Implication of the Occurrence of Minute Biotic Bodies on the Conjoined Nummulites aff. Nummulites acutus (Sowerby) in the subsurface Eocene of Cauvery Basin, India

Conjoined and aberrant adult tests of Nummulites aff. Nummulites acutus (Sowerby) from the subsurface Middle Eocene sediments south of Pondicherry in Cauvery Basin, India show in situ development of minute biotic bodies as rare and unusual incidence. Since usual test of extinct as well as modern nummulitids hardly shows such development, these bodies are studied to know their nature of relation with the host adults. Being indigenous, intact and regular in shape, these forms have diverse mode of occurrence from cluster of embryo-like tiny featureless globular bodies to discrete lenticular bodies with granulated surface and as minute lenticular juvenile tests. The rudimentary and pre-mature morphologic features in the juveniles appear septal filaments, involute spiral sheets, planispiral whorls, quadrangular to rectangular chambers, narrow alar prolongations, marginal cord and septa, whose mature counterparts typify adult Nummulites; therefore, these are the offspring of the adult Nummulites aff. acutus. Their remarkably small size compared to the size of the host test indicates young age whereas their embedded emplacement at crucial morphologic parts of the host test points for a genetic relationship between adult and ‘baby’. With minute initial chamber (<1 µm) followed by several whorls, the offspring resemble microspheric juveniles. Their persistent development only in the ‘conjoined’ and ‘separated’ adult tests indicate that test abnormality is linked with their origin. An entity of conjoined Nummulites aff. Nummulites acutus comprises two megalospheric adult individuals of the species with fused apertures, and resembles ‘plastogamy’ in modern smaller benthic foraminifera, in which two adult individuals of a species unite for sexual reproduction. As in the latter, the two individuals in conjoined Nummulites are fused using secreted lime deposit as adhesive. Since modern larger foraminifera including nummulitids are not known to develop conjoined entity, taking a cue from plastogamy, sexual reproduction is held responsible for the development of the offspring in Nummulites aff. Nummulites acutus. This is supported by the occurrence of minute bodies inside the cavities, pores, and fused apertures and along the fused contact zone, and resemblance of a studied offspring with the offspring of modern nummulitids. The study indicates that sexual reproduction by adult individuals was prevalent in extinct Nummulites aff. Nummulites acutus to generate microspheric forms. This particular life cycle is supposed to prevail in the species towards the end of the Middle Eocene.