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1

New records of Tmaegoceras lacordarii (Michelin, 1835) from the Lower Jurassic Arietenkalk Formation of SW Germany and its systematic relationships

Schweigert Günter, Scherzinger Armin, Gräbenstein Stefan

Volumina Jurassica

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2021

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Vol. 19, no. 1

95--108

EN

New material of the enigmatic cosmopolitan ammonite genus Tmaegoceras Hyatt, 1889 is reported from the upper Lower Sinemurian (Bucklandi Zone, ‛cf. pinguisʼ Biohorizon) of SW Germany. It is better preserved than previously described material. All specimens are assigned to Tmaegoceras lacordarii (Michelin, 1835), an almost forgotten subjective senior synonym of Tmaegoceras crassiceps Pompeckj, 1901. The presence of an extreme rursiradiate ribbing style in some specimens allows a better understanding of this genus and provides a hint for its systematic affiliation. This diagnostic ribbing style is shared with some extremely rare Alpine taxa such as Ammonites salinarius Hauer, 1846 and “Arietites” subsalinarius Wähner, 1891. Tmaegoceras is included here in Pseudotropitinae Donovan, 1973. Pseudotropitinae are possibly an early offshoot of Arietitidae indigenous of the Panthalassa Realm that occasionally spread into the Tethys and its adjacent shelves. A phyletic relationship with Late Triassic Tropitidae Mojsisovics, 1875, however, cannot be excluded either.

2

The Upper Sinemurian ammonite succession in the Sierra Madre Oriental (Mexico)

Meister Ch., Blau J., Dommergues J-L., Schlatter R., Schmidt-Effing R.

Volumina Jurassica

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2009

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Vol. 7, no. 1

31-36

EN

In the Sierra Madre Oriental (Mexico), the biostratigraphic subdivision of the Upper Sinemurian can be refined. It has been possible to differentiate 11 horizons from the upper Obtusum Zone to the top of the Raricostatum Zone which are correlated with the NW European standard zonation. The index species of each horizon is figured.

3

High resolution ammonite stratigraphy of the Charmouth Mudstone Formation (Lower Jurassic: Sinemurian-Lower Pliensbachian) in south-west England, UK

Page K.

Volumina Jurassica

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2009

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Vol. 7, no. 1

19-29

EN

The "Lower Lias" mudrocks of the Charmouth Mudstone Formation inWest Dorset coast are world famous for their ammonite faunas, which range from mid Lower Sinemurian (Semicostatum Chronozone) to Lower Pliensbachian (topmost Davoei Chronozone) in age. The succession includes significant non-sequences, however, and as certain other intervals yield only crushed and relatively poorly preserved material, much of the sequence of ammonite faunas of this interval in south-west England has remained poorly understood. Inland, however, although it has been realised for many years that some of the missing horizons reappear, the Formation is very poorly exposed and as a consequence little has been known about its detailed stratigraphy and palaeontology. The systematic recording over 40 years by Mr H.C. Prudden (Montacute) of temporary excavations in East Somerset (around 20 km north of the Dorset coast), combined with material collected by others from similar exposures has now, however, revealed a virtually complete sequence of ammonite faunas through the interval represented by the Formation including from many of the which are missing on the Dorset coast. In particular, only one subchronozone remains to be conclusively proven in the region, the terminal Sinemurian, Aplanatum Subchronozone (Raricostatum Chronozone). This faunal succession is correlated with that on the coast to provide a detailed synthesis of the sequence of ammonite biohorizons in the region, which is correlated with a contemporary Standard Zonation and high-resolution biohorizonal/ zonule scheme for interval in North-West Europe. The significance for regional and international correlations of the Lower Lias is also discussed.

4

The Sinemurian ammonite genus Dudresnayiceras in the Moroccan High-Atlas

Quartier-la-Tente M., Morard A.

Volumina Jurassica

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2006

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Vol. 4, no. 1

243-244

EN

Dudresnayiceras is a characteristic - though quite rare - ammonite genus of the Tethyan Oxynotum Zone (Upper Sinemurian). We could only count approximately 30 distinct specimens cited in the literature. Four named taxa were attributed to this very peculiar genus with asymmetric ceratitiform suture lines: * D. subcostulatum Schafhäutl, 1854: 1 specimen from the Alpine Oolith of the Bavarian Prealps; * D. suessi Hauer, 1854 (type species of the genus): approximately 20 specimens from the Austrian Hierlatzschichten ("Oxynotum horizon"), from the Apennines and from Tunisia; * D. suessiforme Rakus, 1994: 8 specimens from the Oxynotum Zone of Morocco and Tunisia; * D. tuberculatum Rakus, 1999: 1 specimen from the Austrian Hierlatzschichten. In this study, more than 80 Dudresnayiceras specimens were collected during a bed by bed profiling of two Upper Sinemurian sections from the Moroccan High-Atlas. In both the Bou Hamid (Rakus 1994; Lachkar et al. 1998) and Kadoussa sections, Dudresnayiceras occurs throughout the Oxynotum Zone. This encouraged us to investigate possible changes through time in sutural asymmetry, ornamentation and/or morphology of this genus. Although the shift of the external lobe (and of the siphuncle) principally occurs towards the left flank (56% of investigated specimens), subsymmetrical (19%) and right-sided deviations (25%) are also present. Fluctuations between left- and right-handed deviations in successive samples do not show any consistent trend. The comparison of this material with data from the literature, particularly the type specimens of the few named taxa, confirms the homogeneity of this "anomalous" ammonite group, as well as its important role for correlation purposes. Nevertheless, ontogenetic and intraspecific variability is quite large, mainly affecting whorl section and ornamentation respectively. It is still difficult to determine the phylogenetic affinities of Dudresnayiceras. The simplification of the suture line, as well as its asymmetric configuration, may indicate that this genus suffered stressfull conditions. The very evolute coiling, the quite simple ornamentation and the absence of a keel are other hints to an atavistic morphology.

5

Sinemurian to lowermost Toarcian ammonites of the Brescian Alps (Southern Alps, Italy): biostratigraphical framework and correlations

Meister Ch., Schirolli P., Dommergues J-L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

184-185

EN

A set of 28 ammonite biohorizons or faunal assemblages can be proposed for the Sinemurian, the Pliensbachian and the lowermost Toarcian in the Brescian Alps, in part based on the data from Dommergues et al. (1997) and partly on new results, deriving both from recent field investigations and from the study of the historical collection of the Lower Jurassic ammonites preserved in the Museum of Natural Sciences of Brescia (Northern Italy), placed into their stratigraphical framework (Fig. 1). The biohorizons are referred to the Liassic carbonate succession of the Brescian Alps, outcropping between the eastern surroundings of Brescia (Botticino), to the East, and the Lake Iseo, to the West. Since the Hettangian the region was subjected to the paroxismal phase of the Jurassic rifting and the area of study was located in the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin of the Tethys. At the beginning of Jurassic, an articulated fault-system, composed of N-S master faults (W-dipping) and W-E transfer faults, located from Brescia to the North, separated a western growing basinal area from the eastern Botticino structural high. Since Hettangian to the onset of Toarcian, almost 1000 m of well-stratified cherty marly limestones of the Medolo Group (Gardone Val Trompia Limestone and Domaro Limestone formations) represent the synrift deposition of the Sebino Basin, following the drowning of the Raethian-Hettangian Corna Platform. In contrast, after the Early Sinemurian a coeval reduced sequence (50 m thick), composed by the Rezzato Encrinite (Lower Sinemurian to Upper Carixian) and the overlying thinly-bedded and nodular ammonitic marly limestones of the Botticino Corso Rosso (Upper Carixian and Domerian), covered the Corna Platform in the Botticino High. The recognized ammonite biohorizons and assemblages are quite well integrated and correlable with either the NW European standard zonation or the different zonations given for the Tethyan realm (Appennines, Subbeticas and Austrian and Hungarian Upper Austroalpine). Most of these stratigraphical units are based on Tethyan taxa. Only two horizons (U. cf. jamesoni horizon and P. solare horizon) show NW European affinites. Moreover the E. quenstedti horizon is only known from the northern margin of the Tethys, mainly from the Upper Austroalpine units.

6

Sinemurian ammonite succession from the Sierra Madre Oriental (Mexico)

Blau J., Meister Ch., Dommergues J-L., Schlatter R., Schmidt-Effing R.

Volumina Jurassica

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2006

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Vol. 4, no. 1

147-148

EN

In a series of studies (Blau et al. 2001, 2002, 2003; Meister et al. 2002, 2005) dedicated to Liassic (Sinemurian) ammonites of the Sierra Madre Oriental (Mexico), we have described 42 taxa, amongst them one new genus and five new species. After a critical analysis of Erben’s collection and with our own collections, we propose a synthetic succession of 13 correlable units “biohorizons”. Even though the Sinemurian sediments of the Huayacocotla Basin are very thick only two periods are well represented: the Bucklandi and Semicostatum zones for the Lower Sinemurian and the upper Obtusum and Raricostatum zones for the Upper Sinemurian. These two time intervals can be well correlated with South and North American biostratigraphic schemes. The Lower Sinemurian is characterized by the acme of Arnioceras a genus well distributed on the eastern Panthalassian rim. The Upper Sinemurian allows a much more refined biostratigraphic subdivision and more acute correlations. In the upper Obtusum Zone Euerbenites is an index fossil and can be correlated from Sonora to Peru. In the Raricostatum Zone several horizons allow acute correlation with South America (O. incaguasiense and P. tardecrescens horizons) and with North America (P. harbledownense and P. rothpletzi horizons). Mainly in the Upper Sinemurian there is quite strong endemism of the ammonite fauna in the Huayacocotla Basin (nearly 40%) which may be due to a quite isolated palaeogeographical position of the basin. On the other hand the affinities to Tethyan faunas are obvious. There is little palaeontologic evidence for a Sinemurian connection between Eastern Pacific (Panthalassa) and Western Tethys through the Hispanic Corridor. First palaeontological evidence indicates a possible connection not before the Pliensbachian. If this connection was not established in the Sinemurian, we must imagine other migration ways to explain the Tethyan affinities. There are at least four possibilities or combinations of migration routes: southern peri-Pangean, peri-Asiatic, latudinal trans-Panthalassian, and Boreal (Viking Corridor, Arctic seas). All these migration routes remain hypothetical and the establishment of the Hispanic Corridor already in the Sinemurian cannot be ruled out completely.

7

Microbial mounds from the restricted-marine Sinemurian succession of S. Pedro de Moel (Lusitanian Basin, Portugal): preliminary palaeoecological interpretation

Azeredo A., Duarte L., Cabral M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

77-77

EN

In the Lusitanian Basin the Sinemurian corresponds to a marginal- to restricted-marine succession, representing the early stage of sea flooding in the recently formed basin, following a Late Triassic rifting event. Except for its topmost part, the Sinemurian succession belongs to the Coimbra Fm., composed of dolostones, dolomitic limestones and limestones. This unit is dominated by peritidal facies towards the eastern, more landward zone of the basin, whereas in the W (as at S. Pedro de Moel) more distal (though not quite deep) facies occur. At S. Pedro de Moel the Coimbra Fm. is well exposed and the section displays a succession mostly composed of argillaceous and/or dolomitic limestones and fossiliferous/skeletal limestones (bivalves, gastropods, ostracods), interbedded with a few marly levels. Although fossil remains are commonly present in variable amounts, some beds seem to be azoic, others exhibit rare, deformed burrows and very rare ostracods. Undulating, irregularly bounded, laminar levels are common and, locally, centimetre to decimetre-thick concentrated skeletal/fossil layers occur. However, towards the middle part of the section, well-preserved, dome-shaped stromatolites occur, in clear contrast with the under- and overlying bedded deposits. Towards the top of the succession, the calcareous/clay ratio increases. This section is still under study, so we only make a preliminary palaeoenvironmental approach here. The microbial mounds have an average maximal thickness of 0.75 m and show different fabrics, sometimes within the same mound: laminated, stromatolitic crusts; clotted, peloidal micrite; micritic and sparitic threads; degraded, tuft-like filamentous structures; enhanced fenestral-like porosity; dense, slightly darker micrite. These features suggest that they were formed through hardening of calcified cyanobacterial and other microbial films, whose early disintegration also would have contributed with autochthonous mud for the mounds. However, a more detailed study is clearly needed. It is known from the literature that microbialites, as a whole, may form in a wide range of environmental conditions, though some associations or particular morphologies may give us more accurate ecological information. However, a crucial basic condition is a very low to low background sedimentation. In the present case, it is suggested that the stromatolites grew under low-energy, restricted water-circulation and low rate of mixed terrigenous and calcareous mud deposition. A likely nutrient-poor substrate, the existence of terrigenous material and, maybe, slight (?)hypersalinity would have inhibited a more significant development of epibenthic and heterotrophic organisms, favouring the microbial community. The low energy prevented physical erosion which, coupled with the absence of predators, allowed the development of the well-defined dome morphologies. In contrast, towards the top of the succession a more open setting prevailed, with better water-circulation, probably better oxygenation and somewhat higher sedimentation rate with dramatic decrease of clay material. Though most of the upper deposits are still low-energy ones (biomicrite mudstones and wackestones, with a few ostracods, gastropods, bivalves, rare hyaline forams), skeletal/intraclastic/peloidal packstones and rarer grainstones occur more frequently, attesting for the less protected environment. These conditions did not allow the continuation of microbial growth.

8

High resolution ammonite stratigraphy of the Charmouth Mudstone Formation (Lower Jurassic: Sinemurian – Lower Pliensbachian) in South West England (UK)

Page K.

Volumina Jurassica

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2006

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Vol. 4, no. 1

200-200

EN

The “Lower Lias” mudrocks of the Charmouth Mudstone Formation in West Dorset coast are world famous for their ammonite faunas, which range from mid Early Sinemurian (Semicostatum Chronozone) to Early Pliensbachian (topmost Davoei Chronozone) in age. The succession includes significant non-sequences, however, and as certain other intervals yield only crushed and relatively poorly preserved material, much of the sequence of ammonite faunas of this interval in South West England has remained poorly understood. Inland, however, although it has been realized for many years that some of the missing horizons reappear, the Formation is very poorly exposed and as a consequence little has been known about its detailed stratigraphy and palaeontology. The systematic recording over 40 years by Mr H. C. Prudden (Somerset Geology Group) of temporary excavations in East Somerset (around 20 km north of the Dorset coast) has now, however, revealed a virtually complete sequence of ammonite faunas through the interval represented by the Formation. In particular, many levels have now been identified which are missing in the major non-sequences on the Dorset coast, thereby revealing a much more complete stratigraphical sequence in the region than previously realized. In particular, only one subchronozone remains to be conclusively proven – the Aplanatum Subchronozone of the Raricostatum Chronozone – although this could still be due to collection failure as nodular facies do not appear to be present at this level and near-surface clay exposures are often too degraded to yield determinable specimens. This faunal succession is correlated with that on the coast to provide a detailed synthesis of the sequence of ammonite biohorizons in the region, which is correlated with a contemporary Standard Zonation and high-resolution biohorizonal/zonule scheme for interval in North West Europe. The significance for regional and international correlations of the Lower Lias is also discussed.

9

Ammonite biostratigraphy of Central High Atlas. a reference section for Tethyan Domain

El Hariri K., Dommergues J-L., Chafiki D., Meister Ch.

Volumina Jurassica

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2006

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Vol. 4, no. 1

170-170

EN

The Sinemurian times of the Moroccan Central High Atlas saw important palaeogeographic changes which were of primary importance in the geodynamic evolution of the Atlas Basin. Sedimentation in the subsiding basin was dominated by the hemipelagic, thin bedded sediments of the Aberdouz and Ouchbis formations. The ammonite fauna gives the basis for precise dating of the deposits and recognition of changes of facies along a proximal/distal direction which corresponded to the main breakup of the platform at the turn of Early and Late Sinemurian. This study is concentrated on Sinemurian ammonites of Central High Atlas (Morocco). It refers to four sections namely Bou-Hamid, n’Zala, Tillicht, Foum Zaabel in the High-Atlas of Midelt-Rich and two sections namely Tamadoute, Taguendouf in the High-Atlas of Beni-Mellal. A careful systematic description of 70 taxa belonging to the Phylloceratidae, Juraphyllitidae, Lytoceratidae, Schlotheimiidae, Arietitidae, Echioceratidae, Acanthopleuroceratidae, Oxynoticeratidae, Eoderoceratidae, led to the construction of a detailed biostratigraphical framework for the Sinemurian. Ten horizons were recognized in the Lower Sinemurian (Obtusum Zone) – Upper Sinemurian (Raricostatum Zone) interval up to Lower Carixien (Jamesoni Zone). The bulk of the species show Tethyan paleogeographical affinities and can be correlated with those of the northwestern European standard zonation.