Dommergues J.-L., Corna M. (2014)

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Dommergues J.-L., Corna M. (2014) Ammonites micromorphes à allure de « Cymbites » dans l’Est de la France (Côte-d’Or et Ain): des témoins d’une part méconnue de la paléobiodiversité du Sinémurien // Revue de Paléobiologie, Genève. Vol.33. no.2. P.431-453.

http://www.ville-ge.ch/mhng/paleo/paleo-pdf/33-2/pal_33_2_04.pdf

This work is based on three Lower Sinemurian sections located in eastern France (Burgundy and southern Jura). The Semur-en-Auxois section is the unit-stratotype for the Sinemurian. The degree of stratigraphic accuracy is the bed. Ammonites were systematically collected in order to reduce the usual bias related to taxonomy, size and preservation. Three new micromorph ammonites, corresponding to two new genera and three new species, were discovered during this work. These small ammonites show a more or less obvious Cymbites-like habitus. They are Coronicymbites furcatus nov. gen., nov. sp. (Late Bucklandi Subchronozone, Semur-en-Auxois, Côte-d’Or), Pseudocymbites constrictoides nov. genus, nov. sp. (Early Semicostatum Chronozone, Lyra Subchronozone?, Mavilly-Mandelot, Côte-d’Or) and Agassiceras pseudostriaries nov. sp. (Scipionianum Subchronozone, Boyeux-Saint-Jérôme, Ain). The first two of these taxa are the oldest Cymbites-like ammonites known for the Jurassic. One of them (i.e., Pseudocymbites constrictoides nov. genus,nov. sp.) shows a remarkable ornamentation on the phragmocone, consisting of pseudo-constrictions associated with megastries. Such a feature is unusual among the Arietitidae, the family to which Pseudocymbites constrictoides nov. genus, nov. sp. is tentatively attributed. The discovery of three new micromorph ammonites in the Early Sinemurian of NW Europe enhances the image of a period with great size disparity among ammonite fauna. The very wide range of adult body sizes extends from very small Cymbites-like forms to “gigantic” or at least “very large” taxa belonging to the genera Coroniceras or “Arietites”. Such great disparity in size suggests a correspondingly broad disparity in terms of “traits of life history”.

Middle Jurassic - Lower Cretaceous ammonites & aptychi