Jean-Louis Dommergues

High-resolution dynamics of Early Jurassic marine extinctions: the case of Pliensbachian-Toarcian ammonites (Cephalopoda)

Abstract: The Pliensbachian–Toarcian interval was marked by major environmental disturbances and by a second-order mass extinction. Here, we reappraise the taxonomic, spatiotemporal and selective dynamics of extinctions over the whole interval, by analysing a high-resolution dataset of 772 ammonite species from NW Tethyan and Arctic domains. On average, 40–65% of ammonite species disappeared during each subchronozone, but higher extinction pulses (reaching 70–90%) prevailed from the Margaritatus to the Dispansum Chronozone. The main extinctions, corresponding to the Gibbosus, Pliensbachian–Toarcian boundary, Semicelatum, Bifrons–Variabilis, and Dispansum events, differed in their dynamics, suggesting episodes of ecological stress related to climate change, regression, disturbance in the carbon cycle or anoxia. The multi-pulsed volcanic activity in the Karoo–Ferrar province could well have triggered these ecological changes. In addition, ammonites experienced a morphological bottleneck during the Gibbosus event, 1 Ma before the Early Toarcian diversity collapse. Typically, drops in richness were related both to high extinctions and to declines in origination rates. This feature could result from strengthened ecological stresses related to the temporal overlap of environmental disturbances. After the Early Toarcian crisis, the recovery of ammonites was rapid (2 Ma) and probably influenced by a coeval marine transgression. Supplementary material: Figures showing a comparison of extinction and origination patterns based on different datasets, and variations of the morphospace occupation are available at http://www.geolsoc.org.uk/SUP18381.

Mature Modifications and Dimorphism in Ammonoid Cephalopods

The shell of an ammonoid is a kind of autobiography of the animal that once occupied it. Different parts of the shell tell different parts of the life history. The growth lines and the tiny intervals in between, along with the shape of the shell itself, record what was happening at the anterior end of the body. The septa and their sutures relate the tale of the other extremity.

Differentiation at a microgeographical scale within two species of ground beetle, Carabus auronitens and C. nemoralis (Coleoptera, Carabidae): a geometrical morphometric approach

New morphometric methods, the geometrical morphometrics, offer promising perspectives to appraise morphological variation among organisms and open up, to a large extent, the field of morphometrics for the study of systematics and evolution. Until now, however, few studies have explored the potential of these methods at a microgeographical scale. In the present work, we applied them to quantify morphological (size and shape) differentiation among populations of two forest species of ground beetles: Carabus auronitens and C. nemoralis. We found a significant shape variation among sites, as well as among sexes, for both species. Additionally, for C. auronitens, we found significant positive correlations in both sexes between morphological (shape) and geographical distances between populations. In contrast, significant size differences were found between sexes, but not between sites. We conclude that geometrical morphometric methods provide valuable tools for the study of morphological variation among populations and therefore offer, on the whole, interesting perspectives for the study of biodiversity patterns.

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.

Size patterns through time: the case of the Early Jurassic ammonite radiation

Abstract The shell size of 1236 ammonite species representing all known Early Jurassic faunas is analyzed. Size patterns are studied for the entire period and then at the biozone scale for the first four stages of the Jurassic (28 Myr), during which ammonites recovered from the crisis at the Triassic/Jurassic (T/J) boundary. Our analysis reveals that (1) a size continuum (normal distribution from “dwarfs” to “giants”) exists for all Early Jurassic ammonites; (2) although there are no sustained trends (e.g., no Copes rule), the succession is not monotonous and patterns may differ conspicuously from one biozone to the next; and (3) increases and decreases in size range are the most frequent evolutionary styles of size change. The only pattern that can be connected with a particular episode of Early Jurassic ammonite history is the initial increase in size disparity during the first four biozones attributable to phyletic radiation after the T/J crisis. Subsequent correlations with environmental constraints (e.g., sea-level changes), although suspected, cannot be shown.

Hettangian and Sinemurian magnetostratigraphy from Paris Basin

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148-m-thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300°C) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 65° is interpreted as a present-day field overprint. The characteristic remanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 mT. Eighty-one polarity intervals are recognized in this study. The higher reversal frequency of the late Hettangian/early Sinemurian time interval contrasts with a lower reversal rate observed in the rest of the early Liassic. A rough mean estimate of about 5 reversals/m.y. can be proposed for the earliest Jurassic. These results represent a significant contribution to the magnetic polarity reversal timescale for a time interval hitherto poorly known and add to the magnetic reversal frequency curve of the last 350 m.y.

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.

Early Jurassic normal faulting in a carbonate extensional basin: characterization of tectonically driven platform drowning (High Atlas rift, Morocco)

Abstract: This paper describes a tectonostratigraphic model of the synrift evolution of the Early Jurassic High Atlas rift of Morocco. The model is constrained by mapping of a set of inverted extensional blocks, by facies analysis of carbonate platform and turbiditic to hemipelagic synrift deposits, and by high-resolution (n × 100 ka) biostratigraphy of the Early Jurassic succession. The chronostratigraphic packages of the High Atlas of Rich vary significantly in thickness, facies and architecture from one tectonic block to another. Our study shows how synrift strain varied in space and time over a long time interval (14 Ma) around the High Atlas rift. Initially, in Sinemurian time, the High Atlas rift was affected by low-strain normal faulting that controlled the growth of an extensive, low-gradient carbonate platform, except in the northern domain (towards the rift axis), where hemipelagic deposition related to high-rate faulting prevailed. Subsequently, in Carixian–Domerian time, a rapid increase in accommodation space and block subsidence caused by high-strain normal faulting brought about localized drowning of the carbonate platform and the development of calciturbidites and of starved deposits towards the rift axis. During this interval, high-strain, upper-crustal normal faulting migrated rapidly (over a period of 5 Ma) towards the rift periphery.

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.