Cycloclypeus in the Mediterranean oligocene

Abstract

Fossil assemblages have been studied of the larger foraminiferal genus Cycloclypeus of the family Nummulitidae. The specimens were collected from 28 Oligocene samples from Spain, Italy and Israel. The external and internal morphologies of the tests were examined. In a number of samples a distinct and continuous variation is present from smooth specimens to specimens showing a well developed sculptural relief. Specimens of a restricted number of samples were grouped into classes according to the height of the ornamentation. After sectioning of the specimens the internal morphology was examined. Several parameters were counted and measured, including two parameters for the size of the embryonic chambers, one for the number of preyclic chambers, one for the number of spiral convolutions and one for the maximum diameter of the precyclic stage. In one sample the ontogenetic development of specimens was studied by measuring a number of parameters in successive precyclic chambers. The morphometrical data on internal morphology were treated statistically. Data show that in the Mediterranean region the stratigraphic range of Cycloclypeus comprises the entire Upper Oligocene (P211N2 - P22/N3; NP24 - NP25), possibly extending downwards into the uppermost part of the Lower Oligocene. In the lower and middle parts of its range the genus is represented by the species of the droogeri-lineage, in wich the number of the precyclic chambers is reduced from an assemblage average of about 30 to one of about 16. In the upper part of the Upper Oligocene the representatives of this lineage were replaced by the immigrant species Cycloclypeus eidae. The assemblages of this species are very similar to those reported from the Indo-Pacific province and in one sample even the mixing of two types of megalospheres was recorded, which is typical of the Indo-Pacific occurrences. In the Mediterranean region the genus probably became extinct at the very end of the Oligocene. Data indicate that nepionic evolution in Cycloclypeus primarily concerns the reduction in the size of the precyclic stage. This resulted in the reduction of the number of nepionic chambers, representing the number of growth-steps necessary to reach this nepionic size. In data on internal and external morphology there is no evidence for the existence of two separate contemporaneous lineages in the Mediterranean Oligocene, which would differ in external morphology. Differences in ornamentation of specimens are ascribed to differences in the depth at which specimens lived before their remains were washed together and transported down to the depth of deposition. By their glassy pustules specimens living at greater depth probably made more efficient use of of the restricted lightintensities, enabling the photosynthetic activity of the symbionts living within their tests. In a number of samples it has been checked whether groups of specimens differing in ornamentation and therefore in average depth of habitat, also differ in internal morphology. The results are compared with data on recorded and alleged depth-clines in other groups of living and fossil larger foraminifera, which data are critically reviewed. It is concluded that in the droogeri-lineage a depth-cline in embryon-size was present. It basically consisted of a modest size-increase with increasing depth of habitat, which is thought to have been related to waning light-intensities. Locally or regionally, however, elevated levels of productivity may have induced an increase in average embryon-size in some middle part of the depth range, which was superimposed on the trend of sustained increase with depth. In the Cycloclypei from Israel clinal variation in the number of precyclic chambers is mainly ascribed to the depth-related variation in embryon-size. In European Cycloclypeus a reduction in the number of precyclic chambers is apparent with increasing depth of habitat. This is ascribed to the concurrent reduction in the size of the precyclic stage. Relations between clinal variation and evolutionary development in embryon-size and in nepionic configuration are discussed. Evolution and clinal variation are interpreted in terms of light-intensity, growth-activity and lifestrategy. It is suggested that clinal variation acted as a driving force in the process of nepionic evolution in European Cycloclypeus.