Decrouezina n. gen: Descendant of Ranikothalia in Nummulitacea

Abstract

Decrouezina n. gen. (type species: Assilina praespira var. aegyptiaca Cuvillier 1930) is newly erected from the Upper Libyan Stage (Late Cuisian) at Beni Hassan, Nile Valley, Egypt. It is involute in the earliest ontogenetic stage while most of the test is evolute (operculinoid or assilinoid coiling). It has an enlarged marginal cord, plexus marginalis, transverse trabecules and spindle shaped pillars. Decrouezina carries in the ontogenetic stage the characters of the genus Ranikothalia which decipher the phyletic relationship between the two genera. INTRODUCTION In 1883, Schwager described Operculina cf. canalifera d'Archiac from Gebel Ter in Minia. Cuvillier (1930) remarked that this species is too large to be an Operculina and that it carries whorls which are rather thick; he thus referred it to the genus Assilina and named it Assilina praespira Douville var. aegyptiaca n. var. Strougo et al. (1990) indentified this species as Operculina praespira. Davies (1927) noted (p. 276) the similarities between the Egyptian taxon and Operculina sindensis Davies 1927 (now: Ranikothalia sindensis) with regard to poles, filaments and marginal cord, except that in the former the spire (of six to seven whorls) has the last whorls subequal. Clearly, the generic assignment of the Egyptian form is not agreed upon. Tambareau (1966) described Operculina (Nummulidoides) azilensis from Ariege. She mentioned that the last stage of the test is evolute and considered this species to lie closer to Operculina than to Nummulites. Tambareau (op. cit.) further remarked that the initial ontogenetic involute stage distinguishes azilensis from the classical operculines; she thus assigned it to the subgenus Nummulitoides. Haynes (1981) mentioned that authors have found difficulty in deciding on the generic limits in family Nummulitacea, and he states: more recently Hottinger (1977) has attempted to base generic determination upon the stolon and canal system and goes so far as to claim that the traditional definitions emphasizing the involute or evolute nature of the coiling and presence or absence of secondary septa should be abandoned. Butt (1991) grouped the Ranikothalia species in Pakistan under three types: a) Nummulites like thick forms characterized by the presence of septal filaments and thick marginal cord; e. g. R. nuttalli (Davies) and R. bermudezi (Palmer). b) Operculine complanate forms having evolute expanding spiral with a thick marginal cord and lacking true septal filaments e.g. R. sindensis (Davies). c) Nummulitid forms which appear to be intergrading between the above mentioned groups, e. g. R. sahnii Davies. The above observations of Butt emphasize the fact that some individuals have a tendency to an evolute complanate form externally but still involute in most parts of the test (maturo-evolute of Banner and Hodgkinson 1991). The internal structure of the Egyptian test suggests that it is neither an Operculina nor an Assilina but belongs to a new taxon for which the name Decrouezina n. gen. is proposed. Decrouezina is an evolute form in most part of the test except in the earliest one or two whorls. It may have phyletic relationship with completely involute forms such as Ranikothalia. For the structure of Ranikothalia (= Nummulitoides Abrard 1956 and Chordoperculinoides Ari 1966) see Caudri (1944 and 1975), Abrard (1956), Ari (1966), Tambareau (1966), Butterlin and Monod (1969), Blondeau (1972), Van Hinte and Wong (1975) and Hottinger (1977). The Egyptian material of Decrouezina aegyptiaca is collected form beni Hassan, Nile Valley (text-fig. 1). SYSTEMATIC PALEONTOLOGY Genus Decrouezina Boukhary n. gen. Type species: Assilina praespira var. aegyptiaca Cuvillier 1930 Etymology: After Danielle Decrouez, Museum d'Histoire Naturelle, Geneva, Switzerland. Diagnosis: Decrouezina is a relatively large forminiferal genus belonging to the Nummulitacea, the test is flat and compressed. It has an elevated periphery surrounding a depressed flat spire, giving to the test the appearance of a disc depressed in the middle. The relief of the spiral suture is strong due to a thickening or protrusion of the marginal cord. Transverse trabecules are present; granulations and papillae are noticed in the polar region and are visible in axial sections; the filaments are not continuously connected and do not pass over the marginal cord in most part of the test due to the planispiral evolute nature, except in the early involute part of the test where they are radiate. Test planispiral involute in earliest whorls (1-1), evolute in later part of test. In the microspheric generation, the spire consists of six to seven evolute whorls in which the last three whorls are subequal. Marginal cord prolonged into a gutter comparable to that micropaleontology, vol. 40, no. 1, pp. 95-99, plate 1, text-figures 1-2, table 1, 1994 95 This content downloaded from 157.55.39.215 on Tue, 30 Aug 2016 06:19:09 UTC All use subject to http://about.jstor.org/terms Mohamed Boukhary: Decrouezina n. gen: descendant of Ranikothalia in Nummulitacea