| mhorn | Дата: Thursday, 01.10.2015, 20:24 | Сообщение # 1 |
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| Dommergues J.-L. (2015) Un nouveau genre et une nouvelle espèce d’ammonite micromorphe dans le Sinémurien de l’Est de la France (Côte-d’Or) // Revue de Paléobiologie, Genève (juin 2015) 34 (1). P.151-160
pdf: http://www.ville-ge.ch/mhng/paleo/paleo-pdf/34-1/pal_34_1_10.pdf
A new genus and a new species of Sinemurian micromorph ammonite in eastern France (Côte-d’Or). Hemicymbites tardiornatum nov. gen, nov. sp.- Three micromorph ammonites collected in the area of Semur-en-Auxois – the type-locality of the Sinemurian stage – are studied in the present paper. They are housed in the municipal museum of Semur-en-Auxois (2 spécimens) and in the Natural history museum of Marseille (1 specimen). These three ammonites are attributed to a new genus: Hemicymbites nov. gen. The two specimens of the museum of Semur-en-Auxois are interpreted as a new species: H. tardiornatum nov. gen., nov. sp. and the specimen of the museum of Marseille is here designated as the lectotype of Hemicymbites semicostulatus (Reynès, 1879). These three ammonites are the only currently known specimens belonging to the new genus Hemicymbites. Despite their small adulte sizes, they are characterized by a dramatic change during ontogeny between a smooth Cymbites-like phragmocône and a much more evolute, quasi-serpenticone and coarsly ribbed body-chamber. Even if accurate stratigraphical data are missing, these micromorph ammonites can be plausibly attributed to the Early Sinemurian. They show on the venter some slight indications of a weak keel flanked by faint flat areas. Although rather subtle, these traits coupled with a credible Early sinemurian age are consistent with a possible attribution to the Arietitidae family s.l. These new results support the impression that the ammonite faunas are, during the Sinemurian and especially during the Early Sinemurian, characterised by a remarkable disparity in adult size. Nevertheless, the micromorph Cymbites-like sinemurian ammonites are difficult to understand in terms of palaeobiodiversity. Indeed, these tiny ammonites can be either distinct species characterized by both small adult size and associated distinctive “traits of life history” or microconch morphs belonging to until now unidentified dimorphic couples
Middle Jurassic - Lower Cretaceous ammonites & aptychi
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