Embryo size variation in larger foraminiferal lineages: stratigraphy versus paleoecology in Nephrolepidina praemarginata (R. Douvillé, 1908) from the Majella Mt. (Central Appennines)

Abstract

The Mediterranean Nephrolepidina lineage has been thoroughly investigated in a number of studies. Here, we investigate biometrically two late Rupelian Nephrolepidina populations (E77’, n=54; E76J, n=9) from the northern Majella Mt. (Central Italy), considering three parameters and three factors and their mean values. The Ai factor (degree of enclosure of the deuteroconch on the protoconch) and the parameter C (number of adauxiliary chamberlets) suggest that both populations belong to N. praemarginata (R. Douvillé, 1908). The mean sizes of protoconch and deuteroconch are distinctly larger than other known populations of N. praemarginata. This is interpreted to reflect environmental factors. Increase in embryo size is a general feature among many larger foraminiferal lineages. In addition, studies on recent larger foraminifera indicate that embryo size varies along the depth gradient; however, this variation is poorly investigated. In recent nummulitids, the diameter of the embryonic chambers may either increase up to the ecological optimum and then decrease, or increase linearly with depth. Investigating embryo size may thus be rewarding either in a sequence of populations when sea-level change occurs or when anomalously large values of embryo size are attained in a population. Utilizing evidence from models derived from recent species, although it is uncertain how far they can be extended to extinct radial foraminifers, and sequence stratigraphy, it is inferred that the two populations come from a depth not far from the ecological optimum of the species. Recognition of this ecological optimum in the fossil record is generally hampered by conspicuous transport and mixing along the depth gradient.