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Variability changes in the ammonite family Dactylioceratidae during the Lower Toarcian

100%

Morard A. , Guex J.

tom Vol. 4, no. 1

194-195

The comparison of successive dactylioceratid samples collected in the Moroccan Middle-Atlas, the Spanish Betic Cordillera, the Lusitanian Basin and the Causses Basin, allowed us to better characterize the range of variability of these forms from the very base of the Toarcian to the Lower-Middle Toarcian transition. From a biostratigraphic point of view, the Tethyan subgenus Eodactylites (Schmidt-Effing 1972) clearly precedes the NW European subgenus Orthodactylites – as recorded in the Lusitanian Basin succession for example, the latter then giving rise to the Dactylioceras s.s. Both early subgenera can be distinguished morphologically, and separated in a number of “species”, although isolated specimens may be difficult to identify. The main point is that Eodactylites, as well as early Orthodactylites form complete series of continuous covariation, the latter progressively branching into two main lineages in the Semicelatum Subzone. Following the occurence of some rare forerunners, known from the Middle-Upper Domerian, the evolution of the macroconch representatives of this ammonite family can be summarised in three main steps: 1. A sudden “mass apparition” of Eodactylites defines the base of the Toarcian. Their variability spectrum is immediately quite large, with a covariation series between a densely ornamented, somewhat involute and compressed pole (E. mirabile) and a more robust very evolute form with distant primary ribs and possible tuberculation (E. pseudocommune). 2. Ornamental variability tends to disappear in the outer whorls of Orthodactylites, whereas their inner whorls still display a wide covariation series (from slightly compressed forms with dense simple ribs to stout pantuberculate ones), as already noted by Howarth (1962). Intermediate forms seem to disappear in the Semicelatum Subzone. 3. Two lineages are then clearly distinct, one leading from isocostate to variocostate non tuberculate serpenticones (Dactylioceras s.s.), the second developing depressed whorls prone to tuberculation, particularly in inner whorls (Nodicoeloceras). A further diversification occurs at the Lower-Middle Toarcian transition (Guex 1971). In this new evolutionary frame, divergent views on the taxonomy of the Toarcian Dactylioceratidae can eventually be reconciled, as different authors in the past based their classification schemes only on a limited portion of the succession presented here.

Application of Unitary Associations to biochronological scales based on mammals: the case of Miocene rodents from some localities in western Europe

100%

Martinez J. N. , Guex J.

nr 1

Pliensbachian to Aalenian radiolarian biochronology and global correlation

51%

Carter E. , Gorican S. , Guex J. , De Wever P. , Dumitrica P. , Hori R. , Matsuoka A. , O'Dogherty L. , Whalen P.

tom Vol. 4, no. 1

151-152

A new UA radiolarian zonation for the Pliensbachian to Aalenian interval is established using 145 distinctive, widely-distributed species. The data are from biostratigraphic sections in: Queen Charlotte Islands, NE British Columbia, Baja California Sur, Japan, Oman, Turkey, Slovenia and Austria. A catalogue of 280 species (with revised taxonomy) is completed. For the zonation, about half these species were eliminated from the total dataset, because they are either rare (e.g. Danubea, Farcus, Pseudopoulpus, Rolumbus), long-ranging (e.g. Pseudocrucella, Orbiculiformella, Paronaella) or non-diagnostic with wide limits of variability (e.g. some species of Bagotum, Droltus, Parahsuum). Rich well-preserved radiolarians from thick continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this stratigraphic interval, and all collections are tied with North American ammonite zones or assemblages. An initial sequence of 25 UAs (including ammonite data) was determined from this material only. Subsequently, data from other areas were added and a global sequence of nine radiolarian zones was obtained. These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian (Carter et al. 1998) and the Middle to Upper Jurassic (Baumgartner et al. 1995).