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What is Nummulites lyelli? Evolution in large foraminifera during the Middle Eocene, Egypt

Department of Geology, Faculty of Science, Ain Shams University, Cairo, Egypt, email: mboukhary{at}hotmail.com

Nummulites lyelli D’Archiac and Haime 1853, is described for the first time from photographs of both generations, using material from the species type locality at El-Mishgigah, in the El-Gharaq Formation exposed in the Fayum Depression. Nummulites praelyelli n.sp., described from the Mokattam Formation below the Citadel of Salah El Din, in the eastern part of Cairo, is interpreted to be the ancestor of Nummulites lyelli Archiac and Haime. The main ontogenetic trend in this newly identified lineage is a gradual change from tight to lax coiling during growth, whereas the opposite onotgenetic trend is found in the lineage of Nummulites gizehensis (Forskål) In this work, the group of Nummulites gizehensis and allied species is separated from the group of Nummulites lyelli, partly on this basis but definitively on the basis of granulation and septal morphology. We suggest that at the beginning of the Bartonian, Nummulites lyelli ecologically displaced the Late Lutetian Nummulites gizehensis in both space and time, rather than evolving from it.