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Dommergues J.-L.,University of Burgundy | Meister C.,Museum dHistoire Naturelle de Geneva
Acta Palaeontologica Polonica | Year: 2013

This paper discusses the phyletic interpretation of the genus Phricodoceras and its taxonomic classification at the subfamily, family, and superfamily levels from an historical and critical perspective. First a review of the latest find ings on this taxon is presented and the grounds for the attribution of Phricodoceras to the Schlotheimiidae (Psiloceratoidea) are summarized and illustrated. This review is a synthesis grounded on evolutionary (e.g., heterochronies, innovations), eco-ethological (e.g., assumed shell hydrodynamic capacities) and spatio-temporal pat terns (e.g., bio-chronostratigraphy, palaeobiogeography). Then, the main stages of understanding the taxonomy of Phricodoceras since the early nineteenth century are reviewed. Two main taxonomic concepts alternate over time. The first is based on the "overall resemblance" of Phricodoceras to some coeval Eoderoceratoidea leading to the genus be ing included in its own family or subfamily (e.g., Phricodoceratinae) among the Eoderoceratoidea. The second hypoth esis, recently confirmed by the discovery of an intermediate form (i.e., Angulaticeras spinosus), clearly includes Phricodoceras within the Schlotheimiidae (Psiloceratoidea). Comparison of these two very different conceptions re veals how "overall resemblance" can be misleading and shows that the discovery of intermediate forms is often the key to phyletic reconstructions in ammonites. Copyright © 2013 J.-L. Dommergues and C. Meister. This is an open-access article distributed under the terms of the Cre ative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, pro vided the original author and source are credited. Source

Vullo R.,CNRS Geosciences Laboratory of Rennes | Guinot G.,Museum dHistoire Naturelle de Geneva | Guinot G.,Montpellier University
Die Naturwissenschaften | Year: 2015

Here, we report a novel type of dermal denticle (or placoid scale), unknown among both living and fossil chondrichthyan fishes, in a Cretaceous lamniform shark. By their morphology and location, these dermal denticles, grouped into clusters in the cephalic region, appear to have been directly associated with the electrosensory ampullary system. These denticles have a relatively enlarged (∼350 μm in diameter), ornamented crown with a small (∼100 μm) asterisk- or cross-shaped central perforation connected to a multi-alveolate internal cavity. The formation of such a complex structure can be explained by the annular coalescence and fusion, around an ampullary vesicle, of several developmental units still at papillary stage (i.e. before mineralization), leading to a single denticle embedding an alveolar ampulla devoid of canal. This differs from larger typical ampullae of Lorenzini with a well-developed canal opening in a pore of the skin and may represent another adaptive response to low skin resistance. Since it has been recently demonstrated that ampullary organs arise from lateral line placodes in chondrichthyans, this highly specialized type of dermal denticle (most likely non-deciduous) may be derived from the modified placoid scales covering the superficial neuromasts (pit organs) of the mechanosensory lateral line system of many modern sharks. Source

Giese J.,University of Bern | Berger A.,Copenhagen University | Schreurs G.,University of Bern | Gnos E.,Museum dHistoire Naturelle de Geneva
Precambrian Research | Year: 2011

In central southern Madagascar the crystalline basement is composed of mid-crustal rocks which have experienced polycyclic deformation and metamorphism coupled with repeated granitoid magmatism at the Neoproterozoic-Phanerozoic boundary. Based on the integration of in situ U-Th-Pb dating of monazite and structural relationships, two distinct phases of major ductile deformation, the Andreaba and Ihosy phases can be distinguished in central southern Madagascar. Both these deformation phases occur between ~550 and 520. Ma. Coeval with, and outlasting deformation, granitic plutons and dykes were emplaced. HT/HP granulite facies metamorphism (M1), including migmatisation and anatexis of the crust started at ~585. Ma and lasted until at least ~500. Ma. Monazite growth between 480 and 450. Ma postdates major ductile deformation and might be related to a second, HT/MP metamorphism (M2), indicating that the whole crustal section remained in a mid-crustal position. Metamorphic overprinting related to M2 is spatially limited and is heterogeneously distributed in central southern Madagascar. Newly developed M2 mineral assemblages are preferentially found in areas featuring Ihosy phase vertical foliation planes. Very often these structures show brittle/ductile or brittle overprint and might be considered as preferred pathways for fluid flow, thus localising metamorphic overprint. © 2011 Elsevier B.V. Source

Meister C.,Museum dHistoire Naturelle de Geneva | Dommergues J.-L.,University of Burgundy | Rocha R.B.,New University of Lisbon
Bulletin of Geosciences | Year: 2012

The lowermost Portuguese Pliensbachian is characterized by the association of Apoderoceras dunrobinense Spath, Tragophylloceras numismale (Quenstedt) and Vicininodiceras aff. mouterdei Donovan. This ammonite fauna indicates the lower part of the Jamesoni Chronozone (lower to middle Taylori Subchronozone). Thanks to the numerous specimens collected, the ontogeny and variability of A. dunrobinense Spath, could also be investigated. The paleogeographical distribution of these ammonites underlines the close connections between the Lusitanian Basin and the Euroboreal seas during the Early Pliensbachian. Source

Hardy C.,CNRS Biogeosciences Laboratory | Fara E.,CNRS Biogeosciences Laboratory | Laffont R.,CNRS Biogeosciences Laboratory | Dommergues J.-L.,CNRS Biogeosciences Laboratory | And 2 more authors.
PLoS ONE | Year: 2012

Conservation biologists and palaeontologists are increasingly investigating the phylogenetic distribution of extinctions and its evolutionary consequences. However, the dearth of palaeontological studies on that subject and the lack of methodological consensus hamper our understanding of that major evolutionary phenomenon. Here we address this issue by (i) reviewing the approaches used to quantify the phylogenetic selectivity of extinctions and extinction risks; (ii) investigating with a high-resolution dataset whether extinctions and survivals were phylogenetically clustered among early Pliensbachian (Early Jurassic) ammonites; (iii) exploring the phylogenetic and temporal maintenance of this signal. We found that ammonite extinctions were significantly clumped phylogenetically, a pattern that prevailed throughout the 6.6 Myr-long early Pliensbachian interval. Such a phylogenetic conservatism did not alter - or may even have promoted - the evolutionary success of this major cephalopod clade. However, the comparison of phylogenetic autocorrelation among studies remains problematic because the notion of phylogenetic conservatism is scale-dependent and the intensity of the signal is sensitive to temporal resolution. We recommend a combined use of Moran's I, Pearson's φ and Fritz and Purvis' D statistics because they highlight different facets of the phylogenetic pattern of extinctions and/or survivals. © 2012 Hardy et al. Source

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