|mhorn||Дата: Saturday, 06.03.2010, 11:46 | Сообщение # 1|
|Bonnot A., Marchand D., Courville Ph., Ferchaud Ph., Quereilhac Ph., Boursicot P.-Y. (2009) Le genre Epipeltoceras (Ammonitina, Perisphinctaceae, Aspidoceratidae) sur le versant parisien du seuil du Poitou (France): faunes ammonitiques, biostratigraphie et biozonation de la zone à Bimammatum pars (Oxfordien supérieur) // Revue de Paléobiologie. V.28. no.2.P.413-435. |
The biozonation of the Bimammatum zone (Upper Oxfordian) proposed by the Groupe français d’étude du Jurassique (1997) is based, for the first three sub-zones, on the microconchs (genus Epipeltoceras) range from bottom to top : Semimammatum subzone, Berrense subzone and Bimammatum subzone. Epipeltoceras semimammatum (Quenstedt) is a well known species, because the type is a full adult and a good revision was made by Enay (1962). But it’s not the same for the two other species. The type of Epipeltoceras berrense (Favre) is an adult specimen (the last sutures are telescoped), but incomplete because a large part of the body chamber is missing. This species was not well illustrated and the specimens show a variability in size, morphology and ornamentation. Owing to the preparation of the lectotype, designed by Schmidt-Kaler (1962), the phragmocone ontogeny is now better known and it is clear that a lot of the previous attributions were wrong. The type of Epipeltoceras bimammatum (Quenstedt) is wholly septate and probably immature, the last septa being not approximed ; therefore, the morphology of the body chamber is unknown. This species was frequently illustrated, but not very well understood. The Parisian slope of the Poitou (in the west of the Vienne département) has been known for a long time for its Oxfordian ammonite faunas, in particular Epipeltoceras, which are frequent in the local clays (areas of Doux, Cherves, Maisonneuve). Starting with the Poitou’s ammonites and faunas, Cariou (1966) proposed a biozonation of the Upper Oxfordien pro parte. To make this biozonation with Epipeltoceras more precise, an excavation was carried out close to Frozes (Vienne) for a better understanding of the variability and succession of the Epipeltoceras species. This section was a dig 4.5 m deep in the Marnes à Spongiaires formation between the upper part of the Grossouvrei s.z (Upper Oxfordian, Bifurcatus zone) and the lower part of the Bimammatum s.z. (Fig. 4). The ammonites, collected level by level, belong essentially to three families : Perisphinctidae, Oppeliidae and Aspidoceratidae. In the Aspidoceratidae family, more than 200 specimens of Epipeltoceras have been studied using morphological characters on the phragmocone and the body chamber such as : adult size ; section shape ; rib morphology : trajectory, thickness, density, bifurcations ; morphology, frequency and robustness of tubercules ; aspect of the venter. Four morphologies are traditionally considered as four species, some of them including sub-species or variants. We now know that these species are microconchs of Euaspidoceras and Clambites.
- Epipeltoceras semimammatum is a small species (20-25 mm), with a subcircular section which becomes oval, with primary ribs at first proverse, then radial, retroverse and finally convex, with frequent bifurcations on the phragmocone and body chamber, more or less frequent parabolic tubercules, simple or duplicated, and ribs weakened or interrupted on the ventral axis, with ribs being radial or possibly retroverse. Three forms are interpreted as variants of Epipeltoceras semimammatum (sub-species in Enay, 1962) :
• Epipeltoceras stromeri (Prieser) characterized by frequent parabolic tubercules persisting on the body chamber ;
• E. circumcostatum (Dorn), with its ribs non interrupted on the venter ;
• E. uhligi (Oppenheimer) with quite larger specimens (30 mm), with ribs rarely bifurcated and clearly interrupted on the venter.
Furthermore E. retrocostatum (Dorn) pars (Dorn, 1931, pl. 34, fig. 7a-b) is used for small specimens characterized by an important retroversion of ribs on the upper part of sides and on the venter.
- Epipeltoceras semiarmatum is a larger species (30-40 mm), with a greater thickness, very rare bifurcations and a venter with a clear
smooth band. In this species, we include the greatest specimens of E. retrocostatum (Dorn, 1931, pl. 34, fig. 3a-b);
- Epipeltoceras berrense is yet a larger species (40-45 mm), a large increase in thickness, with very rare bifurcations and a venter with a shallow furrow ;
- Specimens from level 19 in the Frozes section constitute a very homogeneous population, some specimens having evident affinities with Epipeltoceras bimammatum (Quenstedt) : large size (40-60 mm), strong, radial or proverse ribs, deep furrow at the beginning of the body chamber. However, E. bimammatum type is different in morphology by external, frequently double tubercules and by frequent node quite on the ombilic part of the primary ribs. These differences are intraspecific and we attribute the population of Frozes to Epipeltoceras bimammatum morphe alpha nov.
- The section ends before the apparition of typical Epipeltoceras bimammatum and E. treptense (Enay).
So redefined, Epipeltoceras from Frozes section divide themselves as such, from bottom to top (Pl. VII) : a) level with Epipeltoceras semimammatum ; b) level with E. semiarmatum ; c) level with Epipeltoceras berrense ; d) level with Epipeltoceras bimammatum alpha.
Then, we can observe from bottom to top:
- a large increase in size ;
- a centrifugal extension of the proverse or radial ribs stage which has resulted in the disappearance of the retroverse stage (paedomorphosis);
- the progressive disappearance of bifurcations on the phragmocone and the body chamber (paedomorphosis) ;
- however, there is a constant presence of the stage with convex ribs just before aperture (adult character)
- variable morphology of the ribs on the venter, radial or retroverse, in the successive taxons ;
- an evolution of the venter, with successively : ribs weakened in the axis of the venter, interrupted ribs, a clear smooth band, a shallow furrow, then a deep groove. During individual development, the ventral furrow is more developed on the phragmocone and the first part of the body chamber that the venter region is most evolved (peramorphosis).
The first three species constitute a continuous series : E. semimammatum -> E. semiarmatum -> E. berrense, with a gradual change in size, in ribbing and in the morphology of the ventral area. But, E. bimammatum alpha represents the most important morphological leap throughout the lineage : large increase in size, disappearance of the retroverse-ribbing stage and deep ventral furrow. To us, the succession of the Epipeltoceras species is nowhere better documented than in the Frozes section for this part of the Bimammatum zone.
- Semiarmatum sub-zone nov. Three horizons :
• Semimammatum horizon. Index : Epipeltoceras gr. semimammatum (including E. stromeri, E. circumcostatum, E. uhligi and E. cicrumcostatum pars). Associated ammonites fauna : Euaspidoceras striatocostatum, E. hypselum, E. eucyphum ; Orthosphinctes fontannesi ; Passendorferia rosaki ; Ochetoceras hispidiforme and O. hispidiforme with their microconchs « Glochiceras » tectum, O. semifalcatum (rare) ; Taramelliceras pichleri, T. lochense and their microconchs « Glochiceras » microdomum, T. Externnodosum (rare) ;
• Semiarmatum horizon nov. Index : Epipeltoceras semiarmatum. Associated ammonites fauna : Euaspidoceras striatocostatum, E. radisence ; Orthosphinctes sp. ; Discosphinctes jelskii. Ochetoceratinae are rare. Taramelliceratinae are the same as in the prior level ;
• Berrense horizon. Index : Epipeltoceras berrense. Associated ammonites fauna : last of Euaspidoceras striatocostatum, E. costatum ; Orthosphinctes virgulatus. Ochetoceratinae are very rare but the Taramelliceratinae, the same as in the prior level, can be sometimes very frequent.
- Bimammatum sub-zone (emend).
Bimammatum alpha horizon nov. Index : Epipeltoceras bimammatum morphe alpha. Important change of the associated ammonites fauna : Euaspidoceras costatum ; Clambites cf. schwabi ; Aspidoceras sp. ; Lithacosphinctes cf. girardoti ; Orthosphinctes (Pseudorthosphinctes) alternans, O. aff. tiziani, O. colubrinus ; Passendorferia (Enayites) rosaki ; Ochetoceras marantianum macroconch and microconch. However, Taramelliceratinae are the same as in the prior levels.
The study of more recent populations (Cher, Jura, South-East of France, Franconie, Spain…) should permit :
1. a better understanding of the intraspecific variability of Epipeltoceras bimammatum and in particular its relations with E. treptense,
2. to eventually precise the taxonomic position of Epipeltoceras bimammatum alpha and
3. to make the biozonation of the superior part of the Bimammatum sub-zone more precise.
Middle Jurassic - Lower Cretaceous ammonites & aptychi