley. Its rich Melanopsis fauna was found to include five species: buccinoidea , costata, turriformis , phanesiana obediensis and multiformis . These species and their conchiometrics are described. M. buccinoidea and costata occur in the Levant also today whereas turriformis , p. obediensis and multiformis are extinct. These findings suggest considerable faunal connections between the Jordan, the Orontes and perhaps also Rodhos during the lower Pleistocene. During the late Miocene, when the Mediterranean Sea level dropped, coastal-plain freshwater habitats could have expanded and Melanopsis species could then have invaded the Rift Valley via coastal rivers. In addition they could have reached later, via the Rift Valley. Species frequencies of Melanopsis throughout 'Ubeidiya suggest three' Melanopsis landscapes': an early period dominated by buccinoidea , a later one by costata and an upper one by p. obediensis . It is reasonable to speculate that the buccinoidea landscape represents a spring or perhaps a deep lake with a nearby stream; the costata landscape a shallow, nonturbulent, well-aerated lake with a slow current; and the p. obediensis landscape a turbulent, well-aerated lake. Intermediates were found between buccinoidea and costata . Their low frequency and their chrono-distribution suggest they are hybrids, rather than evolutionary transitions between species. The 1.4 Myr old fossils of 'Ubeidiya may be the earliest direct evidence of hybridization among molluscs in nature, that is still going on today in the same region and aquatic system, among the same species. The Melanopsis hybrid populations may perhaps act as filters that prevent the introgression of most genes, but allow others to disperse into the range of the parent species. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 39–57.