Discocyclinidae from Ein Avedat (Israël)

Abstract

Approximately 300 samples were collected from Lower to Middle Eocene sediments of the Avedat Plateau (Central Northern Negev, Israel). Fielddata, microfacies data - qualitative as well as quantitative with respect to PIB ratios and frequency distributions of larger foraminifera - and the analysis of some 30 samples with smaller benthonic foraminifera resulted in a detailed reconstruction of the northern foreslope of the Avedat plateau with special reference to the paleo-bathymetry of autochthonous and allochthonous sediments. Biometric data on the outer and inner morphology of Asterocyclina from about 30 samples and the inner morphology of Discocyclina from 17 samples were analysed. The outer morphological data of Asterocyclina suggest an irregular increase of the test size in the course of the time combined with a relative flattening of the test. The inner morphological data prove distinct heterogeneity in Asterocyclina and Discocyclina. In Asterocyclina two groups are recognized on the basis of the size of the embryon and the degree of enclosure of the embryonic chambers (R). The data suggest an increase in the course of the time of R and the size of the embryon. These parameters serve to subdivide the assemblages into biometric units: A. taramellii and A. stella in the group with a small embryon; A. cuvillieri and A. sp. in the group with a large embryon. Furthermore another "species" cf. Asterophragmina pagoda is separated on the basis of the reduction of radial chamber walls in the median layer. In Discocyclina three groups are recognized on the combinations of outer morphological characters and inner morphology. These are D. varians s.1., D. archiaci and D. sp. None of them shows evolutionary trends, but there are extreme differences in morphology between different samples. The depth profile, the frequency distributions of the larger foraminifera and the morphological characters of A. taramellii, A stella and D. varians s.l. were compared. Asterocyclina shows a preference for the upper foreslope environment. Discocyclina is more frequent in the lower foreslope. The large overlap in depth range suggests a further niche segregation by means of other environmental parameters, possibly by means of substrate or vegetation dependency. In Asterocyclina, and possibly also in Discocyclina the test becomes more flattened with increasing depth. The evolution within Asterocyclina is considered as an example ofquantum speciation. The data suggest that with the change of A. taramellii to A. stella a change of habitat occurred. D. varians is considered as an "elastic" species, which may adapt to widely different subenvironments and show corresponding morphological variation. A hierarchy of morphological features is proposed, which may be used for discrimination on family, subfamily, genus and species level within the Discocyclinidae. Lower ranked features become increasingly environment controlled and may provide information on ecology. The origin of the Discocyclinidae is discussed. It is suggested that they are derived from peripheral isolates of Lepidorbitoides minor and L. socialis which passed the Cretaceous-Tertiary boundary. A subsequent radiation, possibly via A. taramellii and D. augustae resulted in at least three different lineages in the subfamily Asterocyclininae and at least two in the subfamily Discocyclininae, each with its own rate of evolution.