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Un problème de paléobiologie chez les ammonoïdes (Cephalopoda) : croissance et miniaturisation en liaison avec les environnements

Abstract : Two cases of small sized ammonoids are detailed in this paper : - Some Middle Toarcian Hildoceras from the Southern margin of Tethys (Italy, Northern Africa) show ornemental and dimensional particularities, which differentiate them from their NW-European equivalents. Growth rate study, by the way of septal spacing measures, underscores a relation between ammonites growth and the situation of outcrops in the regional paleogeography. During Toarcian indeed, South-Tethyan shelf split up in a string of tilted blocks, demarcating raised areas from narrow basins. In ammonites from this region, time-lags occur during ontogeny (progenesis-type). They are all the more pronounced since the locality is confined. This heterochrony leads to small-sized forms whose life span is shorter. It would appear to be a case of adaptation of these ammonites to contrasting and constraining environments of South-Tethyan margin by the way of a life-history strategy tending towards r-selection. On the other hand, comparison between septal spacing of several members of Hildoceras lineage has shown that this character was different enough between species in question to be used as a taxonomic criterion. - Bajocian outcrops near Asiago (Venitian Alps, Italy) contain a rich and varied marine fauna consisting of unusually minute organisms. Study of size and preservation of ammonites allow to exclude preponderance of mechanical sorting. But septal spacing of Sphaeroceras brongniarti reveal that this species is clearly dwarf compared to its equivalents from other regions (especially from Normandy, France) whose size average is three times greater. These features lead to reconsider the hypothesis according which these organisms have been sorted and transported ashore by storms toward sheltered places. Both examples, although they are very different, demonstrate that ammonoids were sensitive to environmental influence and could react differently depending on circumstances.
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Yannick Mignot. Un problème de paléobiologie chez les ammonoïdes (Cephalopoda) : croissance et miniaturisation en liaison avec les environnements. Paléontologie. Université Claude Bernard - Lyon 1, 1992. Français. ⟨tel-02426776⟩



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