Christian Meister

Evolution of ammonoid morphospace during the Early Jurassic radiation

The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.

Les ammonites du Niger (Afrique occidentale) et la transgression transsaharienne au cours du Cénomanien-Turonien

Resume Notre travail s’inscrit dans le cadre d’une revision de la paleontologie des ammonites liees a la transgression transsaharienne du Cenomanien-Turonien et d’un affinement de la biostratigraphie dans le bassin sedimentaire des Iullemeden (Niger occidental et meridional: Mt Iguelela et Damergou). Plus de 700 ammonites ont ete recoltees avec precision dans 8 localites, Il a ete ainsi possible de mettre en evidence une serie de 9 horizons biostratigraphiques et de decrire egalement 3 taxons nouveaux. En particulier, la presence de Metoicoceras geslinianum (d’ Orbigny ), associe aux Nigericeras gadeni ( Chudeau ) permet de mieux correler les niveaux a N. gadeni avec le nord-ouest de l’Europe ou encore avec les USA (Western Interior + Nouveau Mexique et Texas). La presence d’ammonites comme les Cibolaites (?) africaensis nov. sp. ou les Coilopoceras inflatum Cobban & Hook permettent egalement de bonnes correlations avec l’ouest et le sud des USA. Toutefois les faunes d’ammonites restent essentiellement d’affinite tethysienne comme l’ont deja souligne de nombreux auteurs avec en particulier Nigericeras gadeni ( Chudeau ) et Thomasites nigeriensis ( Woods ) qui montrent un endemisme saharien assez marque (Algerie, Niger, nord-est Nigeria). Bien que l’existence d’une liaison entre la Tethys et l’Atlantique sud, via le fosse de la Benoue pour le Cenomanien — Turonien, soit contestee dans des travaux recents, la repartition des ammonites nous incite, a l’instar des autres ammonitologues, a y voir l’existence d’un bras de mer entre ces deux domaines paleogeographiques. Cette connexion semblerait debuter vers la partie superieure de la zone a Gadeni (Cenomanien superieur) et se prolongerait aux environs de la fin de la zone a Nigeriensis (fin du Turonien inferieur). L’aspect ontogenetique et evolutif est egalement aborde dans cette etude, en particulier pour les Nigericeras gadeni ( Chudeau ) — N. jacqueti Schneegans — N. jacqueti involutus nov. subsp. qui tendent au cours de leur histoire a perdre leur ornementation juvenile par peramorphose alors que, chez le groupe Vascoceras (Paravascoceras) cauvini ( Chudeau ) — V. (P.) proprium ( Reyment ) ou le polymorphisme est exacerbe, la forte costulation des formes adultes tend a s’effacer par une tendance inverse: la paedomorphose. Il en resulte une tendance generalisee, chez les ammonites nigeriennes vivant dans des mers peu profondes, a acquerir des morphologies lisses et des sutures simplifiees en reponse probablement aux contraintes externes (synergie des contraintes d’ordre climatique, ecologique et physiographique) en developpant en fonction de leur contrainte interne (potentiel ontogenetique) des morphologies assez differentes et peut-etre plus performantes pour le groupe (canalisation de la derive morphologique).

A potential global stratotype for the Sinemurian-Pliensbachian boundary (Lower Jurassic), Robin Hood's Bay, UK: ammonite faunas and isotope stratigraphy

A coastal exposure at Wine Haven, Robin Hoods Bay (North Yorkshire, UK) fulfils the criteria for definition as the Global Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic). This marine sequence was deposited during a long-term trans- gression and is relatively expanded stratigraphically. A rich fauna of ammonites above and below the boundary interval allows recognition of the Leptechioceras gr. meigeini, Paltechioceras aureolum and Paltechioceras tardecrescens horizons of latest Sinemurian age, and the Bifericeras donovani, and Apoderoceras gr. aculeatum horizons of earliest Pliensbachian age. A suitable level for the boundary is characterized by the faunal association of Bifericeras donovani Dommergues & Meister and Apoderoceras sp. Strontium-isotope stratigraphy, based on analysis of belemnites, yields a calcite 87 Sr/ 86 Sr ratio for the suggested boundary level of 0.707425 ±0.000021 (combined uncertainties based on line fit to stratigraphic dataset (±0.000004) and measurement of the standard (±0.000017)). Alternative uncertainties of ~±0.000008 are associated with the most extreme interpretation of sedimentation-rate history allowed by the strontium-isotope data (that is, abrupt changes in sedimen- tation rate at precisely the boundary level); however, sedimentological considerations, and measured strontium-isotope values at the boundary, support condensation rather than hiatus. Belemnite oxygen- isotope data suggest a significant temperature drop (~5 °C) across the boundary at this locality.

The recovery and radiation of Early Jurassic ammonoids: morphologic versus palaeobiogeographical patterns

Abstract The recovery and radiation of the Early Jurassic ammonoid morphospace, as represented by nine morphologic groups identified in an earlier study, are traced through the first 36 subzones of the Hettangian to Domerian time interval. A quantitative survey of the dispersion of 436 species over 15 palaeogeographical areas reveals seven palaeobiogeographical patterns, each corresponding to an exclusive set of species exhibiting similar characteristics in terms of distribution and abundance. This study combines morphologic, chronostratigraphical, and palaeobiogeographical data in an attempt to investigate possible connections between morphologic recovery and radiation patterns and the history of ammonoid distribution on a global scale. Two out of nine morphologic groups are found to be prevalent in a single palaeobiogeographical pattern through the major part of the studied time interval, and a single morphologic group prevails in all the marine areas considered during Domerian times. All the other relationships identified are less tightly constrained in space and time and indicate loose ties over more limited periods and narrower distributions. The present results suggest that such complex relationships can be deciphered more successfully by using analytical palaeobiogeographical patterns as proposed here rather than classical palaeobiogeographical units (biochores).

Area of mixed marine faunas between two major paleogeographical realms, exemplified by the Early Jurassic (Late Sinemurian and Pliensbachian) ammonites in the Alps

Abstract The Late Sinemurian and Pliensbachian ammonite faunas of the Alpine Range yield a good opportunity to study an area of mixed marine fauna. The faunas of the 4 major paleogeographical Alpine units (from Southern Alps to Delphino-Helvetic) are illustrated by fifteen recently investigated outcrops. Moreover, information provided by the Alpine units is compared with both northern and southern surroundings. These data areaanalyzed here through a qualitative and quantitative approach. Two main patterns of distribution are recognizable. The first one, mainly Ammonitina versus Phylloceratina and Lytoceratina, shows a rather gradual decrease of Ammonitina to the South and of Phylloceratina-Lytoceratina to the North. The second pattern, well exemplified if we consider Euroboreal vs Tethyan Ammonitina, corresponds to abrupt faunal changes. The pattern of the first suggests that ecological (depth of water, currents, siliciclastic sedimentation, …) constraints are stronger than competition between the two considered groups. In the second case competition between ammonites seems more important than ecological constraints. At last, redundancies in patterns of faunal distribution reveal that only two paleogeographical areas correspond to important faunal changes. The first one is probably located on the Brianconnais area and the second one is situated between Upper Austroalpine and Southern Alps) areas. If the Brianconnais is classically considered as a possible barrier, the second faunal boundary is more enigmatic because palinspastic reconstructions hardly suggested any physical barrier at this place, except perhaps the Vardar ocean.

Late Cretaceous (Cenomanian-Turonian) ammonites from southern Morocco and south western Algeria

On the basis of nine lithostratigraphical profiles, 23 cephalopods taxa (nautilus and ammonites) are described from the Preafrican Trough and the Kem Kem region. Among them, a new species is proposed: Spathites (Jeanrogericeras) asflaensis nov. sp. This fauna is placed in its biostratigraphical framework and correlated with the standard zonation of the late Cenomanian–early Turonian. Moreover, stratigraphic correlations are proposed for the whole Maghreb from the Tarfaya Basin in the west to central Tunisia in the east. Selected taxa reveal the paleogeographical context of the western Tethys; a complex distribution of emerged areas and epicontinental seas impacted by an important marine trangression that constantly modified the costaline.

Breccias of the Adnet Formation: indicators of a Mid-Liassic tectonic event in the Northern Calcareous Alps (Salzburg/Austria)

Stratigraphy, lithology and depositional structures of Liassic red limestone-breccias of the Adnet Formation, including the ‘Adnet Scheck’, were studied at several outcrops of the Northern Calcareous Alps (NCA) south-east of Salzburg. A four-fold lithostratigraphic division is proposed for the Adnet Formation of the Osterhorn Mountains: the hemipelagic Schmiedwirt (Sinemurian) and Kehlbach (Carixian) members are separated from the pelagic Saubach Member (Toarcian) by a layer of amalgamated breccias (Scheck Member, probably Domerian to early Toarcian). Several other breccia beds occur locally from the base of the Kehlbach Member up to the lower Saubach Member. Although the sediments overlying the Scheck Member breccias are of coeval age, the ages of the underlying strata are very different. This can be explained by submarine Liassic erosion during a period of resedimentation from the middle Carixian until the early Toarcian. At least 10–15 m of partly lithified sediments were eroded by gravity flows. The entire Kehlbach Member and up to two-thirds of the Schmiedwirt Member were removed at Adnet. The breccias originated from submarine debris flows. Repeated flows over a long period and the depositional setting exclude a triggering by sea-level fluctuations. Most probably they arose from tectonically triggered slumps and slides of superficial sediments. The ‘Scheck’ was initiated on the steep upper slope of the drowned Triassic Adnet reef and flowed to the north-east. The Pliensbachian to early Toarcian period of tectonic activity indicated by the breccias was the most important during the Liassic in the Osterhorn Mountains and other parts of the NCA. From the large-scale regional distribution of the breccias and in accord with published data, a roughly northeast trending strike-slip fault zone is proposed, crossing the NCA south of the Osterhorn block, with a peaking activity during the Pliensbachian to early Toarcian as the cause of the tectonic movements.

Nouvelles données sur lesammonites du Niger oriental (Ténéré, Afrique occidentale) dans le cadre de la transgression du Cénomanien-Turonien

The new data on the ammonites faunas allow to specify the Cenomanian-Turonian biostratigraphy of the Nigerduring the transsaharian marine transgression with the description of two new horizons: Glabrum horizon and Wallsi horizon. If the endemism tendancy is corroborated mainly for the Damergou and the Iullemmeden Basin, the new results make conspicuous a strong North-South connection between the Tenere and the NE Nigeria with the presence of Vascoceras aff. glabrum (Barber), Neoptychites gr. cephalotus (Courtiller), Thomasites nigeriensis (Woods), Thomasites wallsi (Reyment) et Choffaticeras sp. Moreover a new species Thomelites (?) tenereensis nov. sp. is here describeb.

Unusual faunal associations during Upper Cenomanian-Lower Turonian floodings on the Niger ramp (central West Africa)

Abstract The Upper Cenomanian-Lower Turonian marine deposits of the Iullemmeden Basin and of the northern Chad Basin (Niger) consist of interbedded mudshales and fossil-rich limestones that formed on a ramp as a result of a Tethyan transgression. The faunal content of the limestone beds is characterized by (1) the absence of forms typically found on the Tethyan platforms: rudistids, corals and commonly associated forms (large benthic foraminifers, bryozoans, calcareous sponges, red and green algae); (2) the absence of pycnodonts and of brachiopods, and (3) the abundance of Gryphaeidae and Ostreidae, as well as the relative abundance of ammonites. This unusual faunal composition is ascribed to several factors. Rudistids, corals and associated forms may have failed to occur because of the general decrease in the diversity of these organisms in the Upper Cenomanian-Lower Turonian and because of the lack of suitable shoals on the Niger ramp for reefs to develop. Pycnodonts may have failed to gain a foothold in the face of severe competition from pioneer and opportunistic bivalves (Gryphaeidae, Ostreidae) that quickly colonized the ramp. The absence of brachiopods has not yet been explained satisfactorily; it could be due to several factors, including severe competition with pioneer bivalves and/or excessive turbidity of bottom waters.

Exploration of the Oxynoticeratidae ornamental morphospace using the discrete cosine transform (DCT) to analyze rib patterns.

Abstract The discrete cosine transform (DCT) is a Fourier-related transform widely used in signal processing and well suited to analyzing open outlines such as ammonite ribs. The method is applied here to depict and decipher the ribbing morphospace of a large group of Lower Jurassic ammonites composed of the Oxynoticeratidae and their close ancestors. Because they are clearly associated with buoyancy and/or swimming ability, the usually clearly involute, comparatively smooth and compressed shells of these ammonites may well be misleading taxonomic markers. In this context, quantitative analysis of the ribbing pattern using the DCT may significantly improve our perception of the ornamental patterns expressed within the group. A set of 251 specimens illustrating the worldwide fauna and selected from more than 80 publications is analyzed. Big differences are found in the evolutionary patterns of the two main lineages of Oxynoticeratidae currently accepted in the literature. A previously unsuspected Mediterranean group comprising principally the genus Parasteroceras is identified from its distinctive ornamentation. The northwest European and Mediterranean genera Eparietites, Oxynoticeras, and Parasteroceras do not feature among the American (East Pacific) faunas. This finding calls into question some generally accepted correlations between European and American stratigraphic frameworks. The study shows that the DCT is a valuable tool for discriminating between species within the huge and often puzzling range of ornamental variation of the main genera (e.g., Gleviceras and Radstockiceras).