Bajocian Ammonites Research Articles

Bajocian (Middle Jurassic) ammonites of stratigraphical and palaeobiogeographical importance from Mombasa, Kenya, East Africa

ABSTRACT Collections of Middle Jurassic ammonites made at the Mwachi River locality, near Mombasa in the late 1980′s and early 1990′s are described. They support previous research concluding a Late Bajocian age for the major or the upper part of the ‘Posidonia shales’, immediately below and above the dividing Massive (Coral) Limestone. The ammonite species are figured and briefly described. Two Upper Bajocian ammonite zones are evidenced: a lower assemblage indicates the Strenoceras niortense Zone and a higher one belonging to the Garantiana garantiana Zone. Most of the ammonites are Tethyan forms with abundant phylloceratids and lytoceratids, some indicating connections with coeval Arabian and Sinai faunas. Palaeobiogeographically the assemblages suggest a deep-water position on the East-African Gondwana margin, with strong faunistic influence from the open Tethys and very restricted connections with the Arabian shallow water regions.

Aalenian to Lower Bajocian ammonites from the Qiangtang block (North Tibet)

Aalenian and Bajocian ammonites from the Qiangtang block (North Tibet) are described. Comparisons with successions of similar age in Europe allow the establishment of a correlation at zonal level. The relationship between early stephanoceratids and the otoitid Emileites is discussed. Emileites callomoni sp. nov. is erected. Haplopleuroceras, Fontannesia, Witchellia and Sonninia also occur and their significance highlighted. A lectotype for Haplopleuroceras mundum Buckman is designated.

Lower Bajocian ammonites from the stratotype section of the Etropole Formation kept in the Historical Museum in Etropole (Bulgaria)

The area of the town of Etropole is famous for the well-preserved Lower Bajocian ammonites, which can be still often found in the nearly located exposures. These ammonites were the source of a number of species that enabled the recognition of the Bajocian in Bulgaria. The ammonite faunas and their host rocks were allocated to a series of stratigraphic horizons that composed the stratotype section designated as the reference standard for the definition and characterization of the Etropole Formation. The latter includes an ammonite spectrum which is composed of both Lower Bajocian species of Northwest European distribution and proper Bulgarian dispersal. It contains rare examples of the ammonite families Graphoceratidae and Sphaeroceratidae, frequent ammonites of the family Sonniniidae, and much more common members of the families Oppeliidae, Otoitidae and Stephanoceratidae that define a biostratigraphic interval from the Hyperlioceras discites to the Stephanoceras humphriesianum zones. This paper deals with the Lower Bajocian ammonite specimens hosted in the collection of the Historical Museum in Etropole (Bulgaria). The best preserved and the most valuable examples of this collection are described and figured. Although not purposefully collected, these ammonites give an excellent impression of the relative abundances of what is common and typical for stratotype section of the Etropole Formation. The study of this collection has triggered the rocks of the Etropole Formation (including the type-section) to be biostratigraphically reassessed, but this will be done elsewhere. Considering that both exposures and the fossil-bearing levels previously known from the Etropole area have now nearly disappeared, the ammonite collection of the Historical Museum in Etropole provides authentic specimens to be viewed. Besides, the collection forms an integral part of the overall heritage of the Etropole region and therefore offers both scientific and educational values.

Toarcian and Bajocian ammonites from the Haushi-Huqf Massif of southwestern Oman and the Hawasina Nappes of the Oman Mountains: Implications for paleoecology and paleobiogeography

ABSTRACT New and rare Jurassic ammonites have been found in Oman. A latest Bajocian Arabian Platform-type species was discovered in the Haushi-Huqf Massif autochthon of southwestern Oman, and Bajocian species typical of the Mediterranean Tethys and northwestern Europe were found in the Kawr-Misfah exotic unit of the Hawasina Nappes in the Oman Mountains. The dates provided by the new fauna have resulted in a reinterpretation of the geologic history of the containing rocks, and of their paleoecology and paleobiogeography. It is significant that ammonites from shallow-marine environments of the Arabian Platform are in close proximity to species from open-sea environments of the Mediterranean Tethys and northwestern Europe. This shows that endemism of the Arabian Province resulted from ecological isolation, whereas open-marine environments on the Oman margin, especially the pelagic seamounts off the margin, form part of a migration route between western and eastern Tethys (or Indo-Southwest Pacific), and perhaps far beyond. The occurrences among the Tethyan and pandemic components of ammonite faunas in the Canadian Pacific Cordillera of most of the taxa of the open-marine environments on the Oman margin reopens the question of Pacific biogeography during the Early Jurassic before the Hispanic oceanic corridor was completely open. Among the proposed models, the Pantropic Distribution Model of Newton is examined in the light of the Cretaceous paleobiogeography, with particular reference to rudists.

Diversity dynamics of Bajocian (Middle Jurassic) ammonites and transgressions/regressions in the Caucasian Sea (northern Neo-Tethys Ocean): A case high-resolution test of probable dependence

Ammonites commonly occur in the Bajocian deposits of the Greater Caucasus Basin, from where 180 species and 52 genera are reported. This back-arc basin was located on the northern Neo-Tethyan margin embraced by the Caucasian Sea. An evaluation of diversity patterns for 7 Bajocian ammonite zones documents high diversity in the beginning, middle, and end of the stage with a maximum in the Humphriesianum Zone and two diversity minima. The mid-Early Bajocian impoverishment of ammonite assemblages was caused by both an increase in the rate of disappearance and a decrease in the rate of appearance, whereas the only latter was responsible for the mid-Late Bajocian diversity drop. An analysis of dominating facies types in the Greater Caucasus Basin documents that the total diversity rose during regressions and fell during transgressions. Shoreline shifts corresponded better with changes in the number of appearances and worse with those of disappearances. A comparison of results from this study with those obtained in the Betic Cordillera does not clarify the observed dependence. Changes in the exterior connections of the Caucasian Sea, its depth, and oxygen depletion of its waters are unlikely explanations of the regionally documented coincidence of ammonite diversity dynamics and transgressions/regressions.

Aalenian and Lowermost Bajocian ammonites (Hammatoceratinae and early Otoitidae) from the Western and Central Balkan Mountains (Bulgaria)

Five taxa of the species group, belonging to the subfamily Hammatoceratinae and one species ofthe family Otoitidae are described and figured in the present paper, for the first time in this country. From the subfamily Hammatoceratinae they are Pseudammatoceras subinsigne (Oppel, 1856), Pseudammatoceras cf. ornatum Elmi, 1963, Pseudammatoceras broilii (Renz, 1925), Planammatoceras planiforme Buckman, 1922 (Aalenian, Ludwigia murchisonae Zone) and Parammatoceras boyeri Elmi, 1963 (Aalenian, Graphoceras concavum Zone), and from the family Otoitidae - Docidoceras cf. cylindroides Buckman, 1919 (Lower Bajocian, Hyperlioceras discites Zone).

Bathonian ammonites from CantonAargau, Northern Switzerland: Stratigraphy, taxonomy, and biogeography

Abstract The first Bathonian ammonites found in measured sections in Canton Aargau (Switzerland) are described and figured: they are mainly tulitids, some oppeliids, rare perisphinctids and stephanoceratids. A comparative study of the ontogeny of the tulitids has allowed the number of genera to be reduced to three: Tulites Buckman , 1921 , Bullatimorphites Buckman , 1921 and Morrisiceras Buckman , 1920 . The dimorphism is expressed by the subgenera. Among the oppeliids, Trimarginia Arkell , 1952 , a genus proposed for the Late Bajocian ammonites of the Sinai Peninsula in Egypt, is represented by T. sylviae nov. sp. of Late Bathonian age. These ammonites date from the Middle and Late Bathonian in thin condensed iron oolitic beds that were previously assumed to belong to the Callovian or even Oxfordian ages due to a lack of dating elements. Trimarginia is found for the first time in what are undoubtedly Upper Bathonian beds. This enlarges its previously known biogeographic and stratigraphic range.

Middle Jurassic (Bajocian) ammonites from the Newmarracarra Limestone, Western Australia

Middle Jurassic (Bajocian) ammonites from the Newmarracarra Limestone, Western Australia

Lower Bajocian ammonites (Middle Jurassic; Soninniidae) from the Newmarracarra Limestone, Western Australia

Three species of sonniniid ammonites, previously described under Fontannesia Buckman, are placed in a new genus, Newmarracarroceras; Dorsetensia clarkei Crick is designated type species. These West Australian species differ from other known Fontannesia by having smooth body chambers, flanks which slope gradually toward the umbilical seam without development of an umbilical edge or wall, and simple septal sutures. Biometric studies show that ‘Fontannesia’ whitehousei Arkell is a smaller and more compressed variant of N. clarkei (Crick), with a complete gradation between the two ‘species’ for most morphological characters. While well-dated European Fontannesia occur in the uppermost Aalenian concavum Zone and lowermost Bajocian discites Zone, new dating of the circum-Pacific Pseudotoites faunas, with which Newmarracarroceras occurs in Western Australia, indicates correlation with the higher ovalis or laeviuscula Zones (lower Bajocian). An associated specimen of Sonninia playfordi Arkell is described.

An Upper Bajocian ammonite from the Raasay ‘Cornbrash’

Until quite recently the ‘Cornbrash’ of Rassay has been an anomaly in the regional setting. The identification of a brachiopod fauna by S. S. Buckman as including Cererithyris intermedia (J. Sowerby) led Lee (1920, pp. 65 & 76) to a correlation with the Lower Cornbrash (Upper Bathonian) of southern and eastern England, but the litho- and biofacies of the Upper Bathonian on the neighbouring Trotternish peninsula of Skye are not at all similar (Hudson, in Torrens 1980; Bradshaw and Fenton 1982). After study of the rich microflora extracted from parts of the ‘Cornbrash’ outcrop, Bradshaw and Fenton (1982) were able to date the rocks as late Bajocian, subfurcatum to parkinsoni Zone. This was based primarily on a marine microplankton assemblage. The subfurcatum Zone is preferred by Bradshaw and Fenton because of lithological correlation with units of probable subfurcatum Zone age of south Trotternish (Morton 1976). During my recent undergraduate mapping project carried out in northern Raasay I examined the ‘Cornbrash’ outcrops in some detail. I was fortunate enough to find an ammonite within brown, gritty, fossiliferous sandstone 1.5 km N.E. of Storav's Grave above the middle branch of Glam Burn \[NG 5701 4290\] (Section 2 in fig. 1 of Bradshaw and Fenton 1982). The specimen comprises one third of a whorl in a good state of preservation with the ribs and ventral surface clearly visible. The specimen has been presented to the Royal Scottish Museum, registration number RSMGY. 1983. 16. 1. Prof. J. H. Callomon and Dr. H. S. Torrens . . .

First record of late Bajocian (Jurassic) ammonites in the Fernie Formation, Alberta

An ammonite fauna near Frank, Alberta represents the first record of upper Bajocian strata in the Fernie Formation. The fauna consists of Megasphaeroceras cf. M. rotundum Imlay, Spiroceras cf. S. orbignyi Baugier and Sauzé, and fragmentary stephanoceratids and is correlated with the zone of Megasphaeroceras rotundum (Subfurcatum standard zone; early late Bajocian), which is also known in southern Alaska, eastern Oregon, western interior U.S.A., central Peru, and the Andes of Chile and Argentina. A fragment of Leptosphinctes cf. L. talkeetnanus (Imlay), which is latest Bajocian to early Bathonian in age, occurs 21 m above the Megasphaeroceras rotundum fauna.

Middle jurassic brachiopods from Ellsworth Land, Antarctica

Abstract Flabellirhynchia australensis n.sp. is described from two localities in eastern Ellsworth Land, Antarctica. At one locality it is associated with Middle Bajocian ammonites. Its position in Flabellirhynchia is based on comparison with F. concinna Crickmay. Another rhynchonellid, closely allied to Rhactorhynchia or Cymatorhynchia, is also recorded.

Baryt und ankerit aus Ammoniten des Berner Juras (Schweiz)

Barite has been found for the first time in the Swiss Jura (Hirni, Passwang). Small, white crystals of barite with yellow ankerite grow on calcite in different chambers of a Bajocian ammonite. Chemical data (by microprobe analysis) and lattice parameters (refined by leastsquares from powder patterns) are given for the two minerals. There is a distinct correlation between the cell dimensions and the chemical composition for both of them. The genesis is discussed.

The bajocian ammonites of Western Australia

The Bajocian (Middle Jurassic) ammonites of Western Australia are described on the basis of an extensive collection made in 1952-3 by Phillip E. Playford, who contributes a stratigraphical introduction and a geological map. In this introduction he subdivides the Jurassic sediments (total thickness at outcrop up to 550 ft.), names and defines most of the groups and formations for the first time, and elucidates complications due to lateritic alteration. All the ammonites come from the Newmarracarra Limestone (up to 38 ft. thick). The ranges of the species are determined so far as practicable. The ammonite fauna comprises at least twenty-three species (at least eleven new), now assigned to seven genera. The new collection enables Crick’s type specimens, named in 1894 on the basis of defective and inadequate material, to be reinterpreted, and necessitates complete generic revision. The age of the fauna is Middle Bajocian. Most of it belongs to the Sowerbyi Zone, but in places there is believed to be also a thin representative of the Humphriesianum Zone. A comparison (now possible for the first time) is made with the Bajocian ammonite faunas of circum-Pacific countries and central Asia: New Guinea, the Moluccas, Tibet, eastern Siberia, Alaska, western Canada, western United States, Mexico and the Andes. Photographs are given of comparable ammonites from Tibet, Canada and Argentina, not previously published photographically. Apart from the Moluccas, the peculiar Australian stephanoceratid ammonites, Pseudotoites and their allies, are not known from any of the extensive Bajocian outcrops in the Old World. Hitherto they have been thought to be confined to Western Australia. It is now shown that Pseudotoites occurs in the Moluccas, British Columbia, Alaska and Argentina, and that some rarer allied forms of Western Australia belong to the genus Zemistephanus , hitherto known only in Canada, Alaska and the United States. This distribution is held to indicate free migration across the Pacific Ocean. The regional basis of evolutionary radiation in several groups of Middle Bajocian ammonites is pointed out, and the significance of the facts for evolutionary and systematic theory is briefly stated. The Middle Bajocian stratigraphy of north-west Europe is historically summarized in order to provide a framework and scale of comparison for the Australian and circum-Pacific deposits.

Bajocian Ammonites Collected by Sir Henry Hayden near Kampadzong, Tibet

AbstractSeven species of ammonites from the Lungma Limestone of Tibet are described and figured. The assemblage is Middle Bajocian, of the Sowerbyi and Sauzei Zones, and possible also Humphriesianum Zone, and indicates that the formation is condensed. Its affinities are chiefly with European faunas, but it has one species in common with the Pamirs and another with Western Australia.

Jurassic ammonites from Jebel Tuwaiq, central Arabia with stratigraphical introduction

The Jurassic system as developed along a strike of some 500 miles in the Jebel Tuwaiq, central Saudi Arabia (Nejd), is described and divided into formations, of which the names are here published for the first time, with a synopsis of their lithology and fossil contents. Apart from small collections,'including only one ammonite species, made on camel journeys by H. St J. B. Philby, nothing was hitherto known of these formations; their age (except for the Middle Callovian date given by the ammonite), and the existence of most of them, were unknown until the area began to be mapped and explored geologically about 1940 by geologists of the Arabian American Oil Company, whose results are here summarized. The total thickness of marine Jurassic rocks described exceeds 1000 m., all of it in neritic facies. This sequence is divided, in ascending order, into Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila and Riyadh formations, above which follows the Cretaceous system. Below the marine Marrat is the Minjur Sandstone (315 m.), of 'continental’ facies, with only obscure plant remains, which possibly corresponds to the supposedly Rhaetic-Lower Lias plant-bearing Kohlan Sandstones of the Yemen. The underlying Jilh formation of Saudi Arabia contains marine Middle Triassic fossils near the top. Collections of ammonites establish the presence of marine Jurassic stages from Lower Toarcian (Lower Marrat) to Lower Kimeridgian (Jubaila); the Riyadh has not yet yielded ammonites at outcrop. The ammonites described are of extraordinary interest from the points of view of palaeogeography, correlation, phylogeny and systematics. While most of the faunas contain just enough links with other parts of the world to establish broad correlations, many are entirely new, or confined, outside Arabia, to Sinai. In the Toarcian there was faunal continuity with Madagascar and Baluchistan, in the Bajocian only with Sinai, from the Middle Bathonian onwards with western Europe. Most of the commonest cosmopolitan genera are missing from central Arabia, where there lived a succession of highly peculiar forms characterized at several successive horizons by unstable suture-lines. It has been found necessary to make eight new genera, besides three already named but hitherto not found outside Sinai, Madagascar and Baluchistan, although employing a taxonomic scale so large that some palaeontologists will consider it old-fashioned. This is believed, however, to be more useful at this stage of our knowledge than a proliferation of generic names based on inadequate material; for the area from which the collections were made is about equal to that of all the Jurassic outcrops of England. The paper concludes with a discussion of problems of evolution, speciation, correlation, and palaeogeography raised by the material. In an appendix are described relevant Bajocian ammonites from Sinai, with a hitherto unpublished section of the strata.

Occurrence of Middle Jurassic Rocks in Western Interior of United States

The occurrence of Middle Jurassic rocks in the Western Interior region of the United States is shown by the presence of the Bajocian ammonites, Stemmatoceras and Defonticeras, in the Twin Creek limestone of southeastern Idaho. The occurrence of Middle Jurassic rocks of the Bathonian stage is indicated by the presence of the ammonites, Arctocephalites and Cranocephalites, in the lower part of the Ellis formation of Montana. Marine Jurassic deposits older than the Bathonian appear to have been limited to the central part of the sedimentary trough and will probably be found only along the border area of Idaho and Wyoming, in north-central Utah, and in southwestern Montana.