Campanian-Maastrichtian Rhapydioninidae in the Mediterranean area: Genera Murciella , Sigalveolina n. gen., and Cyclopseudedomia

[摘要] This is the third and last part of a review of the Rhapydioninidae, benthic foraminifera that inhabited almost alone the most central areas of carbonate platforms of the Mediterranean realm. A global schematic table (Fig. 18 , in addition to Figs. 15-17) shows their regional development. The stratigraphic succession of this family seems to have been dominated by three periods of emersion (one being documented, the others only inferred) that limit the time distribution of most of these taxa. Thus it is doubtful that this distribution can be used with certainty in other areas; for example the mention of " Cuvillierinella ? sp." (possibly a Metacuvillierinella ?) in the Upper Maastrichtian of Iran is a hint of such a discrepancy ( Schlagintweit & Rashidi , 2016). Rhapydionininae and Pseudedomiinae subfamilies are summarily considered here, either because the facts have been established previously, or because the knowledge of them is still fragmentary. The Rhapydionininae subfamily, summarily depicted in Figure 18, is characterized mainly by its endoskeleton, lacking the confluent structure, with a wide peripheral zone and a very early unrolling trend (either cylindrical: Rhapydionina , with small proloculus, or flabelliform: Fanrhapydionina , with large proloculus). The milioline origin of this subfamily is indicated by species of the R. dercourti group, which are faintly streptospiral in the initial stages of both generations ( Fleury , 2014). The Pseudedomiinae subfamily is not the subject of these studies, being essentially represented by taxa in more Eastern regions. Besides, nothing definite can be deduced from the minimal knowledge of the group (see review in Mavrikas et al. , 1994), until the type of the type species is still poorly known. Cuvillierinellinae subfamily ( Fleury , 2016) is made up of six genera, including Cuvillierinella from which the others are clearly derived. But the origin of this genus remains uncertain. However, the presence of Pseudonummoloculina aff . kalantarii in our samples enables the consideration of what would be such an ancestor and how it could be named. We can try to imagine it from a simple principle and a few observations. The principle is the independence of evolutionary characters, commonly observed elsewhere in the family but quite obvious in the type population of Cuvillierinella salentina (see De Castro , 1988; Fleury , 2016), in which coexist the well-known type and specimens of overall same appearance, either streptospiral and devoid of partitions, or planispiral with a highly specialized endoskeleton. The observations are those that can be performed on species described by various authors under the name of Pseudonummoloculina. These organisms change from streptospiral to planispiral stage during their development. Some have a unique "notched" opening, others a double row of openings (and complex combinations of these two rather indistinct types) and some have no endoskeleton while others display rudimentary partitions. Varying combinations of these characters defy the conventional criteria of classification, each combination (changing during the ontogenetic development) being likely to justify a generic or specific status according to the arbitrary ordinary standards. These organisms are however in all cases isolated in time and most often deprived of clearly identifiable descendants (with the exception of P. pecheuxi Fourcade & Fleury , 2001, which is the origin of the species of Praechubbina genus). They probably do not constitute a unique group, but represent a step in an iterative evolution, leading members of a group of Miliolacea to the Alveolinacea, as Reichel (1936, 1937) and Pêcheux (2002) conceived it and as Fleury and Fourcade (1990) illustrated. We are thus led to apply the generic term Pseudonummoloculina to indicate a transitional state, that is to say an evolving stage. We are here confronted with the fundamental difficulty of adapting a nomenclature of creationistic essence to the perpetual reorganization of living organisms. It is therefore futile to characterize by terminology the temporary results; the term Pseudonummoloculina seems better to express a vague and essentially unstable notion related to the realities of evolution, ... at least as long as evolving networks of various known stages are not identified. The five genera derived from Cuvillierinella have a wide range of morphology ( Fleury , 2016, Fig. 13). Each genus is characterized by a dominant evolutionary trend, variously associated with secondary independent features. Metacuvillierinella and Pseudochubbina genera are treated superficially in this text and are probably derived from Cuvillierinella or its supposed close ancestor. Metacuvillierinella is the closest, comparable in its young stage to typical streptospiral tests of C. salentina , comprising an early well developed wide meshed endoskeleton. It is distinguished by its adult advolute coiling without final uncoiling and its very low dimorphism of generations, restricted to the initial coiling. Pseudochubbina is linked to Cuvillierinella perisalentina , also hesitating on its coiling mode ("Slow Axial Rotation" in Fleury , 2016), and sharing the "Scattered Secondary Chamberlets" ( Fleury , 2016), which are the only likely criteria for the origin of this genus. Murciella , Sigalveolina n. gen. and Cyclopseudedomia , are reviewed here from the types and various new populations, which allows deepening and renewing of the knowledge of them. They are all fully planispirally coiled in the A generation and to varying degrees display an endoskeleton that previously was called "helicoidal structure", here renamed 'confluent structure'. Murciella is very close to Cuvillierinella as evidenced by the coexistence of specimens having characteristics of both types in the population of the Cuvillierinella type species. The existence of populations including both Cuvillierinell a and Murciella , without morphological intermediaries, as well as the existence of populations containing only one or the other of these taxa emphasizes their independence. The six various populations assigned to M. cuvillieri , known from Spain to the Eastern Mediterranean sea, are likely to belong to the same zone (CsB6a) and were never associated with species of the overlying zone. These populations constitute a homogeneous set close to that of C. salentina appearance, with a little wider range of the A proloculus and a finer endoskeletal mesh. Tests of the B generation in several populations suggest that the holotype of the type species represents this generation, which appears to be less influenced by uncoiling than C. salentina . Sigalveolina n. gen. has been long confused with Murciella , too incompletely known to be distinguished before now. This new genus characterizes younger levels (zone CsB6b). This study shows that the high degree of confluent structure that gives its individuality coincides with a morphological diversity illustrated by four distinct species, including a new one, Sigalveolina reicheli . This new species probably corresponds to the mythical material, known as " Cosinella ", formerly discovered by Schlumberger from the Italo-Slovenian border. A tests of the four species are larger in size than those of Murciella , at diameters of proloculus of the same order of magnitude; they are only rarely uncoiled, with the apparent exception of Sigalveolina reicheli n. sp., which adopts this trend only at a very late development stage. B tests also tend to be involute at a later stage, in contrast to the early uncoiling of previous and next genera. Cyclopseudedomia is characterized by a very large A proloculus and an extremely reduced involute stage resulting in a longer uncoiled uniserial stage. This outstanding evolutionary trend is modulated in four species, including three already described and a new one, C. mavrikasi from the Italo-Slovenian border and Southern Italy. This trend is well marked in tests of both generations. In A tests, the involute part never exceeds 1.5 whorls around a large proloculus, and the uncoiled part is either cylindrical or flabelliform. B tests are mainly made of large evolute flabelliform flange, reaching a discoidal stage in the type species only; these flanges are always flat and increase in thickness very slightly towards the periphery. Finally, it must be noted that data in Figure 18, reduced to the A tests, are imprecise inasmuch as the genera generally include the features of several species, which are distinguished by nuances affecting their characters, but also because the time span of the genera and the included species likely does not correspond to the entire period of time covered by the zone in which they are recognized. This means that Figure 18 represents a state of schematic knowledge that can be improved only by specific careful field studies.

[学科分类] 地质学