Evolutionary aspects of the Miogypsinids in the Oligo-Miocene carbonates near Mineo (Sicily)

Abstract

The investigation is based on a well exposed sequence of 120 to 130 metres of bioclastic limestones, located at the northwestern edge of the Hyblean Plateau on Sicily, 2 to 4 km NE of the village of Mineo. The carbonates were deposited in a shallow, open marine environment during the Chattian to Burdigalian time interval. Tests of the larger foraminifera Miogypsina are abundant in the lower and upper parts of the carbonate succession. The internal morphology of these tests has been thoroughly investigated. Biometric research reveals that both the oldest and the youngest species of the Miogypsina lineage known from the Mediterranean province, are present in the Mineo carbonates. These species are M complanata and M mediterranea respectively. The morphometric series of Miogypsina is not complete, however; sedimentary hiatuses and changes in the local environment caused breaks in the biorecord of the Miogypsinids. The absence of the globally widespread M. globulina is remarkable. Long records have been found for the Aquitanian M. gunteri and for the Burdigalian M intermedia and M. cushmani. The Early Miocene associations of Miogypsina s.s. are often accompanied by associations of Miogypsinoides. Most of these are close to M bantamensis. On the basis of the mean embryon size two types of assemblages of Miogypsinoides could be distinguished in the Aquitanian sediments. Type I with the smaller embryon (D1 = 110-125 p,m) resembles associations reported from African localities and type II with the larger embryon (D1 = 210-230 p,m) conforms to associations known from the European Miogypsinoides stock. Type II is restricted to the Aquitanian, whereas the range of type I extends into the Burdigalian. The data set for the main lineage of Miogypsina exhibits a distinct overall change in morphology of the nepiont, which change is in agreement with the 'principle of nepionic acceleration' as defined by Tan Sin Hok. Detailed information on the Burdigalian Miogypsina s.s. shows that this trend is not the result of a gradual unidirectional change, but that the course was staggered. In smaller stratigraphical intervals the nepionic variable V may exhibit stasis or shifts opposite to the overall positive trend. Part of the observed changes in the nepiont may have been caused by variations in sediment accumulation rates or in a variable degree of reworking of older faunal elements. No evidence has been found for a direct control of the environment on the configuration of the nepiont. There is no proof for a functional dependence of the nepionic variables on the changes in mean embryon size; in the literature the latter size is sometimes related to some depth-linked environmental factor. A substantial part of the morphological changes in Burdigalian Miogypsina s.s. are associated with intervals, in which this subgenus is present in low relative frequencies. This suggests that the evolution of Miogypsina s.s. was controlled by bottlenecks in the size of the standing stock. By contrast with Miogypsina s.s. no net change has been observed in the Burdigalian sequence of Miogypsinoides assemblages; stasis prevails inthis subgenus.