New nummulite (Foraminiferida) species from the Eocene of Northern Oman

Three new species of nummulites; Nummulites minutus sp. nov., N. omanensis sp. nov. and N. schaubi sp. nov., are described and illustrated from the Eocene of Northern Oman. N. omanensis sp. nov. and N. schaubi sp. nov. are shown to range from Early to Middle Lutetian whilst N. minutus sp. nov. ranges from Late Ypresian to Early Lutetian. N. minutus sp. nov. is far smaller in all major dimensions than any species of Nummulites previously described and if found in isolation would be assumed to be very primitive and probably dated as Late Palaeocene. However, N. minutus sp. nov. was found in association with an unreworked Lutetian fauna. The commonly held belief that proloculus size and test size are smallest in the most primitive (i.e. oldest) species must therefore be treated with some degree of caution.


INTRODUCTION
During a detailed study of the nummulitid fauna of Northern Oman some sixty-six species of nummulitid, including 38 belonging to Nummulites, were identified, described and illustrated (Racey 1988, Racey, in press). Of these, three are new and constitute the basis of this paper. All three species Ivere found in Early Lutetian shallow marine, ramp limestones of the Seeb Limestone Formation (formerly called the Dammam Formation, a term widely used throughout the Arabian Peninsula) at Wadi Rusayl in Northern Oman (Fig. 1). One of the species, N. minutus sp. nov. was also found in the southern foothillsatWadiBaniKhalid ( Fig. 1). Theassociatednummulite and assiline fauna is shown in Fig. 2. Previous work on the Tertiary nummulitids comprises a single paper by Montenat et. al. (1977) who cited nineteen species of Nummulites and three of Assilina, of which only three species were illustrated and none described.

SYSTEMATIC PALAEONTOLOGY
For the purpose of this study the following generic definition (from Blondeau, 1972 andSchaub, 1981) has been used.

Remarks
This section is used to :-draw attention to the differences between morphologically similar species and to discuss taxonomic and nomenclatural problems.

Faunal Associations and Stratigraphic Range
Faunal associations refer to those taxa which occur with the new species in Oman, and are restricted to the associated nummulitids.   A-form. Test very small, inflated lenticular, biconical with a rounded periphery. Septal filaments simple, radiating, narrowly spaced and gently curved; polar pillar present (0.14 mm in diameter at surface P1.1 fig. 6). Spire regular opening uniformly ( Fig. 3; P1.1, figs. 6 -7) ; chambers regular, compact isometric to slightly higher than long; septa straight to slightly inclined or curved. Marginal cord uniform < 1/3 chamber height. Proloculus very small, 0.06 to 0.08mm in diameter. ( Test lenticular with a rounded rarely flexed periphery. Septal filaments radiating, slightly flexed, tending to become slightly subreticulate towards the periphery; pillars arranged spirally on marginal cord (seen on decorticated specimens) and over remainder of test on and between the septa1 filaments. Spire regular, loosening slightly after the fifth/sixth whorl, and tightening in the last few whorls of larger individuals, forming a slight tripartition of the spire (Fig. 5; P1.1; fig. 5); chambers subrectangular to slightly falciform, 1.5 x higher than long in the early whorls becoming 2-2.5 x longer than high in the outer whorls ( Pl. 1 fig. 4); septa compact, regular, inclined and gently curved in the early whorls becoming more curved, inclined and widely spaced in the outer whorls. Marginal cord generally 1 /2 -1 /3 of chamber height, often thicker in the middle whorls (P1.1, fig. 4). (27 measured  Remarks. The spiral arrangement of pillars, and the fairly compact, regularly inclined and curved septa with slightly falciform septa in the initial whorls suggest that this species belongs to theN. partschiGroup sensu Schaub (1981). However, the spire is markedly tighter than in any other member of this group. In addition, the development of slight subreticulation and lengthening of the chambers in the outer whorls suggests partial affinities with the laeuigatus and/or brongniarti groups of Schaub (1981).  Fig. 5. B-form, equatorial section (slide P52427, holotype), X7; Fig. 8. B-form, equatorial section (slide P52428, paratype), X7; Fig. 9. B-form, axial section, (slide P52431, paratype), X5; Fig. 10, B-form, oblique section, parallel to equatorial plane, note crude spiral arrangement of pillars, (slide P52430, paratype), X5.  Description. B-Form Test flattened lenticular, with a fairly thin, rounded to sharp, slightly flexed periphery. Septa1 filaments S-shaped to slightly swirling; pillars on and between septa1 filaments, occasionally elongated along them, less abundant towards the periphery. In axial section, pillars are long, thin and concentrated in the polar regions, rarely reaching the last whorl (PI. 1, fig. 1). Spire uniform, tight and regular with occasional irregularities in the outermost whorls (Pl. 1, fig.2; Fig. 6); chambers regular, fairly numerous, initially 1.5-2 x higher than long, rectangular or slightly falciform becoming up to 2 x longer than high after the tenth/eleventh whorl; septa compact, initially straight to slightly curved, less compact and more curved in the outer whorls. Marginal cord thick, up to 1/2 chamber height.

Dimensions in m m (23 measured specimens)
Max

A-Form Not Found
Remarks. These specimens appear to be more evolved species of N . arnii as figured by Schaub (1981, pl. 47, figs. 31-37) from the basal Lutetian of Libya. They differ from N. arnii in that they are larger, possess more whorls and have a tighter spire. Faunal Associations and Stratigraphic Range. The probable precursor to this species, N . arnii, has been recorded from the Basal Lutetian of the Syrte Basin in Libya. N. schaubi sp. nov. is found in Oman in association with N. beneharnensis, N. gizehensis, N . obesus, N . discorbinus, N. omanensis, sp. nov., N. cuvillieri, Assilina spira abrardi, A papillata, A. spira indicating a late Lower Lutetian to early Middle 1 Lutetian age (Fig. 2).

DISCUSSION
1. Three new species of nummulite are described from the Early to Middle Lutetian part of the Seeb Limestone of Northern Oman.

2.
In terms of the palaeobiogeographic importance of these new species and the associated nummulitid fauna, the following points are of note : a) The Middle Eocene nummulitid fauna of Northern Oman shows a pronounced mixing between Western Tethyan and Eastern Tethyan species and, more exceptionally Indo-Pacific species. This mixing became most pronounced in the latter part of the Middle Eocene (Late Lutetian) (for a full discussion see Racey, in press and Racey 1988).
(b) It is interesting to note that the new, possibly endemic species described herein developed during the period of maximum species diversity development in the Oman nummulitid fauna, immediately prior to the pronounced mixing of Eastern and Western Tethyan and Indo-Pacific faunas which followed in the Late Lutetian.
3. ThepresenceofN. minutus sp.nov. inunreworkedLutetian sediments is difficult to explain in the context of the generally accepted views of the evolutionary development of the genus. It is easy enough to understand how such a small species could easily be "overlooked", in that it is common practice to use the larger more easily recognised and better described 8-forms for the biostratigraphic dating of nummulite-bearing sequences. N. minutus sp. nov. is smaller in all major dimensions by a factor of approximately50%whencompared with thesmallest (most primitive) species previously described. The presence of this small new species from the Lutetian raises a number of interesting questions: (a) Could this new species be a stunted variety of morphologically similar Lutetian species suchas N. discorbinus? This is unlikely since none of the associated nummulite species show any evidence for stunting, with the palaeoenvironment suggesting optimal conditions for nummulite development both in terms of numbers and species diversity.
(b) Could N. minutus sp. nov. be part of a trimorphic life cycle of another already described nummulite species as observed for the living nummulitid Heterostegina depressa by Rottger (1987)?. Trimorphism in the fossil record would be virtually impossible to detect and to the best of the author's knowledge has not been described for any fossil nummulitid; consequently it will not be discussed further.
(c) Could N. minutus sp. nov. be a true "primitive" species which has simply not evolved? Although sufficiently morphologically similar to primitive Late Palaeocene species to have been potentially ancestral to such species, N. minutus has notbeenfoundinOmanbelow theLateYpresian(0r above the Early Lutetian) despite extensive studies of nummulite-bearing Late Palaeocene to Priabonian limestones. The range of this species given herein as Late Ypresian to Early Lutetian is therefore considered to be close to the probable absolute range for the species (at least in Oman).
(d) Are we simply wrong to assume that such small species are primitive and therefore misguided in assuming that the two dominant trends in nummulite evolution from Late Palaeocene to end Middle Eocene are an increase in test size of B-forms and an increase in proloculus size in A-forms (with a general, but not so marked increase in test size)?. Such generalizations only really hold for individual lineages and cannot therefore be applied to all species of similar age. For example, A-forms of N. discorbinus (N. rotalarius lineage) from the Middle Lutetian of Oman have test diameters of 2.5-3.6mm and proloculus diameters of 0.15-0.3Omm whilst coeval N. gizhensis (N. partschi lineage) A-forms, from the same bed, have test diameters of 3.67-6.41mm and proloculus diameters of 0.66-1.25mm. N. minutus sp. nov. cannot be easily incorporated in any existing lineages and therefore is most likely to be part of a new lineage.