The taphonomy of gastropod shell accumulations in large lakes: an example from Lake Tanganyika, Africa

Abstract

An investigation of shell morphometrics of the prosobranch genusParamelaniafrom Lake Tanganyika shows striking contrasts between some live and dead populations from identical localities. Living populations and surficial dead shells were collected from 15 sublittoral-profundal localities along the east side of the lake. Interpopulation variability in this endemic gastropod is clinal (N-S) for several shell characters. Intrapopulation variability of dead shell populations frequently exceeds that of live populations. Distinctive morphs may be present in a local dead shell population which are absent in the live population from the same locality. However, the phenodeviant dead shell morphology may occur among live snails elsewhere in the lake. Phenodeviant shells may be encrusted, but are unabraded and show no preferred orientation.Radiocarbon ages of >2000 yr b.p. on dead phenodeviants suggest that surficial shell accumulations in Lake Tanganyika are strongly time-averaged. Although wave activity and biogenic concentration are important processes in Lake Tanganyika, neither is sufficient under modern lake conditions to account for these admixed assemblages. Winnowing andin situstratigraphic condensation during periods of lowered lake levels is the most likely explanation for the occurrence of phenodeviant surficial shells. This hypothesis is consistent with the observation that extensive shell lags are exposed on the lake floor in areas currently inhabited by the same snails.A model of biogeographic range fluctuations for particularParamelaniamorphs during the Holocene, in concert with lake level fluctuations, can account for the shell assemblages seen on the lake floor today. The complex history of these accumulations suggests that taphonomic admixtures may obscure the interpretation of evolutionary sequences in the lacustrine stratigraphic record.