Répartition biostratigraphique des ammonites dans I'Oolithe ferrugineuse de Bayeux (Bajocien) à Feuguerolles-sur-Orne (Calvados) Eléments nouveaux pour une révision des Garantianinae

Abstract

A detailed analysis of the vertical distribution of the ammonites in the "Oolithe ferrugineuse de Bayeux" Formation (Humphriesianum Zone - basal Parkinsoni Zone) is described at Feuguerolles-sur-Orne (Calvados, France). The principal results concerning the new stratigraphical data exposed in a preliminary note (H. Gauthier et al., 1995) are discussed. Reelaboration is obsewed between the base of the Blagdeni Subzone and the Humphriesianum Subzone. Taxonomic precisions in favour of a better subdivision of the subfamily Garan tianinae are founded on the centimeter by centimeter sampling of numerous specimens in this quarry. The abundant Garantianinae collected from the Feuguerolles site give a clear example of their evolution, new for Normandy, and their comparison with casts of the type-specimens of known species, especially German and English ones, leads us to briefly reexamine the classification and taxonomy of the Garantianinae. The ammonite fauna of Feuguerolles commonly do not allow all the ontogenic stages to be studied. Therefore, our conclusions are founded essentially on observations of the outer whorls, which are well preserved and in some cases also on the nucleus. Distribution and evolution of the Garantianinae at Feuguerolles-sur-Orne. The primitive untuberculate Orthogarantiana, with a tubular peristome, only distinguishable from the Cadomites by a narrow ventral interruption of the ribs (0. haugi group),appear at the base of the median horizon of the Polygyralis Subzone. Orthogarantiana with lateral tubercles (0. densicostata group Quenstedt - R. Douvillé; 0. schroederi group Bentz) and rectiradiate untuberculated secondary ribs appear simultaneously, or a little later the ventral furrow on their last whorl progressively enlarges into a smooth band and the ventral end of the ribs is reinforced by a weak radial crest (0. densicostata orthogona subsp. nov.: pl. IX, fig. 1 a-c). At the top of the Polygyralis Subzone, this crest lenglhens out radially while the secondary ribs become lightly proverse; the ventral band enlarges and becomes lightly concave (0. densicostata obliqua subsp. nov.: pl. IX, fig. 4a-c; 6a, b). The peristone remains always tubular. The original specimen of A. densicostatus Quenstedt (1886, pl. 71, fig. 9) is close to the subspecies orthogona; the large specimen of R. Douvillé (191.5, pl. III, fig. 1) is intermediate between the subspecies orthogona and obliqua: the specimen represented by Buckman (1925, pl. DLXXXI) as Baculatoceras baculatum is actually an Orthogarantiana densicostata obliqua. In the overlying Baculata Subzone, Garantianinae appear with true ventral tubercles and a broad ventral band; i.e. the earlier Garantiana, still bearing welldelineated lateral tubercles, of the G. baculata (Quenstedt) and G. althoffi Bentz groups. At the top of the Baculata Subzone, the last true Orthogarantiana disappear but not Orthogarantiana conjugata (Quenstedt), which corresponds to a species apart in the subfamily, with its uninterrupted secondary ribs on the venter. The Garantiana of the group G. garantiana (d'orbigny), with well-marked ventral tubercles but with lateral tubercles decreasing or disappearing completely, appear in the Dichotoma Subzone; their smooth ventral band is large and their peristome is a mere wrinkle along the ribs. The species platyrryma Buckman belongs to this group. Specimens with only bifurcated ribs and always with lateral tubercles, can be found in the same general morphology, as can other specimens with thin and dense ribbing, closely connected to the form figured as "Ammonites parkinsoni densicosta" by Quenstedt (1886, pl. 72, fig. 2) but which Callomon, Dietl et al. recently (1987, p. 16) related to a finely ribbed Garantiana of the Garantiana Zone. In the middle part of the Garantiana Zone and especially at the top, certain specimens have a larger size and show height compressed whorls, the section of which quickly increases wilh their diameter; the last ribs at the end of the body chamber cross over the siphonal line. These.forms lead to Garantiana tetragona Wetzel which characterizes the top of the Garantiana Zone and is the last species of this branch of the Carantianinae. Finally, in the Garantiana Zone (top ?), one finds Garantianinae with a Sharp and deep ventral groove, and much more involute and massive whorls, the acme of which is in the Acris Subzone at the same time as the early Parkinsonidae. These mark the extinction of the Garantianinae. Odontolkites longidens (Quenstedt) belongs to this group. Although limited to one site, the detailed study of this subfamily leads to a better understanding of the evolutive trend of the Garuntianinae and to a clearer distinction between principal and secondary taxonomic criteria. The ventral morphology is very important, as already observed elsevvhere in the Oppeliaceae (Elmi, 1967). The form of the peristome, the absence or presence of ventral tubercles, the involution of the whorls, the shape of the section and its increasing of height and thickness seem also to be fundamental elements in their evolution. One very constant evolutive trend, which has yet been noted (Wetzel, Bentz, etc.), is the increasing proversity of the ribs front an initial rectiradiate form to an increasingly pronounced bending forward such that the ventral chevron pattern is more and more acute forward. This evolution appears earlier and earlier in the ontogenesis (centripetal evolution). On the other hand, the lateral tubercles do not show a constant trend. The density of the ribbing, the height of the division point of the ribs and the costal division index seem to be only features of interspecific differentiation (or even of intraspecific variability). Systematic remarks The synonymy of the species G. subgaranti Wetzel, 1911 with the species G. garantiana d'Orbigny was recently (H. Gauthier et al., 1995) recognized by comparison of the cast of the holotype from Wetzel with the Arkell's lectotype (1956), and also with the original drawing of d'Orbigny (1845, pl. 123, fig. 1, 2). The species garantiana had been chosen as type for the genus Garantiana (Hyatt 1900, Mascke 1907), and later as type for the subgenus (Garantiana) Bentz 1928. It had previously been designated, as a zonal index (S. Buckman, 1893). Its concept has gained wide recognition in the scientific community and was further specified by the designation of a lectotype by Arkell (1956). A difficulty avises from the synonymy of these two species since they are type- species of two different subgenera both created by Bentz (1928): (Garantiana) and (Subgarantiana). In fact, the original definition of the subgenus (Subgarantiana) given by Bentz does not at all apply to the species subgaranti even though he chose it as a type. Bentz defines (Subgarantiana) as the "jüngeren Garantiana", with the ventral ribs bent forward and making a significant angle on the venter and with a Sharp ventral furrow bordered on each side by an outer row of tubercles. That original definition applies only to the Wetzel species (1911) alticosta, coronata, cyclogaster, depressa, pompeckji and subangulata ofthe Lower Parkinsonia beds, and not to the species subgaranti, suevica, trauthi, and tetragona which have a very broad ventral band and outer ribs gently bent forward; these latter species are related to the subgenus (Garantiana) as created and defined by Bentz himself in the same pape. Nevertheless, this author includes these species in the same subgenus, thas making a composite subgenus, as revealed by the synonymy of the two type-species. The subgenus (Subgarantiana) Bentz was set by Arkell (1957) in synonymy with the earlier subgenus (Hlawiceras) S. Buckman, 1921, of which the type species platyrrymum, in fact, has no affinity with the (Subgarantiana) such as described by Bentz, but is actually a (Garantiana). On the other hand, the genus Odontolkites Buckman 1925 (type-species Ammonites parkinsoni longidens Quenstedt 1846, p. 144, 1849, pl. 11, fig. 10; 1886, p. 592, pl. 71, fig. 6) is suitable for the species group described by Wetzel (1911) and taken up again by Bentz in his definition. Moreover, it would be certainly better ou divide the Garantianinae macroconques into only three genera without subgenera: (Orthogarantiana, Garantiana, Odontolkites) since they are already joined together in the same subfamily. The separation between Orthogarantiana and Garantiana has already been adopted by certain authors (Sturani 1971, Pavia 1973, for instance). The three macroconch genera of Garantianinae can therefore be redefined as follows: Genus Orthogarantiana (Bentz, 1928) = ex-subgenus (Orthogarantiana) Bentz, 1928: Sharp ventral groove; secondary ribs not ending in a ventral tubercle (or only one feeble radial crest on the body chamber); rectiradiate or very slightly bent forward; these charac- ters are the same on the inner whorls; with more evolved species the ventral sulcus widens on the body chamber and external ribs end in a radial crest, whorls generally slightly involute and their section generally increases slowly in height as the specimen grows in diameter; tubular peristome. Type-species: Orthogarantiana schroederi Bentz 1924. Principal species: 0. fredericiromani (Roché), 0. haugi Pavia, 0. densicostata (Quenstedt), 0. crassa Bentz, 0. schroederi Bentz, 0. conjugata (Quenstedt). Stratigraphical range: Niortense Zone, extreme base Garantauna Zone. Genus Garantiana Hyatt 1900 = ex- subgenus (Garantiana) Bentz 1928 + ex-subgenus (Subgarantiana) Bentz 1928 (pars): band lined with well-shaped tubercles for both the outer and the inner whorls; secondary ribs slightly inclined forward except at the end of the last whorl where they become more prorsiradiate; slight or middle involution of the whorls, height of their section more rapidly increases with the diameter; peristome constituted by a simple fold. Type-specles (Bentz): Garantiana garantiana (d'Orbigny). Principal species: G. baculata (Quenstedt) more involute, althoffi Bentz, G. densicosta (Quenstedt), G. platyrryma (Buckman), G. trauthi Bentz, G. garantiana (d'Orbigny) and its morph suevica (Wetzel), G. intermedia Bentz (transitional to Odontolkites?), G. tetragona Wetzel (last morph of this branch, with rapid increase in the height of the section). Stratigraphical range: uppermost part of the Niortense Zone, whole of the Garantiana Zone and extreme Lowermost part of the Parkinsoni Zone. Includes Baculatoceras Mascke and Hlawiceras Buckman. Genus Odontolkites S. Buckman, 1925 = ex-subgenus (Subgarantiana) Bentz, 1928 (pars): deep narrow ventral groove, secondary ribs ending on ventral acute crest; prorsiradiate ribs, generally very sharp; involute whorls (overlapping involution to half height); thick section with rapid increase in its height peristome reduced to a simple fold. Type-species: Odontolkites longidens (Quenstedt 1846, 1849). Principal species (Wetzel's species 1911): 0. subangulatus, 0. alticostus, 0. coronatus, 0. cyclogaster, 0. depressus, 0. pompeckji, 0. quenstedti. From the drawing of its ventral groove, Ammonites bifurcatus Schlotheim might very well belong to this species. Stratigraphical range: Upper part of the Garantiana Zone (?) - lower part of the Parkinsoni Zone.